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Birds Select Fruits with More Anthocyanins and Phenolic
Compounds During Autumn Migration
Author(s): Jessica A. Bolser , Rebecca R. Alan , Adam D. Smith , Liya Li ,
Navindra P. Seeram , and Scott R. McWilliams
Source: The Wilson Journal of Ornithology, 125(1):97-108. 2013.
Published By: The Wilson Ornithological Society
DOI: http://dx.doi.org/10.1676/12-057.1
URL: http://www.bioone.org/doi/full/10.1676/12-057.1
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BIRDS SELECT FRUITS WITH MORE ANTHOCYANINS AND
PHENOLIC COMPOUNDS DURING AUTUMN MIGRATION
JESSICA A. BOLSER,
1
REBECCA R. ALAN,
1
ADAM D. SMITH,
1,3
LIYA LI,
2
NAVINDRA P. SEERAM,
2
AND SCOTT R. MCWILLIAMS
1
ABSTRACT.—We evaluated whether fruit selection by autumn-migrating birds at an important stopover site in southern
New England was related to water-soluble antioxidant content of fruits. We measured total anthocyanins, total phenolics,
and total antioxidant capacity in fruits from common native and non-native plant species and related this to estimates of
fruit selection by free-living birds. Birds selected certain fruits over others, with arrowwood (Viburnum recognitum,V.
dentatum) consumed at the highest rate, followed by Virginia creeper (Parthenocissus quinquefolia) with much lower
consumption of other fruits (e.g., oriental bittersweet [Celastrus orbiculatus], multiflora rose [Rosa multiflora], winterberry
[Ilex verticillata]). Antioxidant concentrations primarily differed by shrub species and less so between sites. Arrowwood
spp. had the highest total antioxidants, followed by Virginia creeper, northern bayberry (Myrica pennsylvanica), chokeberry
spp. (Aronia prunifolia, A. melanocarpa), multiflora rose, winterberry, and oriental bittersweet. These results are consistent
with the hypothesis that free-living birds select fruits based, in part, on antioxidant content. We suggest birds may actively
select polyphenol/anthocyanin-rich fruits during autumn migration to protect themselves against the potentially damaging
effects of oxidative stress caused by long-distance fasting flight. Received 4 April 2012. Accepted 29 June 2012.
Key words: anthocyanin, Block Island, fall migration, fruit antioxidants, fruit selection, Viburnum.
All aerobic organisms produce reactive oxygen
species (ROS) as unavoidable byproducts of
respiration and animals have evolved several
mechanisms for coping with this oxidative burden
(Finkel and Holbrook 2000, Pamplona and
Costantini 2011). Birds, in particular, are exposed
to especially high levels of oxidative stress as a
direct consequence of long-distance flight (Cost-
antini et al. 2007). One way migratory birds might
mitigate oxidative stress is through consumption
of dietary antioxidants. Many birds that are
primarily insectivorous during the breeding sea-
son switch to eating seasonally-abundant fruits
during autumn migration. Many of these fruits are
excellent sources of dietary antioxidants and may
provide an important defense against oxidative
stress for both fruiting plants and their consumers
(Garcia-Alonso et al. 2004, Smith et al. 2007b,
Catoni et al. 2008). Most antioxidants in fruits are
secondary metabolites that include flavonoids and
allied phenolic and polyphenolic compounds
(Crozier et al. 2006). These compounds derived
from diet may reduce oxidative damage and are
referred to as ‘dietary antioxidants,’ although
these compounds often have many other physio-
logical functions and their antioxidant benefits to
date are usually only measured in vitro (Halliwell
and Gutteridge 2007). Wu and others (2004)
analyzed phenolic content, specifically anthocya-
nins, of 100 plant-based foods, and found the
three foods containing the highest levels were all
fruits: chokeberry (Aronia melanocarpa), Amer-
ican elder (Sambucus canadensis), and wild
blueberry (Vaccinium corymbosum). High antiox-
idant activity measured in vitro was also observed
in other fruits from the genera Rubus, Ribes, and
Aronia (Benvenuti et al. 2004). These studies
provide insight into the antioxidant content of
specific fruits but do not address how antioxidant
composition might explain patterns of fruit
selection by birds.
