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Plasma Testosterone Profile of Male Tawny Owls Strix aluco in Relation to Breeding Density, Breeding Experience, and Offspring Provision
Abstract
The role of plasma testosterone in territorial behavior and breeding success, which has not previously been examined in nocturnal raptors, was studied in male Tawny Owls. Blood was collected for testosterone analysis during the territorial and nestling periods from owls breeding in nest boxes in Duna-Ipoly National Park, Hungary. Testosterone levels, defense activity, prey supply for broods and reproductive performance were related to breeding density and breeding experience of males. Defense activity, as measured by responses to broadcasted hooting calls and dummy owls during the territorial period, correlated positively with testosterone concentration. Males with more breeding experience had high testosterone levels and occupied better territories in dense breeding areas than less experienced males which had low testosterone concentrations. Mate fidelity was linked male quality: females were more likely to be tenacious in good territories. Testosterone levels dropped between territorial and feeding periods, however males with higher concentrations fed nestlings more frequently with high mass prey per night because food was more abundant in territories they had secured. We suggest that differences in testosterone levels in the provisioning period are related to the males' abilities to acquire territories of different quality.
... Parental age was determined by the pattern of the primaries and secondaries (Petty 1992). In a previous study we found that male tawny owls supplied the females and chicks with more preys and produced more fledglings in those broods reared at lower altitudes (Sasvári et al. 2009). This higher breeding success was attributed to the fact that the main alternative prey species, the birds, were more abundant around the owl nests at low than high elevations (Sasvári & Hegyi 2005). ...
... In previous studies we have shown with three relevant relationships that the different territory qualities might be separated by their altitude range in the breeding area of the tawny owls. 1) We found that the main alternative prey species, the songbirds and woodpeckers, which are particular important for the owls when snow cover the ground in incubation and brooding period, were more abundant around the owl nests in low rather than high elevations (Sasvári & Hegyi 1998, 2005. 2) Males delivered more prey items with higher mass of prey to females and chicks and produced more fledglings in the broods reared at lower altitudes (Sasvári et al. 2009). 3) Since superior habitats are the most desirable, territory quality may be measured by the occupancy rate of the breeding area (review: Sergio & Newton 2003). ...
... 3) Since superior habitats are the most desirable, territory quality may be measured by the occupancy rate of the breeding area (review: Sergio & Newton 2003). The higher settlement densities reflected better territory quality in low elevations for tawny owls in our study area (Sasvári & Hegyi 2005, Sasvári et al. 2009). ...
Nest boxes for breeding tawny owls (Strix aluco) were located in a mixed oak-hornbeam-beech (Quercus-Carpinus-Fagus) forest in the Duna-Ipoly National Park, 30 km northwest of Budapest, Hungary during the period 1992-2010. The 550 m altitude range (120-680 m) of the study area was divided into low and high elevations: < 400 m, and > 400 m, each containing 88 nest-boxes. We marked 77 males in their first breeding year and evaluated the lifetime territory occupation and reproductive performance for individuals which reached five, six, seven, eight or nine years old. The fledgling production of tawny owl males increased in their first, second and third years, reached a high level in years four and five, but declined once they were six years old. The relative low reproductive performance in the early and late years of the males' lives may be attributed to the fact that the majority of males were unable to occupy high quality territories, and the rate at which individuals skipped breeding was high. We suggest that both the reduced ability to acquire high quality territories and declining fledging production in late years of males reflect senescence patterns in tawny owls.
... In temperate zone birds, the androgen testosterone has been suggested to be an important mediator in the trade-off between paternal effort and mating effort (Wingfield et al. 1990; Ketterson and Nolan 1999;Hau 2007) for several reasons. Firstly, in socially monogamous birds, male testosterone concentrations are typically much higher around territory establishment and the female fertile period than during the paternal care phase (e.g., Wingfield et al. 1990;Pinxten et al. 2007;Sasvári et al. 2009). Secondly, testosterone implant experiments generally find that males with artificially elevated testosterone concentration during provisioning feed their young at a lower rate than control males (e.g., Saino and Møller 1995;De Ridder et al. 2000;Van Roo et al. 2004;Lynn et al. 2009; but see Van Duyse et al. 2000;Lynn et al. 2005). ...
... McGlothlin et al. (2007) found that in dark-eyed juncos (Junco hyemalis), male feeding rate was not related to testosterone but was negatively correlated with the magnitude of increase in testosterone after injection with gonadotropin-releasing hormone (GnRH). In tawny owls (Strix aluco), males with higher testosterone fed nestlings more frequently and with larger prey (Sasvári et al. 2009). In the owl study, it is unclear whether this was a direct effect of testosterone or whether this resulted from males with high testosterone having previously secured the best territories in terms of food availability. ...
... Repeatability (r) of male testosterone concentrations was calculated after Lessells and Boag (1987). Individual testosterone concentration may vary as a result of differences in extrinsic factors at the time of sampling, such as sexual state (fertile or not) of the social mate (Wingfield et al. 1990), time of day (Balthazart 1976), or breeding density (Sasvári et al. 2009). At the start of the breeding season, we caught and blood-sampled males during 6 different mornings, the first being 30 April and the last being 12 May. ...
Males may face a trade-off between caring for offspring and pursuing additional matings. In birds, the androgen testosterone has been suggested to be a key proximate mediator in this trade-off for several reasons. At the population level, high testosterone is typically associated with the period of intense male--male competition over females, whereas low testosterone is associated with the period of paternal care. In addition, males with experimentally elevated testosterone during provisioning feed their young at a lower rate than control males. Nearly all studies observing these patterns, however, ignore the tremendous variation in endogenous testosterone concentration that exists within a population of males, even during the same breeding stage. Because selection acts at the level of individual, this variation has to be taken into account when studying proximate mechanisms mediating the paternal and mating effort trade-off. Studying barn swallows we here show that, within males, testosterone concentrations were not higher around the fertile period of the social mate than during nestling feeding. More importantly, 30% of males showed no decrease in testosterone concentration between these 2 periods. Further, male feeding effort was not related to testosterone concentration during feeding. These results indicate that, at least in barn swallows, endogenous testosterone is not a key mediator in the trade-off between paternal effort and mating effort. Our results also stress that to understand how selection has shaped temporal testosterone profiles and action, it is crucial to study the relationship between testosterone and traits contributing to fitness at the level of the individual. Copyright 2011, Oxford University Press.
... As predicted by the 'challenge hypothesis' (Wingfield et al., 1990), aggressive interactions between males should result in an increase in circulating T. This prediction has been confirmed in some species (Moore, 1984;Wingfield, 1984;Wikelski et al., 1999;McGlothlin et al., 2007), but not in others (Van Duyse et al., 2004;Landys et al., 2007Landys et al., , 2010. Aggressive male-male interactions are expected to increase as conspecific density increases, contributing to the positive association between T and conspecific density in some species (Wingfield and Hahn, 1994;Sasvári et al., 2009;Horton et al., 2010). Because of its role in mate attraction, T secretion also increases as a male's partner becomes fertile (Johnsen, 1998;Schwabl et al., 2005) and as the pool of fertile females within a population grows (Peters et al., 2001). ...
... Our detection of an inverse relationship between T and conspecific density is, to our knowledge, unique in that other studies have either reported no relationship (Eikenaar et al., 2011b) or an increase in T under conditions of high density (Wingfield and Hahn, 1994;Sasvári et al., 2009). The latter is the expectation under the assumption that T mediates aggression and that malemale conflict increases when nesting density is high (Wingfield et al., 1990). ...
We documented parental behaviour and paternity of eastern kingbirds (Tyrannus tyrannus) to test the predictions that paternal care would decline with increasing loss of paternity, increasing nesting density (a proxy for probability of paternity loss), male quality, and number of fertile females available in the population. Extra-pair young were found in 58% of 45 nests for which behaviour was recorded and a higher proportion of young were extra-pair as nesting density increased. Male feeding rate declined with increasing nesting density and male quality, but neither feeding rate nor a composite measure of paternal behaviour varied with number of fertile females or paternity. Although alternative explanations exist, one interpretation of the reduced paternal care at high nesting density was that it was a response to perceived threats of paternity loss. The ultimate basis for the lower paternal effort of higher quality males is unclear but we discuss several possible explanations.
... As predicted by the 'challenge hypothesis' (Wingfield et al., 1990), aggressive interactions between males should result in an increase in circulating T. This prediction has been confirmed in some species (Moore, 1984;Wingfield, 1984;Wikelski et al., 1999;McGlothlin et al., 2007), but not in others (Van Duyse et al., 2004;Landys et al., 2007Landys et al., , 2010. Aggressive male-male interactions are expected to increase as conspecific density increases, contributing to the positive association between T and conspecific density in some species (Wingfield and Hahn, 1994;Sasvári et al., 2009;Horton et al., 2010). Because of its role in mate attraction, T secretion also increases as a male's partner becomes fertile (Johnsen, 1998;Schwabl et al., 2005) and as the pool of fertile females within a population grows (Peters et al., 2001). ...