Many factors influence patterns of avian
frugivory (Snow 1970, Stiles 1980, Levey and
Martinez del Rio 2001) including macronutrient
and mineral composition (Herrera 1987, Smith
et al. 2007b), fruit and seed size and shape
(Herrera 1984, Witmer and Van Soest 1998), fruit
abundance and spatial arrangement (Denslow
1987, Sargent 1990, Carlo and Morales 2008),
and anti-nutrient secondary metabolites (Cipollini
and Levey 1997a, b; Levey and Cipollini 1998;
Struempf et al. 1999; Tsahar et al. 2002; Schaefer
et al. 2003). However, the influence of antioxidant
levels on fruit preference has only recently
1
Program in Wildlife and Conservation Biology, Depart-
ment of Natural Resources Science, 105 Coastal Institute in
Kingston, University of Rhode Island, Kingston, RI 02881,
USA.
2
Bioactive Botanical Research Laboratory, Department
of Biomedical and Pharmaceutical Sciences, College of
Pharmacy, 41 Lower College Road, Fogarty Hall, Univer-
sity of Rhode Island, Kingston, RI 02881, USA.
3
Corresponding author;
e-mail: rebecca_alan@my.uri.edu
The Wilson Journal of Ornithology 125(1):97–108, 2013
97
received attention from ornithologists. Schaefer
and others (2008) measured anthocyanin and
carotenoid content in 100 fruit species and found
that Eurasian Blackcaps (Sylvia atricapilla) pre-
ferred artificial diets supplemented with anthocy-
anins over foods without these supplements.
Catoni and others (2008) showed, in behavioral
choice trials, that Eurasian Blackcaps preferred
food with flavonoids over that without flavonoids.
Senar and others (2010), in a similar diet
preference trial, artificially enriched mealworms
with carotenoids and found Great Tits (Parus
major) consistently chose carotenoid-enriched
mealworms over non-enriched mealworms. This
research suggests birds prefer food supplemented
with antioxidants (Schaefer et al. 2008, Catoni
et al. 2008), and birds are able to detect dietary
antioxidants without visual signals (Senar et al.
2010); however, the positive effects of dietary
antioxidants on oxidative stress have not yet been
demonstrated.
No previous studies have examined the extent
free-living migratory birds select wild fruits
containing higher levels of antioxidants. Assess-
ments of fruit ‘selection’ by free-living birds are
complimentary to studies of diet or fruit ‘prefer-
ence.’ Diet ‘preference’ is evident when birds
consume more of certain diets when given equal
access to alternative diets. Diet ‘selection’ refers
to when free-living birds consume more of certain
foods when availability of alternative diet choices
may be quite different (Frazer and McWilliams
2002). Diet ‘selection’ is a function of the
interaction between diet preference and availabil-
ity of alternative diet choices. Our objectives were
to: (1) examine relative patterns of avian fruit
consumption of seven fall-fruiting shrub species
during autumn migration, (2) qualitatively relate
patterns of avian fruit selection to availability of
antioxidants in fruits, and (3) examine island-scale
spatial variation in antioxidant concentration of
fruits for a select subset of fall-fruiting shrub
species.
METHODS
Study Area.—All fieldwork was conducted on
Block Island, Rhode Island (41u139N, 71u339W),
a 2,900-ha, teardrop-shaped island 15.5 km south of
the Rhode Island coast and 22.5 km northeast of
Long Island, New York. Migrating passerines,
especially hatch-year birds, fly south to the
southern New England coast and are often pushed
out to sea by northwest winds associated with
passing cold fronts during autumn migration (Ralph
1981). Coastal locations, such as Block Island, are
especially important stopover sites for many of
these birds (Able 1977). The main study site was in
the northeast portion of the island within the Bay
Rose and Clay Head preserves. Previous research
has shown this is an area where migratory birds
concentrate on Block Island and it also supports
numerous fall-fruiting shrub species (Able 1977;
Parrish 1997, 2000; Hammond 2002; Reinert et al.
2002; Smith et al. 2007b; Smith and McWilliams
2010). The boundaries of the 27-ha site were
arbitrarily chosen: Corn Neck Road to the west,
Mansion Road to the south, the coast to the east,
and Kurz Road to the north.
A variety of habitat types occur in southern
New England, but most of the study site can be
classified as a maritime shrubland habitat (Ham-
mond 2002). Salt spray and wind disturbance are
important determinants of the plant species
composition. Shorter-statured northern bayberry
(Myrica pennsylvanica), poison ivy (Toxicoden-
dron radicans), and downy goldenrod (Solidago
puberula) dominate the more exposed areas,
whereas northern arrowwood (Viburnum recogni-
tum), southern arrowwood (V. dentatum), purple
chokeberry (Aronia prunifolia), black chokeberry
(A. melanocarpa), winterberry (Ilex verticillata),
and Virginia creeper (Parthenocissus quinquefo-
lia) are common in areas with more protection.
Two non-native species, oriental bittersweet
(Celastrus orbiculatus) and multiflora rose (Rosa
multiflora), are also common throughout this site.
Avian Fruit Consumption.—The fruit consump-
tion study was conducted at seven randomly-
chosen locations within the main study site on
Block Island. The locations were chosen by
overlaying the study site with a numbered grid
consisting of 300 3300-m cells and randomly
choosing cells within the grid. Locations were
accepted if they contained at least one fruiting
plant of each of seven species within 150 m of the
center of a selected grid cell. The seven species
included: northern bayberry, arrowwood spp.,
winterberry, chokeberry spp., Virginia creeper,
oriental bittersweet, and multiflora rose. Identifi-
cation of plant species was based upon criteria
from multiple sources (Symonds 1963, Petrides
1988, USDA 2008). We selected the nearest plant
from the central point that had a pair of branches
with at least 25 fruits per branch if there was more
than one fruiting plant of a given species at a
location. One branch on the selected plant was
98
THE WILSON JOURNAL OF ORNITHOLOGY NVol. 125, No. 1, March 2013
covered with plastic mesh (0.5-cm grid), follow-
ing Smith et al. (2007b), that prevented access to
birds but not most invertebrates. A partner branch
was marked and remained uncovered. Only nine
terrestrial mammal species occur on Block Island,
none of which are known to commonly consume
fruits (Lang and Comings 2001). Thus, songbirds
are the primary consumers of fruits on Block
Island during fall, and the majority of these birds
are stopping over during migration.
We counted the number of fruits on each
marked branch beginning in early September and
every 7–14 days thereafter for a total of five
counts. The procedure involved repeated counting
of the number of fruits on each branch until a
consistent number was attained. We assessed the
percent ripeness of fruits during each count by
visually estimating the percentage of fruits on
each branch that were unripe, ripe, and over-ripe
so the sum was 100%. The mesh net was carefully
removed from the netted branch prior to each
count for northern bayberry and replaced after-
wards because we were unable to accurately count
these clustered fruits through the net.
Fruit Collection.—Fruit samples were collected
from the same seven shrub species and locations.
Fruit was harvested during peak ripeness for each
species (early Sep for chokeberry spp., arrow-
wood spp., and northern bayberry; early Oct for
winterberry and Virginia creeper; and early Nov
for oriental bittersweet and multiflora rose). Ripe
fruits from three plant species that were widely
available on Block Island (chokeberry spp.,
southern arrowwood, and northern bayberry) were
also collected within maritime shrubland habitat
at two additional locations on the southern portion
of the island to examine spatial variation in fruit
composition on an island-scale. All collected fruit
samples were kept frozen on Block Island until
they were transported to the University of Rhode
Island where they were stored at 280 uC until
chemical analysis.