... Our detection of an inverse relationship between T and conspecific density is, to our knowledge, unique in that other studies have either reported no relationship (Eikenaar et al., 2011b) or an increase in T under conditions of high density (Wingfield and Hahn, 1994;Sasvári et al., 2009). The latter is the expectation under the assumption that T mediates aggression and that malemale conflict increases when nesting density is high (Wingfield et al., 1990). ...
... mous and single-brooded species which is resident in the Palearctic region. An earlier study showed that the breeding success of Tawny Owls varied with the quality of territories found at different altitudes (Sasvári et al. 2009). At low elevations, where food was in plentiful supply and owls bred at high densities, the parents produced more fledglings than at higher elevations. ...
... This would mean that the detected high and low settlement densities of Tawny Owls colonies at low and high altitudes respectively indicate that territory quality differs between altitudes. The presumed high quality of occupied territories at lower elevations is indeed reflected in the prey supplied to the broods (Sasvári et al. 2009). A previous study comparing the reproductive performance of different bird species over a large altitudinal range (0-4000 m) found no differences in the risk of nest predation between low and high elevations (Badyaev & Ghalambor 2001). ...
Nest predation was studied in a population of Tawny Owls Strix aluco in the Duna-Ipoly National Park, Hungary during the period 1992–2009, and related to the choice of nesting sites at different altitudes, the breeding experience of males, and weather conditions. The use of nesting sites at high elevations reduced the risk of nest predation in comparison to lower elevations, where breeding territories occur at a higher density. Also, the risk of nest predation decreased as the local breeding experience of males increased. Adverse weather conditions, i.e. long-lasting snow cover, increasing breeding density, and later laying dates enhanced the probability of breeding failure by nest predation. Broods were depredated in the nestling period rather than during incubation.
... Individual testosterone concentration may vary as a result of differences in extrinsic factors at the time of sampling, such as sexual state (fertile or not) of the social mate [1], time of day [38] or breeding density [39]. Consequently, extrinsic factors could mask a potential relationship between male testosterone and fertilization success. ...
... Variation in male elevated testosterone concentration may contribute to sexual selection through other routes than extra-pair paternity. A positive correlation between male elevated testosterone concentration and copulation or fledgling success may, at least in part, result from males with higher testosterone being able to secure higher-quality territories (red-winged blackbirds [15]; tawny owls (Strix aluco) [39]), occupy the superior positions in a lek (black grouse [2]), or build higher-quality bowers (satin bowerbirds [14]). Testosterone implant experiments may elucidate whether the above mechanisms are truly a cause of the observed relationship between testosterone and reproductive success. ...
In most monogamous bird species, circulating testosterone concentration in males is elevated around the social female's fertile period. Variation in elevated testosterone concentrations among males may have a considerable impact on fitness. For example, testosterone implants enhance behaviours important for social and extra-pair mate choice. However, little is known about the relationship between natural male testosterone concentration and sexual selection. To investigate this relationship we measured testosterone concentration and sexual signals (ventral plumage colour and tail length), and determined within and extra-pair fertilization success in male North American barn swallows (Hirundo rustica erythrogaster). Dark rusty coloured males had higher testosterone concentrations than drab males. Extra-pair paternity was common (42% and 31% of young in 2009 and 2010, respectively), but neither within- nor extra-pair fertilization success was related to male testosterone concentration. Dark rusty males were less often cuckolded, but did not have higher extra-pair or total fertilization success than drab males. Tail length did not affect within- or extra-pair fertilization success. Our findings suggest that, in North American barn swallows, male testosterone concentration does not play a significant direct role in female mate choice and sexual selection. Possibly plumage colour co-varies with a male behavioural trait, such as aggressiveness, that reduces the chance of cuckoldry. This could also explain why dark males have higher testosterone concentrations than drab males.
... However, results of testosterone manipulations are in contrast to published studies relating the naturally occurring variance in testosterone levels with paternal care: post-capture testosterone concentrations (termed 'baseline testosterone' for the remainder of this article) were not related to paternal care in barn swallows [36], black redstarts (Phoenicurus ochruros; [37]), dark-eyed juncos [38], eastern bluebirds (Sialia sialia [39]), European starlings [40], and northern cardinals (Cardinalis cardinalis; [41]; note, however, that in dark-eyed juncos maximum testosterone was related to pre-capture levels of paternal care [38]). Male tawny owls (Strix aluco) with higher testosterone levels showed even higher provisioning rates than males with lower circulating levels of the hormone [42]. Thus, there is little indication that males with higher baseline levels of testosterone are less paternal than males with lower baselines. ...
A long-standing tenet of evolutionary endocrinology states that testosterone
mediates the life-history trade-off between mating and paternal care. However,
the support for a role of testosterone in suppressing paternal care is
mixed: implantation studies in birds suggest that high-level testosterone
implants suppress paternal care, but circulating levels of testosterone and
paternal care are typically not correlated. Because any trade-off in real life
must be realized with hormone levels that are within an individual’s reaction
norm, it is crucial to show that natural changes in the hormone can modulate
behaviour. Here, we used GnRH-injections to alter testosterone levels of freeliving
male black redstarts within each individual’s hormonal reaction norm:
individuals experiencing a short-term peak in testosterone resumed feeding
their offspring later and showed a stronger suppression of offspring-feeding
behaviour than control males. For the first time, this study demonstrated that
short-term peaks in testosterone within the hormonal reaction norm of individuals
can suppress paternal behaviour. Our findings reconcile previous
seemingly contradictive effects that testosterone implants had on paternal
care and the absence of correlations between circulating testosterone levels
and paternal care, and demonstrate that the differential production of testosterone
within the hormonal reaction norm of single individuals can indeed
function as a mechanism to mediate a potential trade-off between mating
and parenting. On a broader note, our results suggest that natural and
short peaks in testosterone can elicit adaptive behavioural changes.
... Tawny owl parents display clear division of duties during reproduction. Males deliver food to their offspring and partner (Sasvári et al., 2009), while females guard the brood and distribute the prey items among the progeny. Our brood size manipulation experiment is primarily expected to increase male parental effort and reduce female food resources. ...
Oxidative stress, determined by the balance between the production of damaging reactive oxygen species (ROS) and antioxidant defences, is hypothesised to play an important role in shaping the cost of reproduction and life history trade-offs. To test this hypothesis, we manipulated reproductive effort in 94 breeding pairs of tawny owls (Strix aluco) to investigate the sex- and melanic-specific effects on markers of oxidative stress in red blood cells (RBCs). This colour polymorphic bird species shows sex-specific division of labour and melanic-specific history strategies. Brood sizes at hatching were experimentally enlarged or reduced to increase or decrease reproductive effort, respectively. We obtained an integrative measure of the oxidative balance by measuring ROS production by RBC, intracellular antioxidant gluthatione levels (tGSH) and membrane resistance to ROS. We found that light melanic males (the sex assuming offspring food provisioning) produced more ROS than darker conspecifics, but only when rearing an enlarged brood. In both sexes, light melanic individuals had also a larger pool of intracellular antioxidant glutathione than darker owls under relaxed reproductive conditions (i.e. reduced brood), but not when investing substantial effort in current reproduction (enlarged brood). Finally, resistance to oxidative stress was differently affected by the brood size manipulation experiment in males and females independently of their plumage coloration. Altogether, our results support the hypothesis that reproductive effort can alter the oxidative balance in a sex- and colour-specific way. This further emphasizes the close link between melanin-based coloration and life history strategies.
... Similar results were reported by Silverin (1998) when comparing the territorial behaviour and T levels of male pied flycatchers breeding in optimal and suboptimal habitats. Although the biological relevance of such differences in T levels in females is little known, similar differences in males have been shown to have a biological significance in relation to breeding density (Ball & Wingfield 1987;Smith et al. 2005;Sasvari et al. 2009), survival (Mart ınez-Padilla et al. 2014 or oxidative stress (Alonso-Alvarez et al. 2007). High T levels may be adaptive when nesting sites are limited because they prepare the individuals for intense competition with conspecifics (Moss et al. 1994). ...
Nesting holes are a scarce resource for obligated cavity nesting birds and an important selective force for the evolution of aggressive female behaviours, which may be mediated by testosterone (T) levels. It is known that during periods of intense intrasexual competition such as initial breeding stages, females are highly aggressive towards intruding females. Here, we studied the implications of T levels for female-female competition by comparing levels of aggressiveness towards simulated female intruders (decoys) in two populations of the pied flycatcher (Ficedula hypoleuca) with a marked difference in breeding density. To this end, we exposed free-living females to simulated territorial intrusions during 30 min when nest construction was almost complete. T levels of females were measured at the beginning of incubation under the assumption they are positively associated with T levels during nest building. We also related aggressiveness to T levels in both populations. Furthermore, we aimed at detecting if variation of T levels may explain female incubation attendance. Females showed higher T levels in the populations where pied flycatchers were exposed to a higher likelihood of conspecific interactions (high breeding density) than in the population with low breeding density. Female territorial presence, vigilance at the nest-box and proximity to decoys were negatively related to circulating T levels in the high density population, but not in the low density population. Differences in T levels between populations did not result in differences in female incubation attendance, but T levels were negatively related to the incubation attendance in females from the population showing high T levels. In our populations, T levels in females prior to laying reflect the need to defend nesting cavities which is higher at high breeding density and in subdominant females. High T levels are costly in terms of incubation attendance.