Sample Preparation.—Preparation of the frozen
fruit for analysis involved manually de-seeding
10–15 g of wet fruit from each sample with the
exception of chokeberry spp. and multiflora rose
which were analyzed whole. These fruits contain
hundreds of small seeds that are not easily
extracted from the fruit pulp. Fruit was then
freeze-dried, weighed, and homogenized using a
Wiley mill. Each dried/ground sample was
divided into triplicate sub-samples that were
analyzed separately to obtain an estimate of
within-sample variation in chemical composition.
Aqueous extracts from each fruit subsample were
obtained using a double methanol wash method
(Garcia-Alonso et al. 2004) in 10 ml of acidified
(0.1%HCl) methanol collected into a glass vial.
Samples of northern and southern arrowwood fruits
were washed a third time because, unlike the other
fruit species, the supernatant from the second wash
was not visibly lighter than the first. We removed
the solvent from the supernatant under a stream of
nitrogen gas and under vacuum. Dry mass of the
sample was recorded and the extract was frozen at
280 uC until chemical analysis.
Chemical Analysis.—A variety of measures has
been used to assess antioxidant status in biological
samples, the advantages and disadvantages of
which have been extensively reviewed and
discussed (Prior and Cao 1999, Sanchez-Moreno
2002, Schlesier et al. 2002, Haung et al. 2005,
Halliwell and Gutteridge 2007). The chemical
analysis of the aqueous extracts in our study
included a spectrophotometric assay (DPPH) that
measures total antioxidant capacity of the sample
(Ozgen et al. 2006, Seeram et al. 2008). The
DPPH assay involves dissolving a sample in
dimethyl sulfoxide and measuring its ability to
reduce 50%of the DPPH radical (EC
50
) based on
standard calibration curves (Brand-Williams et al.
1995, Molyneux 2004). High DPPH values
indicate only a small quantity of the radical has
been reduced while lower DPPH values indicate a
higher amount of radical reduction. The extract
from a fruit with a higher total antioxidant
capacity, and a higher ability to quench free
radicals, will exhibit lower DPPH values. Total
phenolics (TPH) in the fruit were measured
spectrophotometrically following the widely-used
Folin-Ciocalteau method (Singleton et al. 1999);
total monomericanthocyanins (TA) in the samples
were measured with the pH-differential method
(Giusti and Wrolstad 2001). All assays were
conducted in duplicate or triplicate and were
repeated when the coefficient of variation be-
tween samples was .15%.
Statistical Analysis.—We constructed a log-
linear (Poisson) model to compare numbers of
fruits on a branch by species and treatment
(branch enclosed or open) in SAS (GENMOD
procedure; SAS Institute 2009). We inflated
standard errors of parameter estimates with a
dispersion parameter (i.e., Pearson’s Chi-square
statistic divided by the degrees of freedom) to
account for over-dispersion in the data.
Bolser et al. NFRUIT ANTIOXIDANTS
99
We calculated two metrics to evaluate the fruit
preferences of birds: percentage of remaining fruit
on each enclosed and open branch at each count
period for each plant species, and a consumption
index (CI) (Smith et al. 2007a) for each pair of
branches on the same plant at each count period,
where CI 512(%remaining on open branch/%
remaining on enclosed branch). These dependent
variables did not conform to the assumptions of
normality, even when transformed. Thus, we
convertedthemtorankvaluestoevaluate
differences using repeated-measures ANOVA
models with three main effects (species, sampling
site, and count period). We also considered two
interaction terms (species*count period, sampling
site*count period). Sampling site was used as a
fixed effect in the model to assess regional-spatial
patterns in bird fruit preferences. We used the
best-fitting covariance structure to account for
multiple observations from the same individuals
using second-order Akaike’s Information Criteri-
on (AIC
c
) values (Burnham and Anderson 2002);
a compound symmetric covariance structure was
the best fit for all models. Non-significant
interactions were removed from the full model.