Empirical studies often support the notion that testosterone inhibits paternal behaviour in animals, although most studies have focused on high, as opposed to low, levels of paternal care. We studied the relationship between plasma testosterone levels and incubation in male Japanese Barn Swallows Hirundo rustica gutturalis, in which some males participate in incubation. Contrary to our prediction, the probability of incubation by males increased with plasma testosterone level. Male incubation behaviour might have emerged as part of nest monitoring behaviour by high‐testosterone males in some subspecies that experience high nest predation rates.
It has recently been demonstrated that male ornaments feedback on their physiological state and this is expected to be followed by behavioral alterations in paternal care. However, only a few studies have shown the effects of experimental manipulation of ornamentation on paternal care behavior. The present study investigated the effects of experimental manipulation of the size of a male ornament, the red throat patch, on the feeding rates of nestlings by male and female Barn Swallows (Hirundo rustica gutturalis). First, we observed a dynamic change in male throat patch height during the breeding season, indicating that behavioral alterations in response to changes in throat patch height may be beneficial for male Swallows. Second, males with an experimentally reduced throat patch, but not their mates, fed their nestlings significantly more often compared with control individuals. In addition, we found a significant interaction between treatment and male body size: i.e., the effects of a reduced throat patch height varied depending on male body size. Among males with a reduced throat patch, male feeding rate increased with body size, but this was not the case in control males. This interaction is consistent with the idea that small males have low behavioral flexibility, and might contribute on positive correlation between throat ornamentation and male body size, given that behavioral flexibility is adaptive. The current finding indicates that males can increase paternal care in response to a seasonal decrease in ornamentation and this differential behavioral flexibility among males may facilitate the honesty of ornamentation.
Although paternal care is generally rare among vertebrates, care of eggs and young by male birds is extremely common and may take on a variety of forms across species. Thus, birds provide ample opportunities for investigating both the evolution of and the proximate mechanisms underpinning diverse aspects of fathering behavior. However, significant gaps remain in our understanding of the endocrine and neuroendocrine influences on paternal care in this vertebrate group. This review focuses on proximate mechanisms of paternal care in birds, with an emphasis on specific hormones that vary predictably and/or unpredictably during the parental phase in both captive and wild birds: prolactin and progesterone are generally assumed to enhance paternal care, whereas testosterone and corticosterone are commonly-though not always correctly-assumed to inhibit paternal care. In addition, because endocrine secretions are not the sole mechanistic influence on paternal behavior, potential roles for certain neuropeptide systems (specifically the oxytocin-vasopressin nonapeptides and gonadotropin inhibitory hormone) and social and experiential factors in influencing paternal behavior in birds are also explored. Ultimately, mechanistic control of fathering behavior in birds is complex, and I suggest specific avenues for future research with the goal of narrowing gaps in our understanding of this complexity. Such avenues include (1) experimental studies that carefully consider not only endocrine and neuroendocrine mechanisms of paternal behavior, but also the ecology, phylogenetic history, and social context of focal species; (2) investigations that focus on individual variation in both hormonal and behavioral responses during the parental phase; (3) studies that investigate mechanisms of maternal and paternal care independently, rather than assuming that the mechanistic foundations of care are similar between the sexes; (4) expansion of work on interactions of the neuroendocrine system and fathering behavior to a wider array of paternal behaviors and taxa (e.g., currently, studies of the interactions of testosterone and paternal care largely focus on songbirds, whereas studies of the interactions of corticosterone, prolactin, and paternal care in times of stress focus primarily on seabirds); and (5) more deliberate study of exceptions to commonly held assumptions about hormone-paternal behavior interactions (such as the prevailing assumptions that elevations in androgens and glucocorticoids are universally disruptive to paternal care). Ultimately, investigations that take an intentionally integrative approach to understanding the social, evolutionary, and physiological influences on fathering behavior will make great strides toward refining our understanding of the complex nature by which paternal behavior in birds is regulated.
Copyright © 2015. Published by Elsevier Inc.
Abstract The roles of testosterone (T) during reproduction are often complex and vary among and within vertebrate taxa and locations, making general hypotheses relating T to breeding behavior and success difficult to integrate. In birds, T is thought to influence degrees of territoriality and associated aggression in males to maximize breeding success. Importantly, most work supporting these ideas has been conducted in the Northern Hemisphere. However, accumulating work on tropical species has shown divergent patterns of T in association with breeding behavior. The compilation of work from northern temperate and tropical species suggests that the function of T in relation to breeding behavior varies across latitude and environmental conditions. We investigate the patterns of T in relation to breeding behavior in a subspecies of the rufous-collared sparrow Zonotrichia capensis australis breeding at high latitude in the Southern Hemisphere (55°S). We then compare the T profiles and breeding behaviors of male Z. c. australis to conspecifics breeding in the tropics and congeners in North America to test the hypothesis that environments with breeding seasons of similar lengths will drive similar patterns of T in relation to breeding behavior. We found that Z. c. australis have high levels of T during the early-breeding periods when territories are being established and low levels of T during the parental phase of breeding, similar to temperate and Arctic birds in the Northern Hemisphere but unlike tropical Zonotrichia capensis costaricensis. In contrast, we found that Z. c. australis also exhibit similar aggressive behaviors in early breeding and midbreeding, unlike many birds in the Northern Hemisphere.
Nest and territory defence are risky and po-tentially dangerous behaviours. If the resolution of life history trade-offs differs between individuals, the level of defence may also vary among individuals. Because melanin-based colour traits can be associated with life history strategies, differently coloured individuals may display different nest and territory defence strategies. We investigated this issue in the colour polymorphic tawny owl (Strix aluco) for which plumage varies from dark to light reddish melanic. Accordingly, we found that (1) our presence induced a greater response (flying around) from dark-coloured than light-coloured females and (2) dark reddish males suffered lower nest predation rates than light-coloured males. In experimentally en-larged broods, the probability that females reacted after we played back the hoot calls of a stranger male was higher if these females were lighter reddish; the oppo-site pattern was found in experimentally reduced broods with dark parents being more reactive than light parents. Finally, darker females alarmed more frequently when paired with a light than with a dark male, suggesting that partners adjust their behaviour to each other. We also tested whether colouration is used as a signal by conspecifics to adjust the level of their defensive behav-iour. Accordingly, breeding females responded more vigorously to a dark than a light reddish stuffed tawny owl placed beside their nest. We conclude that melanin-based colouration is a signal of alternative nest and territory defence behaviour that depends on ecological factors.
Tawny Owl diet was studied in the primeval deciduous forests of Białowieża National Park (eastern Poland), where the Tawny Owl was the most numerous predator and the forest rodents were non-cyclic with recurrent waves of outbreaks-crashes (triggered by heavy seed crops at 6-9-year intervals). The study (1985-1992) covered both noncyclic and outbreak-crash years. Analysis of 1,116 pellets showed that Tawny Owls were specialised on forest rodents, mainly the yellow-necked mouse Apodemus flavicollis, but that they also exploited secondary resources: birds (especially Turdus, Parus and Carduelis), insectivores, and frogs. Between-year changes in the Tawny Owls' diet were mainly influenced by the abundance of yellow-necked mice, which thus determined the proportion of bank voles Clethrionomys glareolus taken and of alternative prey such as birds, amphibians, and shrews. Snow cover decreased the efficiency with which owls hunted for bank voles but not for mice and shrews. The alternative prey hypothesis, proposed to explain the linked cyclicity of microtines, shrews, and small game in northern Europe, may also be applied to animal population fluctuations in the temperate oak-dominated forests of Europe. During rodent crash years, predation by Tawny Owls may heavily affect the population dynamics of other small animals.
A combination of field and laboratory investigations has revealed that the temporal patterns of testosterone (T) levels in blood can vary markedly among populations and individuals, and even within individuals from one year to the next. Although T is known to regulate reproductive behavior (both sexual and aggressive) and thus could be expected to correlate with mating systems, it is clear that the absolute levels of T in blood are not always indicative of reproductive state. Rather, the pattern and amplitude of change in T levels are far more useful in making predictions about the hormonal basis of mating systems and breeding strategies. In these contexts we present a model that compares the amplitude of change in T level with the degree of parental care shown by individual males. On the basis of data collected from male birds breeding in natural or captive conditions, polygynous males appear less responsive to social environmental cues than are monogamous males. This model indicates that there may be widely different hormonal responses to male-male and male-female interactions and presumably equally plastic neural mechanisms for the transduction of these signals into endocrine secretions. Furthermore, evidence from other vertebrate taxa suggests strongly that the model is applicable to other classes
Some secondary sexual traits (SSTs) such as structural characteristics are semi-permanent or static, while others, such as court- ship display, are more labile or dynamic. In this paper we report results from two experiments designed to test the relative attractiveness to female dark-eyed juncos (Junco hyemalis, Passeriformes, Aves) of a relatively static plumage trait, the amount of white in the tail, and a relatively dynamic behavioral trait, courtship intensity. The experiments derived from a study showing that female juncos prefer males that court more vigorously. We asked whether females also base their preferences on plumage traits and how they respond when presented with a choice between attractive traits that are either static (plumage) or dynamic (courtship) in nature. In the first experiment we presented males to females in paired mate-choice trials and found that males enhanced with more white in their tails were more attractive to females than controls with unenhanced tails. Females spent more time with enhanced males and directed more sexual displays toward them. In the second experiment we tested whether females preferred males with enhanced tails (a static SST) or males with enhanced hormone-mediated courtship behavior (a dynamic SST). In this experiment females did not demonstrate a consensus preference for either the static or the dynamic trait. Instead, some females preferred the male whose courtship performance was enhanced with testosterone, while others preferred the male with an enhanced tail. We conclude that both kinds of traits are important in junco mate choice, but that some females apparently weigh static traits more heavily than dynamic ones, while other females use opposite weightings. Key words: dark- eyed juncos, Junco hyemalis, lability, mate choice, secondary sexual traits. (Behav Ecol 10:91-96 (1999))
Territorial intolerance can limit bird numbers by preventing subordinates from holding territories and breeding. We report the first controlled and replicated experiment in a natural population of territorial animals which shows that population density can be decreased by implants of testosterone inducing increased aggressiveness. Implanted territorial cocks expanded their territories at the expense of unimplanted neighbours, some of which left the study area. The probability of an unimplanted cock leaving the study area increased with his number of implanted neighbours. Implanted cocks had better breeding success but poorer subsequent survival than control cocks.
SUMMARY Monogamous male birds typically allocate less e¡ort to courtship and more to parental behaviour than males of polygynous species. The seasonal pattern of testosterone (T) secretion varies accordingly. Mono- gamous males exhibit a spring peak in plasmaT followed by lower levels during the parental phase, while males of polygynous species continue to court females and maintain T at higher levels. To determine whether testosterone underlies the trade-o¡ between mating and parental e¡ort, we treated male dark- eyed juncos (Junco hyemalis) with exogenous Tand compared the reproductive success (RS) of T-treated males (T-males) to that of controls. T-males had lower apparent annual RS than controls, probably because elevated Treduced parental care. Nevertheless, annual genetic RS of the treatment groups was similar because (i) T-males su¡ered fewer losses in genetic RS due to extra-pair fertilizations (EPFs), and (ii) T-males gained more genetic RS throughtheir own EPFs.This is the ¢rst hormonal manipulation of an avian phenotype shown to in£uence male RS through EPFs. Together with other studies, it suggests that testosterone may have mediated the evolution of inter- and intraspeci¢c di¡erences in allocation of repro- ductive e¡ort to mate attraction and parental care.
Hormones influence many aspects of organismal behavior, physiology, and morphology, and thus hormones may lie at the root of many life-history trade-offs. By manipulating hormones we can create novel phenotypes (i.e., perform phenotypic engineering) and attempt to relate phenotypic variation to fitness. We report the effect of testosterone treatment on parental behavior and vocal behavior of adult male dark-eyed juncos. Testosterone partially suppressed paternal behavior and increased the frequency of song. When we compared treated males and controls for nine potential correlates of fitness (offspring growth and survival to the age of 10 d, condition of females, length of the interval between consecutive nestings, size of subsequent clutches and broods, mate retention within and between breeding seasons, and survival rate), we found no statistical differences. In some measures treated males outperformed controls, but in most the reverse was true. The power of some of our tests was not great enough to detect small differences. At this stage of our investigation, three interpretations of our results seem almost equally probable: (1) a broad range of behavioral phenotypes is selectively neutral in the junco, (2) male parental behavior is beneficial to males only in some years or habitats, or (3) we have yet to measure the correlates of fitness that are most strongly affected by the behavioral changes induced by elevated testosterone.
The number of songs in a male's repertoire, and the amount of time he spends singing, have been shown to correlate with territory size and quality, reproductive success, parental care and parasite load in some passerine species. In addition, females of some species use song rate and com- plexity as a cue to mate choice and are more responsive to more frequent and complex songs. Few studies, however, have examined the influence of body size and parasitic infections on the sound fre- quency (pitch) and structure of vocalizations of birds. The Tawny Owl (Styix aluco) hoot is important in communication between birds at night when visual information is limited, and is simple enough to allow a quantitative analysis of its structure. Six temporal and four frequency measures of the hoots of 50 Tawny Owls were taken, and compared to body mass, wing length, breeding success and number and intensity of parasitic infections of the singers. There was a decrease in call frequency with increasing body mass and the vibrato tail of the last note was longer in larger birds, but there was no part of the call that correlated with breeding success. There was an increase in call frequency as the number of parasitic infections increased, and there was a decrease in the length of calls as the intensity of parasitic infections increased.
Time-budget estimates of parental investment in captive populations of zebra finches Poephila guttata demonstrate that relative attractiveness correlates with the apportionment of parental investment as predicted by the differential-allocation hypothesis: attractive birds incur smaller parental expenditures; unattractive birds have higher parental investment per offspring, shorter life spans, and lower long-term reproductive success. -from Author
We approach conceptual issues in evolutionary biology from an endocrinological perspective, noting that single hormones typically act on several target tissues and thereby mediate suites of correlated phenotypic traits. When several components of such a suite are beneficial, an important evolutionary question is whether all are adaptations or some are exaptations. The answer may depend on whether the traits arose in response to selection on variation in systemic levels of the hormone on variation in responsiveness of target tissues to invariant levels of the hormone. If the former, se- lection probably acted directly on fewer than all traits; beneficial traits arising indirectly would be exaptations. In contrast, multiple beneficial traits that arose out of independent changes in target-tissue sensitivity to invariant hormone levels could all be adaptations. Knowledge of specific hormonal mechanisms as well as of historical selective regimes will be necessary to draw such distinctions. En- docrine constraints on evolution can be studied experimentally by applying hormones systemically and measuring interdependent re- sponses of beneficial and detrimental traits to selection (phenotypic engineering with hormones). Supposing that alteration of one trait in isolation would enhance fitness, cases in which the net effect of endocrine alteration of multiple traits is to depress fitness provide evidence for constraints. We briefly report results of recent studies employing hormonal manipulations, stressing our own work on the dark-eyed junco (Junco hyemalis: Emberizidae).
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Bird song structure may honestly reveal the health and vigour of individual males to potential mates and competitors. If this is the case then song may reflect the level of parasitic infections in males. We initially examined the relationship between blood parasite infections and the time taken to respond by 22 male Tawny Owls to a broadcast hoot. We then examined the call structure (total length and frequency) in relation to parasite infection, an index of owl condition and an index of food abundance. Owls with higher parasite loads responded more slowly to an intruder, although this relationship was not significant once condition and vole abundance were controlled for. We found no relationship between call length and any of the measured variables. However, the high frequency and the range of frequencies used in calls decreased with increasing parasite load. Thus, there was the potential for individuals to assess male parasite load from the speed of response and the structure of the call. Experimental tests of these relationships are now required.
Sexual selection arises when variance in male reproductive success is non-randomly related to phenotypic characters of males. Song can be considered as such a phenotypic character and several studies have shown that song complexity and/or song output are important in competition among males or in partner choice. In the blue tit Parus caeruleus a peak in male singing activity occurs at dawn during the female fertile period, i.e. after pair formation. The function of this dawn chorus is not well understood. In this study 20 male blue tits were recorded at dawn and song complexity and output were expressed as versatility, mean strophe length, mean percentage performance time and bouts with or without drift, i.e. with or without a systematic decline in percentage performance time. Females mated to males with a higher mean percentage performance time (output) and a higher versatility (complexity) started to lay eggs earlier, but the latter was not significant. Females mated to males that showed no drift in their song bouts laid significantly larger clutches. Our results thus suggest that in the blue tit, song output at dawn, rather than song complexity, might be a trait under sexual selection.
In this study, we investigated patterns of natural covariation between testosterone and reproductive status in a cooperatively breeding bird species, the azure-winged magpie (Cyanopica cyanus). To assess the relationship between testosterone and breeding behavior, we also manipulated testosterone (T) levels early in the season (before breeding started) using testosterone-filled or empty implants. Our results do not support the hypothesis that circulating testosterone levels affect the occurrence of helping behavior in the azure-winged magpie. Helping males had similar T levels to breeding males. Furthermore, experimentally augmented plasma T did not affect the likelihood of becoming either a helper or a breeder. Overall, these results are consistent with previous findings in other bird species and only give some support to the behavioral suppression hypothesis, suggesting that helping in the azure-winged magpie is a flexible behavioral option moderated in the short-term by social and ecological factors. Experimentally elevated testosterone levels, however, reduced the reproductive success of male breeders because of markedly lower levels of paternal care, had similar effects on their mates, but had the contrary effect on helpers, which raised levels of parental effort. We suggest increases in the share of paternity and in social prestige as possible explanations for these results.