We report the least squares mean rank estimates
of the response variables in the event of a
significant count period*species interaction.
The three measures of antioxidant composition
(TA, TPH, and DPPH) did not conform to the
assumptions of normality even when transformed,
and we converted TA, TPH, and DPPH to rank
values (ties averaged) to examine correlations
among the variables using Spearman’s rho. All
three variables were significantly correlated, and
we used the Vegan package (Oksanen et al. 2011:
R package Version 2.0-2) in R (R Core Develop-
ment Team 2011: R Version 2.14.1) to examine
multivariate differences in the three antioxidant
variables among plant species, nested within sites.
We performed a permutational multivariate anal-
ysis of variance (MANOVA; McArdle and
Anderson 2001) of raw (untransformed) antioxi-
dant values. Differences among species were
evaluated in the permutational MANOVA with
an F-test based on 5,000 permutations.
We evaluated regional differences (northern vs.
southern Block Island) in antioxidant values for
three species (arrowwood spp., chokeberry spp.,
and northern bayberry) using a permutational
MANOVA. The initial model contained species,
region (north or south), and the species*region
effects. We also related the consumption index
with the three antioxidant variables in another
permutational MANOVA to investigate the rela-
tionship between fruit antioxidant composition
and consumption by birds. We used the CI
estimate from the count period when the fruit
sample was collected; however, chokeberry spp.
and arrowwood spp. were collected at the initial
count period when it was not possible to calculate
the CI, and we used the CI estimate for the second
count period for these two species. We ran an
additional permutational MANOVA model for CI
that omitted DPPH to ascertain the specific
association of total anthocyanins and total pheno-
lics with fruit selection by avian consumers.
We considered effects significant at a50.05.
We used R (R Core Development Team 2011) to
test for normality, correlation, and for the
permutational MANOVA models. All other sta-
tistical analyses were conducted using SAS 9.2
(SAS Institute 2009).
RESULTS
Avian Patterns of Fruit Consumption.—The
initial number of fruits on open and enclosed
branches was not significantly different within a
species (x
2
50.87, df 51, P50.35), although
there were large differences among species in the
initial number of fruits per branch (x
2
54.70, df
56, P,0.001). Plant species differed in fruit
phenology as indicated by both the rate of natural
abscission and percent of ripe fruit at each count
period (Fig. 1). Percent of fruit remaining on the
enclosed branches decreased during the six count
periods indicating regular loss from natural
abscission that was consistent among sampling
sites (count: F
5,205
5437.25, P,0.001; site:
F
6,35
50.40, P50.87; Fig. 1). Plant species
differed in the rate of natural fruit loss (species:
F
6,35
513.43, P,0.001; species*count: F
30,205
57.83, P,0.001). Early-fruiting species, such
as chokeberry spp., had higher abscission rates
during the first few count periods, whereas later-
ripening species, such as winterberry, had higher
abscission rates during later count periods.
The percent of remaining fruit on the open
branches accessible to birds rapidly decreased
during the six count periods and this pattern was
consistent among sampling sites (count: F
5,205
5
358.42, P,0.001; site: F
6,35
50.90, P50.51).
Plant species differed in the rate of fruit decline
(species: F
6,35
521.48, P,0.001; species*
count: F
30,205
57.33, P,0.001) indicating that
birds were selectively consuming specific species,
100
THE WILSON JOURNAL OF ORNITHOLOGY NVol. 125, No. 1, March 2013
FIG. 1. Median percent of fruit remaining on open and closed branches and fruit ripeness over six count-periods during
autumn 2008 on Block Island, Rhode Island; the interquartile range is indicated by whiskers. Species, sorted approximately
from increasing to decreasing consumption, include (A) arrowwood spp., (B) Virginia creeper, (C) chokeberry spp., (D)
northern bayberry, (E) winterberry, (F) oriental bittersweet, and (G) multiflora rose.