Previous investigations have shown that malemale interactions over territory boundaries result in an increase in plasma levels of testosterone (T). In order to determine the time course of this increase following an agonistic challenge, male song sparrows, Zonotrichia (= Melospiza) melodia, were exposed to simulated territorial intrusions. Responding males were captured at intervals after onset of the intrusion for up to 60 min. Plasma levels of T were elevated by 10 min after onset of the challenge, and remained high in birds sampled 10–60 min after onset of the intrusion. In addition, plasma levels of luteinizing hormone (LH) increased and were significantly elevated after 10 min of stimulation. The results suggest that at least the prolonged elevation of T secretion precipitated by a male-male interaction was mediated via an increase in LH release. However, the initial surge of T levels in plasma may be LH independent. p Investigations of captive birds confirmed that circulating LH and T levels increased when male song sparrows were challenged by a conspecific male. Challenges involving a male house sparrow, Passer domesticus, had no effect suggesting that responses to male intrusions were species characteristic and not a result of general arousal. Further, auditory stimuli (vocalizations) alone, and visual stimuli (a devocalized male) alone were less effective in stimulating an increase in secretion of T than a combination of auditory and visual stimuli.
In temperate-zone birds, testosterone (T) influences male behavior during the breeding season. The elevation of plasma levels of T to a breeding baseline is necessary for basic reproductive behaviors, but it is still unclear whether variation in T levels above this critical threshold influences the intensity of these behaviors. Such a relationship between T and sexually selected traits is a critical assumption of the immunocompetence handicap hypothesis. We here experimentally elevated T levels in blue-tit males above the natural mean (T-males) during the period of nest building and egg laying, without manipulating hormone levels during chick feeding. T-males neither interacted more often with other males, nor did they respond more aggressively to a dummy intruder, compared to males with control implants. T-males did not guard their social mates more closely, but they were more likely to interact with potential extra-pair mates. Females mated to T-males did not change their behavior during egg laying and the treatment did not significantly affect male and female feeding rates. Despite this, nests of T-males produced larger and heavier fledglings in one study year. Our observations suggest that T levels above the natural mean during the mating period do not increase aggressive or territorial behavior in male blue tits. However, if females perceived T-males as high-quality mating partners, superior offspring development in nests of T-males might be caused by higher maternal investment. Hence, male behaviors involved in mate attraction may have been influenced by T levels above the natural mean.
Superb fairy-wrens are cooperatively breeding birds that combine stable, socially monogamous pair bonds and high levels of paternal care, with extreme levels of extra-pair mating and high levels of sexual competition. Our aim was to determine which testosterone correlates would prevail in such a life history that combines features that are conventionally associated with divergent hormone profiles. Unlike the situation in other species with monogamous pair bonds and high levels of paternal care, testosterone was elevated for a very long period of several months. During breeding there was a broad peak in testosterone followed by a gradual decline: this resembles the profile found in polygynous and promiscuous species. We found that three factors correlated with testosterone: development of the sexually selected nuptial plumage, social status and extra-group mating opportunities. Testosterone started increasing months prior to breeding, when the males that are later preferred as extra-group sires develop their nuptial plumage. Although these males did not have higher testosterone levels during breeding, they sustained high testosterone for much longer, and this might lend reliability to this sexual signal. Dominant males in groups had higher testosterone than pair-dwelling males and subordinate helpers. This was not due to differences in age, reproductive capability or mating opportunities, but was presumably associated with the assertion of dominance. In contrast to findings in other species, male testosterone level was not correlated with whether the resident female was fertile or had dependent nestlings. However, testosterone was strongly correlated with the total number of fertile females in the population, and hence with the opportunities for extra-group mating.
Previous studies have suggested that testosterone (T) profiles of male birds reflect a trade-off between mate attraction behaviours
(requiring high T levels) and parental care activities (requiring low T levels). In this study, we experimentally elevated
T levels of monogamous males in the facultatively polygynous European starling (Sturnus vulgaris), and compared mate attraction and paternal behaviour of T-treated males with those of controls (C-males). T-males significantly
reduced their participation in incubation and fed nestlings significantly less often than C-males. Females paired to T-treated
males did not compensate for their mate’s lower paternal effort. The observed reduction in a male’s investment in incubating
the eggs was accompanied by an increased investment in typical female-attracting behaviours: T-males spent a significantly
higher proportion of their time singing to attract additional females. They also occupied more additional nestboxes than C-males,
although the differences just failed to be significant, and carried significantly more green nesting materials into an additional
nestbox (a behaviour previously shown to serve a courtship function). T-males also behaved significantly more aggressively
than C-males. During the nestling period, the frequency of mate-attracting behaviours by T-treated and control males no longer
differed significantly. Despite the reduced paternal effort by T-males and the lack of compensation behaviour by females,
hatching and breeding success did not differ significantly between T- and C-pairs.
In males of socially monogamous birds, plasma testosterone (T) typically declines to low levels during the parental phase.
Studies on multiple-brooded species indicate that high T may be incompatible with high-quality paternal care. The length of
the breeding season may affect the costs and benefits of high T and its effect on paternal care. We studied the effect of
experimentally elevated T on paternal care in a single-brooded species with a short breeding season, the Lapland longspur
(Calcarius lapponicus). We monitored T levels and parental behavior in 16 males with subcutaneous T implants, 14 males with empty implants, and
14 unimplanted males. We videotaped nests when nestlings were 2–3 days old and again at 4–5 days. T males with 2- to 3-day-old
young visited nests and fed young less often than control males, and the mates of the T males compensated with elevated visits
and feedings. However, when nestlings were 4–5 days old, T males visited their nests at normal rates – though feeding movements
remained below normal – and T females visited and fed at normal rates. Nestling mass and nest success were similar in both
groups. Overall, high T suppresses paternal care in Lapland longspur males. The partial improvement of paternal care when
nestlings are older, despite high T, may be related to the short 6-week breeding season of this arctic species, and the consequently
reduced benefits of sexual behavior late in the breeding season.
Testosterone has been proposed to serve as the mediator that controls the relative effort that an individual male bird will
devote to mating effort versus parental effort. Here, we demonstrate a testosterone-influenced trade-off between parental
and mating efforts in male house finches. Male house finches with experimentally elevated testosterone fed nestlings at a
significantly lower rate, but sang at a higher rate than males without manipulated testosterone levels. Females mated to testosterone-implanted
males fed nestlings at a significantly higher rate than females mated to males without testosterone implants, resulting in
similar feeding rates for both treated and untreated pairs. The effects of testosterone on male house finches, however, were
not as dramatic as the effects of testosterone observed in some other socially monogamous species of birds. Because extra-pair
copulations are uncommon in house finches and males provide substantial amounts of parental care, these more modest effects
may be due to differences in how the allocation of reproductive effort affects the costs and benefits of different reproductive
behaviors.
Abstract. Oystercatchers breeding on Schiermonnikoog divorced, because one of the mates deserted (leaving the partner as a victim of its choice), or because one or both mates were chased from the territory by usurpers. Though the majority of divorced birds were classified as victims, among birds that retained breeding status females more often chose to divorce than did males. Females changing mates were also more likely to move between territories than were males. A much larger database from Skokholm showed that the reproductive success of new pairs, which depended on the breeding history of the female only, was lower than that of old pairs. The suggestion that females benefited from divorce depended on the interpretation of the decrease in reproductive success with the new mate for widowed birds as an efficiency cost of mate change. However, the increase in reproductive success with duration of the pair bond for female first-time breeders and divorced females (but not for widowed females) may be linked to an increase in breeding age of the female, instead of a re-bound from initial inefficiency in reproduction with the new mate. Hence, competition for good mates and/or good territories appears the primary constraint on options for mate change for both males and females. The role of active choice and competition for good mates and/or good territories has been neglected in previous studies of divorce. The better option hypothesis takes account of both.
In many vertebrates, elevated levels of plasma testosterone (T) are important for reproduction and territorial aggression. However, many tropical birds reproduce and defend territories while plasma T-levels are basal. We studied how aggression and T-levels are regulated in male neotropical spotted antbirds, which defend territories year-round in the Panamanian rainforest. Although spotted antbirds reproduce seasonally, T-levels of individual males often remained at baseline (0.2 ng ml-1) throughout the year, even in courting males. On the other hand, T-levels were elevated (maximally to 1.57 ng ml-1) during periods of social instability at any time of the year, even when males had entirely regressed gonads. Experimental territorial intrusions (broadcast of conspecific song) confirmed these observations by showing that T-levels increased after about two hours of playback time. Our data suggest that spotted antbirds avoided the potential costs associated with constantly high plasma T-levels (e.g. increased mortality rates). Contrary to temperate zone birds, spotted antbirds had the potential to react to social challenges with an increase of plasma T year-round. These results are, to our knowledge, presently unique, but may apply to many vertebrate species that inhabit the tropics.
Examines the effect of different densities of vole Microtus arvalis on the diets of Strix aluco, Asio otus and Tyto alba. Seasonal variability of the diet and the variability due to environment are also discussed. Specialization of owls in capture of voles and its consequences are discussed. -from Author
Nest boxes for breeding Tawny Owls (Strix aluco) were located in a mixed oak-hornbeam-beech (Quercus - Carpinus - Fagus) forest located in the Duna-Ipoly National Park, 30 km northwest of Budapest, Hungary, during the period 1992-2004. We marked the parents individually in the first known breeding year of the females and recorded their reproductive performances through 5 subsequent breeding years. Reproductive performance of females increased with increasing breeding experience; they laid more eggs and reared more fledglings with subsequent breeding years. However, no significant differences were found in reproductive performance between the third and fifth breeding years. Fledging success was higher when the males were older than the females, but hatching success was not influenced by the age of the males. Parents achieved higher hatching and fledging success in years without snow cover than those with snow cover. Fledging success was higher than hatching success in the females' first and second breeding years, but hatching success was higher in third, fourth, and fifth breeding years, which indicates age-dependent change in offspring production limitation by parents. Pairs changed nest sites and moved to lower altitudes in years with snow. As a consequence, the majority of older parents bred at low elevations. Based on the greater mass loss by females than males in adverse weather conditions, we concluded that males reduced the amount of prey brought to their mates to ensure their own survival in conditions in which food was scarce. Females raised lighter fledglings in snow years than in years without snow cover during their first and second breeding season, but the influence of snow cover on fledglings' condition was not present in the broods of experienced parents.
Plasma levels of testosterone (T) were measured by radioimmunoassay in Mountain White-crowned Sparrow (Zonotrichia leucophrys oriantha) males of known age and natural history during four consecutive reproductive seasons in the Sierra Nevada of California. Because nesting occurred comparatively early in all of these seasons and mean schedules varied by only 11 days, data were lumped to show the seasonal pattern of T. This pattern was unimodal with a maximum coinciding with the period of competition for territories and mates and with mate guarding. This was followed by a decrease toward a minimum that coincided with the period of parental care and, eventually, postnuptial molt. When these data were analyzed by stage of the reproductive season (rather than calendar date) and by age, two unexpected results emerged. First, T levels decreased significantly between the stages of nest building and laying. Female mates should have been fertilizable and soliciting copulations during both of these stages and intermale conflicts associated with mate guarding should have been frequent. In both situations, high T levels would be predicted by current paradigms. Second, T levels were considerably lower in yearling males than in older males prior to the laying stage. Reasons for this age-related difference are unknown but it was observed that yearling males tended in all comparison intervals to have lower body masses, shorter wings, and shorter testis and cloacal protuberance lengths than older males--they were smaller birds. Yearling males were also less successful in obtaining mates than older males but, once paired, reared just as many offspring.
Male Satin Bowerbirds court females with complex displays that include stick structures built adjacent to a decorated display court. Here we report circulating levels of testosterone and corticosterone in a natural population of Satin Bowerbirds and their relation to the development of complex mating displays. We found that (1) males without bowers had lower levels of testosterone than bower holders, (2) testosterone, but not corticosterone, levels were significantly correlated with male mating success, and (3) levels of testosterone, but not corticosterone, were consistently correlated with the quality of several important display characters that have been shown to be important in affecting male mating success. These results suggest that testosterone level is an important component of male mating success. We consider reasons why variation in testosterone levels persists among mature male bowerbirds given its potential to affect male mating success.
1. Parallel population studies were done on a predator, the tawny owl (Strix aluco), and on its two main prey species, the wood mouse (Apodemus sylvaticus) and the bank vole (Clethrionomys glareolus) in deciduous woodland on the Wytham Estate, near Oxford, from October 1954 to August 1956. 2. The study area of 260 ac (104 ha), in Wytham Great Wood, contained a trapping grid of 120 ac (49 ha) on which, approximately every 2 months, mice and voles were caught, marked with numbered metal leg-rings and released. On this area the tawny owl population, consisting of eleven pairs, was censused and their breeding success recorded; only two of them had territories entirely within the trapping grid. 3. The territorial boundaries of these owls were mapped from observations made at dusk. A check on the observations was rendered possible by searching for pellets regurgitated by the owls and containing the indigestible parts of their prey, including marking rings. Of 1992 rodents marked, 160 were traced subsequently in pellets. 4. Since the home ranges of the rodents are small, the distribution of the locations where the recovered rings had originally been put on the prey delimits the areas in which each pair of owls hunted. The territory boundaries indicated from these data show very close agreement with the boundaries plotted from observations. This confirmed the validity of the observational method, which was being used for long-term census work on the whole estate (1000 ac, 405 ha). 5. A map of the trapping grid was prepared in terms of four categories of density of ground vegetation, ranging from almost bare ground under heavy canopy, where hunting is easy, to ground covered with a dense and high growth of bracken and bramble where hunting would be very difficult. 6. The trapping results showed that about five wood mice per ac were caught in each of the cover categories. The catches of bank voles were of this order in the barest covertype but increased with density of vegetation until they were three times as numerous in the thick bracken-bramble cover. 7. The recovery of marking rings from owl pellets showed that the owls caught bank voles in the four cover-types in the same proportions as did the traps. However, of wood mice they caught proportionately many more than the traps on the bare areas and many less than the traps in the densely vegetated areas. There was no evidence of selection by the owls between the sexes or size classes of the prey. 8. These results suggest that owl territories with plenty of bare ground will provide conditions for the most successful hunting when wood mice are abundant, whereas the territories in which the intermediate cover types predominate will be better when bank voles are abundant. 9. During the period of study both species of prey were scarce in the 1955 breeding season, whereas in the 1956 season the voles had increased in numbers markedly, the wood mice only slightly. 10. In 1955 only two pairs of tawny owls on the area studied even attempted to breed, though both fledged young successfully. In 1956 seven pairs attempted to breed, of which three fledged young. Breeding performance is a sensitive index to availability of prey in spring. 11. A comparison of breeding performance in these 2 years with features of each territory suggests that, in addition to the level of prey density, other characteristics, viz. territory size, the distribution of ground cover, and the previous experience of territory owners all contribute significantly to breeding success.
We collected 354 blood samples from territorial and nonterritorial male Red-winged Blackbirds (Agelaius phoeniceus) to determine plasma levels of testosterone (T) and corticosterone (B) of a polygynously breeding bird, and to determine the potential influences of steroid hormone levels on obtaining breeding territories. In contrast to monogamous species, we found no peak in average T levels early in the breeding season and a five-week-long high plateau in T levels during the period when most nests were initiated. These endocrinologic characteristics may result from the long breeding season and resource defense polygyny of Red-wings. Territory owners had higher T levels during the breeding season than adult floaters. Subadult floaters had the lowest levels. Also, territory owners generally had the highest circulating levels of corticosterone during most of the breeding season. This implies greater energetic demands and stress on owners than on floaters. During early April the T levels of males for which we had complete breeding and territorial history information correlated significantly with their annual fledging success, and males with more breeding experience tended to have higher peak T levels. There was also a positive correlation between B levels in early April and eventual fledging success. Circulating levels of steroid hormones may affect territory ownership and reproductive success in this species.
The density of great tit Parus major L. and blue tit Parus caeruleus L. was artificially increased by placing nest-box colonies for these species in the vicinity of the nests of breeding tawny owls during 1993–1997. Bird prey composition in the owl nests, the proportion of parents disappearing from the breeding tit populations and the reproductive performance of the widowed parents were analysed. The frequency of predation on tits by tawny owls was greater in areas where tit density had been artificially increased. Owls preyed more on tits during the feeding period of owlets than during the incubation period and more in years when snow covered the ground during the incubation period than when it did not. Mortality due to predation was male biased and more females lost their mates in populations breeding near tawny owl nests. Reproductive performance of the widowed parents was lower and their body weights were lighter at the end of the nestling period than those found in birds rearing youngs with their mates. Predation by owls increased the between-year turnover in the breeding tit population: widowed parents did not return to the nesting site for the next breeding season.
Circulating levels of testosterone in male Zonotrichia leucophrys increase when their mates are sexually receptive. Males with sexually receptive females are more aggressive than males with nonreceptive females under labratory conditions. Experimental elevation of plasma levels of testosterone results in increased territorial defense. This response appears to function as mate-guarding behavior.-from Author
Breeding male House Sparrows (Passer domesticus) were implanted with testosterone (T), the antiandrogen flutamide (F), or an empty capsule as a control (C). Parental feeding rates by C-treated males were high until nestlings reached 10 days of age, then declined significantly. This is the typical temporal pattern of parental behavior for free-living males. In contrast, F-treated males fed young at a high rate throughout the nestling stage, while T-treated males fed young much less frequently and were more involved in male-male competition during this period of time. There was a significant decrease in the breeding success of T-treated males resulting from increased starvation of their nestlings. Despite lowered levels of testosterone, F-treated males were able to maintain control of their nest boxes and exhibited normal sexual behavior. During the subsequent brood, breeding success of T-treated males again was reduced by nestling starvation. Our results demonstrate that high levels of testosterone inhibit the expression of parental care in male House Sparrows. Moreover, they suggest that the typical pattern of testosterone levels in males (high when mate guarding and low when feeding young) represents an optimal compromise between allocation of effort to male-male competition vs. parental care.