Bolser et al. NFRUIT ANTIOXIDANTS
101
such as arrowwood spp. Arrowwood spp. and
Virginia creeper had the highest consumption
indices (Fig. 2A, B), which suggests these fruits
were selected by birds relative to other available fruit
species (species: F
6,35
511.05, P,0.001; count:
F
5,205
575.78, P,0.001; species*count: F
30,205
5
4.27, P,0.001; site: F
6,35
51.02, P50.43).
Antioxidant Composition of Fruits.—Total an-
thocyanins (TA), total antioxidant capacity (DPPH),
and total phenolics (TPH) were significantly cor-
related with one another (TA*TPH: r50.63, n5
55, P,0.001; TA*DPPH: r520.78, n549, P,
0.001; TPH*DPPH: r520.81, n549, P,0.001).
Fruit species varied in antioxidant content (Fig. 3A–
C; permutational MANOVA: F
6,32
526.3, P,
0.001). Arrowwood spp. contained the most
anthocyanins, whereas multiflora rose, oriental
bittersweet, and winterberry contained essentially
no anthocyanins (Fig. 3A). Fruit from northern
bayberry, chokeberry spp., and arrowwood spp.
contained relatively more total phenolics, but
there was overlap in values across species
(Fig. 3B). DPPH, a reciprocal measure of total
antioxidant capacity, was highest for fruit species
with the lowest amount of antioxidants. For
example, winterberry, oriental bittersweet, and
multiflora rose had the lowest TA, but the highest
DPPH values among species (Fig. 3C). This
suggests these three fruits would be unlikely to
offer much antioxidant protection to consumers.
Antioxidants and Avian Consumption of
Fruits.—Avian consumers exhibited marginal
selection of fruits associated with antioxidant
content when considering all three measures
simultaneously (permutational MANOVA: F
1,37
53.1, P50.060). However, selection of fruits
was particularly strong when considering only
total anthocyanins and total phenolics but not total
DPPH (permutational MANOVA: F
1,37
58.9, P
,0.001). For example, arrowwood spp. had the
highest consumption index and relatively high
anthocyanin and phenolic values, whereas oriental
bittersweet had the opposite pattern. Fruits with
the lowest DPPH values were also selected most
commonly by birds (Fig. 3C). However, DPPH
was the least effective of the three antioxidant
measures in explaining patterns of avian fruit
consumption; DPPH was also the least specific
measure examined in this study. We did not find
any consistent regional differences in antioxidant
values for those species tested (region: F
1,35
5
2.32, P50.11; species: F
2,35
56.35, P50.002;
Fig. 4).
DISCUSSION
Birds consumed certain fruit species (arrow-
wood spp.) more rapidly than others (oriental
bittersweet, multiflora rose, winterberry). These
patterns of fruit selection suggest birds consume
fruits with high antioxidant content more rapidly
than those with low levels of antioxidants.
Antioxidants in fruits varied somewhat among
sites on Block Island, although these were
relatively subtle compared to differences in
antioxidants between plant species.
Patterns of Avian Fruit Consumption.—Birds
rapidly consumed northern and southern arrow-
wood fruits in the maritime shrubland habitat of
northern Block Island. Previous work on Block
Island repeatedly documented arrowwood as the
most readily consumed fruit species (Parrish
FIG. 2. Median fruit consumption index for seven
species of fruiting shrubs over six count-periods during
autumn 2008 on Block Island, Rhode Island; the inter-
quartile range is indicated by whiskers. (A) Two species
(arrowwood spp. and Virginia creeper) were consumed
more regularly than (B) northern bayberry, oriental
bittersweet, chokeberry spp., multiflora rose, and winter-
berry. A higher consumption index indicates a larger
proportion of fruit consumed; consumption index (CI) 51
2(%remaining on open branch/%remaining on enclosed
branch). Interquartile range omitted from (B) to
preserve clarity.