Several of the reported costs and benefits of testosterone-induced changes in avian breeding behaviour may be mediated by the hormone's effects on spatial activity. Therefore, radiotelemetry was used to monitor the effects of experimentally elevated testosterone on the movements of free-ranging male dark-eyed juncos. Juncos were located every 0·5 h over periods of 3-5 days. During the incubation period and while feeding nestlings, males with testosterone-filled silastic implants (T-males) showed significant changes in spatial activity compared with males with empty implants (C-males). On average, T-males had home ranges approximately 200% larger, and within these home ranges occupied core areas (areas encompassing 95% or 50% of all activity) over 300% larger, than those occupied by C-males. T-males were at the nest less often, were found at greater maximum distances from the nest, and sang more often than C-males. Males with experimentally elevated testosterone are less attentive to their nests and young, but may benefit from reduced intrusion rates by other males onto their territories, attraction of females to their large territories, or greater opportunities for extra-pair copulations within their expansive home ranges.
We investigated the role of testosterone (T) in territory establishment and maintenance in male red-winged blackbirds (Agelaius phoeniceus) by implanting exogenous T or pharmacological agents that block the action of T in nonterritorial floaters and territory owners. Floaters with artificially elevated plasma T levels were unable to obtain territories. Territory owners implanted with T did not expand their territories, although they engaged in more aggressive behavior than did control males. Flutamide, which binds to T receptors in target areas, had no effect on territoriality. However, most territory owners given a combination of flutamide and ATD, an aromatization blocker, lost portions of their territories even though they actively defended them. Our results 1) suggest that, although T influences aggressive behavior, elevated plasma levels alone are insufficient to overcome previously-established social relationships between territory owners or between owners and floaters; 2) indicate that impairing the action of T leads to reduced abilities of territory owners to maintain territories against vigorous challenges: and 3) support recent findings that T acts on reproductive behavior in birds through both androgenic and estrogenic metabolites.
Territory quality may affect individual fitness and contribute to density‐dependent reproduction, with repercussions on population regulation. We investigated the probable causes and population consequences of spatio‐temporal variations in territory quality, measured by occupancy, in eight black kite Milvus migrans Boddaert populations, one of them studied for 10 years (Lake Lugano) and the rest for 4–5 years.
Over a period of years, the occupation rate of territories varied from a random pattern. Some territories were preferred while others were avoided. On return from migration, males and females settled earlier on high‐occupancy territories.
The positive association between territory occupancy and breeding performance held in all years of study at Lake Lugano, and in six of seven tested populations. As a result, high‐occupancy territories contributed most of the young produced by each population.
The occupation rate of the overall 225 territories was related positively to food availability and negatively to mortality risk, measured as proximity to the nearest eagle owl Bubo bubo Linnaeus nest.
At the population level, spatial variation in mean occupancy was positively correlated with spatial variation in mean productivity, suggesting that mean occupancy could be used as a measure of overall habitat quality and population performance.
In the Lake Lugano area, a higher proportion of low quality territories was occupied in years of higher density and annual productivity was related negatively to its coefficient of variation. However, annual productivity was not related significantly to the proportion of low quality territories occupied, so support for the theory of site‐dependent population regulation was only partial.
In a review of 22 studies of territory occupancy in 17 species, occupancy always deviated from a random pattern in species in which it was tested and was always correlated with productivity and/or with some other measure of territory quality. Our results confirm the importance of prioritizing conservation of high quality territories. Occupancy may be a reliable method of quality assessment, especially for populations in which not all territories are always occupied, or for species in which checking occupancy is easier than finding nests.
In male birds, the gonadal hormone testosterone (T) is known to influence territorial and mating behaviour. Plasma levels of T show seasonal fluctuations which vary in relation to mating system and social instability. First, we determined the natural T profile of male blue tits Parus caeruleus during the breeding season. We found that plasma levels of T increased at the onset of nest building. Thus, the increase in circulating T was not associated with territory establishment, nor with the fertile period of the males’ mates. In most individuals, T levels dropped to values close to zero during the period of chick feeding. Second, we investigated the relationship between plasma levels of T and male age, size, and singing behaviour. During the mating period, T levels did not differ between 1 yr old and older males and did not correlate with body size or condition. However, song output during the dawn chorus tended to be positively correlated with T levels. Therefore, if high T levels are costly, song output might be an honest indicator of male quality in blue tits. Finally, we show that plasma levels of T are significantly higher during the night than during the day. This pattern has also been observed in captive non-passerine birds, but its functional significance remains unknown.
This example is provided so that non-theorists may see actual applications of the theory previously described. The Dickcissel sex ratio is employed as an indirect index of suitability. A sex ratio index was found to be correlated positively with density. This is consistent with the hypothesis that territorial behavior in the males of this species limits their density. This study provides a valid example of how the problem can be approached and offers a first step in the eventual identification of the role of territorial behavior in the habitat distribution of a common species.
Previous research has suggested that parental condition may affect offspring mortality patterns by affecting offspring testosterone levels. Accordingly, we hypothesized that there is a relationship between offspring testosterone concentration and survival during the early nestling period, and that both are influenced by parent age/experience and by prey availability. We tested our hypothesis on tawny owls Strix aluco in their first and third known breeding seasons, when they bred either in adverse or mild weather conditions, in Duna-Ipoly National Park, Hungary. Plasma testosterone concentrations of the nestlings were analyzed and related to parental condition, hatching order and nestling mortality. Inexperienced parents breeding in all weather conditions and experienced parents breeding in adverse conditions were both in poor condition compared to experienced parents breeding in mild conditions. Parents in poor condition produced broods with large between-sibling differences in testosterone concentrations and their later-hatched nestlings (which had low testosterone levels) died during the early nestling period, whereas parents in good condition produced broods with lower variation in offspring testosterone concentrations and all offspring survived the early nestling period. We discuss environmental influences on the amount of testosterone deposited in eggs, and also how maternal testosterone might induce those mechanisms producing testosterone in the nestlings.
Breeding behaviour and spermatogenesis of all males in a population of pied flycatchers, Ficedula hypoleuca, were experimentally prolonged by long-acting testosterone. The behaviour of these birds was compared with that of birds from an untreated population and a population where the males were given the vehicle only. The testosterone treatment prolonged territorial behaviour, and a higher percentage of nests without a feeding male during the nestling period was found in the experimental area. The proportion of polygamous males did not however differ between the experimental and control areas. The feeding activity of those testosterone-treated males returning to their home territories was markedly depressed. The female of such a pair did not compensate for the male's low feeding activity, but fed the nestlings as she would have done with a normal untreated male at the nest. On the other hand, females without a male present during the nestling period compensated for their mates' absence by increasing their own feeding frequency. It was also shown that the song of the male pied flycatcher is dependent upon androgens. The testosterone-treated males produced fewer fledglings than the control males.
Differential allocation occurs when reproductive investment is influenced by mate attractiveness. Recently, wide-ranging empirical support for differential allocation has been obtained. These data suggest that mates can affect the payoffs from reproduction, thus making sacrifices of reproductive value worthwhile when breeding with an attractive mate. As an example of an adaptive parental effect, the existence of differential allocation has some interesting implications for empirical studies of sexual selection and for predicting evolutionary responses to selection.
Plasma levels of testosterone and corticosterone were measured in free-living male bush warblers captured on their breeding ground at different times of the breeding season. Their territoriality was also estimated from their singing response to song playbacks. The pattern of change detected in the levels of plasma testosterone was different from that of “typical” monogamous species but similar to that of polygynous species. In “typical” monogamous species, plasma testosterone levels elevated during territory settlement and courtship behavior and then declined to low, stable levels during incubation. In bush warblers, plasma levels of testosterone were already high (1–2 ng/ml) upon arrival in late March and peaked (2.5–4 ng/ml) in early June. They then decreased but relatively high levels were maintained until early August. In late August the testosterone concentration was 0.03 ng/ml or less. Plasma levels of corticosterone also showed a seasonal change, being highest in May to July and declining in late August. Territoriality showed clear seasonality, reflecting the levels of circulating testosterone. Upon arrival, latency periods for responses to song playback were long and singing activity was rather low but this behavior was soon stabilized and a high degree of territoriality was maintained to late August. These results suggest that high levels of circulating testosterone and corticosterone allow males to pursue a polygynous breeding strategy, to hold a territory, and to maintain breeding activity for a prolonged period, characteristics which are likely to be adaptations to dense bushes with high rates of predation and brood parasitism of this species.
When there is a direct relationship between testosterone level and payoff in reproductive success through aggression, testosterone levels should be elevated. Elevated testosterone, however, has fitness costs, particularly a decreased tendency to display parental care. Thus the pattern of testosterone secretion in males should vary with the social and mating system. Western scrub-jays, Aphelocoma californica woodhouseii, form monogamous pairs on territories during the breeding season. Mexican jays, A. ultramarina, live in large, stable groups and up to five females within a group attempt nesting each spring. In both species, testosterone levels rose rapidly in March and peak levels did not differ. Elevated testosterone levels were only observed for about 3 weeks in the monogamous western scrub-jay, but were observed into May in Mexican jays, a reflection of prolonged opportunity for males to mate with multiple females and continual interaction with other competing males. In Mexican jays, nonbreeding yearlings had lower testosterone levels than all other age groups. Testosterone in males owning nests did not differ from that in other adult males, many of whom engage in extrapair fertilizations. Testosterone was elevated throughout the incubation phase, but was significantly lower when chicks were present in any nest in the group. Nearly all birds in the group fed all chicks. These observations support the hypothesis that testosterone is elevated when male–male competition is frequent and mating opportunities depend on the outcome of that competition, and testosterone is decreased when the necessity for parental or alloparental care would make its effects deleterious.
Investigations of the effects of testosterone on male mating effort have focused on behaviours associated with intra-sexual interactions. We tested whether testosterone may also affect male reproductive behaviour by mediating the intensity and frequency of male courtship. In two simultaneous choice experiments, we assessed the relative attractiveness of testosterone-implanted males, whose testosterone levels were manipulated to match naturally occurring spring peaks, and empty-implanted control males, whose testosterone levels were lower and resembled those of males caring for young. In both experiments, testosterone-males outperformed control-males, giving both more frequent and more exaggerated displays. Females showed a significant preference for the peak-testosterone males as measured by both female attendance and female courtship displays. We concluded that variation in testosterone can affect male attractiveness to females and thus male copulatory success. Thus, testosterone has the potential to mediate all aspects of male reproductive behaviour and may provide a mechanism through which males adjust their full range of breeding behaviours (i.e. inter-sexual as well as intra-sexual and parental behaviour) to fit their current status. In a second experiment, we tested the effect of oestradiol treatment on female mating preferences. No evidence was found that oestradiol changed the preferences of females.
Tawny owls Strix aluco breeding in nest-boxes were studied in a mixed oak/hornbeam/beech forest located in the Duna-Ipoly National Park 30 km north-west of Budapest, Hungary, during the period 1992–1999. Diet composition, prey mass, breeding performance and body mass of the parents of known age were recorded. Older males had a greater ability to choose alternate prey, delivered a greater mass of prey with a higher feeding frequency and achieved higher productivity than younger males when the availability of the preferred prey declined. The reproductive cost was paid only by young parents. We suggest that the lowest breeding performance, which was observed with young parents in adverse weather conditions, may be due to both the lower ability of these younger birds to exploit alternative prey and to their poor body condition which resulted in them providing fewer resources to their offspring because of their need to provide for their own survival.
Territorial species, such as the spotted owl (Strix occidentalis), are predicted to follow an ideal despotic distribution. However, debate exists on whether wild populations actually meet the assumptions of an ideal distribution, such as perfect perceptual abilities (i.e., the ability to recognize high- and low-quality sites without error). Because this hypothesis has important life history ramifications for spotted owls, we investigated whether occupancy rates of California spotted owl (S. o. occidentalis) territories in the San Bernardino Mountains of southern California positively correlated with a qualitative "potential fitness" (denoted by λ-sub-pf) estimated from survival and reproduction of territorial owls. Spotted owls in our study tended to occupy territories with the highest λ-sub-pf, supporting the assumption of ideal perceptual abilities within this population. However, this relationship was noisy, and we suggest that some individuals do not assess site quality accurately because of perceptual limitations, prey dynamics, and large territory sizes. Furthermore, dispersal processes, high survival rates, and long life spans of spotted owls may be other key factors preventing some individuals from selecting sites of the highest quality and, consequently, our ability to precisely estimate λ-sub-pf. Copyright 2003.
Behavioral and endocrine changes associated with reproductive events were studied in free-living female house sparrows (Passer domesticus). Circulating levels of luteinizing hormone (LH) and estradiol (E2) were maximal during egg-laying, declined during incubation, and began to rise during the nestling stage. This pattern was repeated three to five times during the prolonged breeding season of this species. Multivariate statistical analysis demonstrated that elevated levels of LH and E2 were associated with maximal sexual activity, high rates of intrusion at nests by conspecifics, and elevated rates of agonistic activity. Patterns of sexual activity varied during the season and perhaps reflected differing strategies for protecting paternity adopted by the male. Curiously, elevated levels of testosterone (T) and dihydrotestosterone (DHT) were not associated with high levels of intrusion or agonistic response, but rather were associated with incubation stages. As in males, concentrations of corticosterone (B) were high during each egg-laying and nestling stage and were associated with high or rising levels of reproductive hormones, suggesting that reproductive activity was not overly stressful to these birds. Body mass and fat depots declined during the season and may regulate termination of reproduction. It appears that access to abundant food resources found in association with human dwellings allows house sparrows to maintain reproductive activity longer than most other temperate-zone dwelling bird species.
Cock testes incubated with labeled pregnenolone or progesterone as substrates produced testosterone as the main compound. The formation of 17α-hydroxyprogesterone was greater with progesterone as precursor, but the production of radioactive 20β-dihydroprogesterone and androstenedione was similar with either substrate. Testes from 1-, 21-, and 40-day-old chicks incubated with progesterone as a substrate had a very low testosterone: androstenedione production ratio (0.05). In mature animals, however, more testosterone than androstenedione was produced from radioactive progesterone and the production ratio testosterone: androstenedione was 24.
Endocrinologic investigations of free-living populations of song sparrows, Melospiza melodia, have revealed temporal patterns of secretion of reproductive hormones that differ from those of other monogamous avian species. Males arrive in the breeding area in March whereas females arrive 1-2 weeks later. In males the periods of territory establishment and attraction of a mate are characterized by high circulating levels of luteinizing hormone (LH) and testosterone (T), whereas testis mass is low, and growth of the cloacal protuberance (CPL) is just beginning. In April, testes and CPL develop rapidly reaching a peak in early May when females are laying eggs, and when most copulations occur. Plasma LH and T decline in early April, but increase for a second time in late April and early May coincident with the egg-laying period. Thereafter, circulating LH and T decline during the parental phase, but not to basal levels. Although there is an increase in LH during the egg-laying period for the second clutch, there is no change in T levels. Testis mass, CPL, plasma LH, and T all decline to basal levels simultaneously in late July and August. The high levels of LH and T in March, followed by a temporary decline and resurgence in April and May, indicate that environmental factors in addition to the well known effects of increasing day length can regulate secretion of these hormones. In females, plasma levels of T and dihydrotestosterone (DHT) are also elevated in March and early April, and decline to low levels as the nesting phase progresses. A further decline, to basal levels, occurs in August during the molt. LH and estradiol (E2) titers in females show two peaks coincident with the egg-laying periods for each clutch. Plasma levels of corticosterone (B) increase during the breeding season in males, but not females. In both sexes B levels are basal during the moult and increase in October after moult is completed. Body mass and fat depot decline in males as the nesting phase progresses, and then increase dramatically after breeding is terminated. As expected, females show two peaks of body mass and fat depot coincident with the two periods of egg laying. The postbreeding increases in body mass and fat depot are much less pronounced in females than in males.
Plasma samples from naturally breeding populations of spotted sandpipers, Actitis macularia, were analyzed by radioimmunoassay for circulating levels of immunoreactive luteinizing hormone (irLH), and four steroid hormones. Plasma levels of irLH, corticosterone (C), and estradiol-17 beta (E) did not differ between males and females. Males had curiously high levels of E (0.64 pg/ml in nonnesting males; 0.35 pg/ml in nesting males) which are similar to those of nonlaying females (0.23 pg/ml). Testosterone (T) and dihydrotestosterone (DHT) were higher in nonnesting males (0.95 ng/ml for T; 0.20 ng/ml for DHT) than in females (0.18 ng/ml for T; 0.12 ng/ml for DHT) or nesting males (0.17 ng/ml for T; 0.09 ng/ml for DHT). These data do not support earlier suggestions that in polyandrous mating systems, females have higher circulating levels of androgens than males.
Male red-winged blackbirds were given implants of testosterone or flutamide in order to manipulate the level of androgen reaching receptor sites. Implants of cholesterol were given as controls. Radioimmunoassay showed higher levels of testosterone and DHT in the plasma of testosterone-implanted birds than in the plasma of the pooled flutamide-implanted and cholesterol-implanted birds. Dominance was significantly correlated with effective androgen level. Androgen level was not correlated with rates of supplantations per subordinate but was correlated with rates of escalated attacks per subordinate. Thus androgen level influences dominance in male red-winged blackbirds by influencing one component of aggressiveness.