102
THE WILSON JOURNAL OF ORNITHOLOGY NVol. 125, No. 1, March 2013
1997, 2000; Smith et al. 2007b). The latter authors
measured avian consumption of three fruit species
(arrowwood spp., black chokeberry, and American
pokeweed [Phytolacca americana]) during autumn
migration on Block Island in 2004. They found
arrowwood spp. had the highest consumption index
of the three species they measured. However, there
were differences in the timing of fruit removal
during fall and the overall abundance of fruit
between 2004 (Smith et al. 2007b) and our study,
conducted in 2008. Fruits were abundant in 2004,
all arrowwood was ripe by the first census on 9
October, and .70%of fruits on open branches was
consumed by 16 October (Smith et al. 2007b). In
contrast, fruits were much less abundant in 2008, all
arrowwood was ripe by mid-September, and at least
70%of fruits on open branches were consumed by
the first days of October. It is evident that, even in a
year with low fruit yield, as 2008 was for northern
arrowwood, birds still sought these fruits and
consumed them at a higher rate than those species
whose fruits were more abundant, despite differenc-
es in the timing of fruit consumption between years.
Consumption rates for the remaining five fruit
species were relatively low (,0.2; Fig. 2A),
indicating minimal avian consumption of these
FIG. 3. (A) Total anthocyanins, (B) total phenolics, and (C) total antioxidant capacity (DPPH) for seven species of
fruiting shrubs during autumn 2008 on Block Island, Rhode Island. High DPPH values indicate only a small quantity of the
radical has been reduced while lower DPPH values indicate a higher amount of radical reduction. Fruit extracts with higher
antioxidant capacity will have lower DPPH values. Arrowwood spp. (AW) fruits consistently contained the highest
antioxidant measures. Antioxidant measures from Virginia creeper (VC), chokeberry spp. (CB), and northern bayberry
(NB) were lower than those in arrowwood but relatively higher than those of winterberry (WB), oriental bittersweet (OB),
and multiflora rose (MR). Box plots show the median, interquartile range, and 10th and 90th percentiles; outliers are
indicated by unshaded circles.
Bolser et al. NFRUIT ANTIOXIDANTS
103
species. Smith and others (2007b) found birds ate
high-fat fruits, such as northern arrowwood, more
frequently than fruits with more carbohydrates,
such as black chokeberry, which agrees with our
results. A similar study found that, of all the
fruiting plant species available to birds on Block
Island, fruits of northern arrowwood, northern
bayberry, and American pokeweed predominated
in fecal samples of a variety of migratory bird
species (Parrish 1997). The fruiting phenology of
later-ripening species, such as oriental bittersweet,
might partially explain why birds did not
selectively consume these fruits. Additional
studies are needed that document fruit selection
of specific bird species in relation to timing of
their migration, the distribution and abundance of
fruit over this same migration period, and the
chemical composition of these fruits.
Antioxidant Composition of Fruits and Spatial
Variation.—Antioxidant concentration has fre-
quently been measured in fruit species regularly
consumed by humans, but only rarely for fruits
more commonly eaten by wildlife. We found that
arrowwood spp. had relatively high levels of
antioxidants compared to other species. This is the
first study to our knowledge to report the
antioxidant content of arrowwood fruits. In
contrast, the antioxidant composition and health
FIG. 4. Regional variation in (A) total anthocyanins, (B) total phenolics, and (C) total antioxidant capacity (DPPH) in
fruits from three plant species during autumn 2008 on Block Island, Rhode Island. We detected no consistent regional
differences in any antioxidant measures within these three plant species. Samples were collected from the main study site
on the northern end of the island (gray) and from three sites in the southern half of the island (white). Box plots show the
median, interquartile range, and 10th and 90th percentiles; outliers are indicated by unshaded circles.
104
THE WILSON JOURNAL OF ORNITHOLOGY NVol. 125, No. 1, March 2013
benefits of black chokeberry have been exten-
sively reported (Jurgonski et al. 2008); however,
direct comparison of our results to those of other
studies is challenging because analytical methods
and standards vary. Jurgonski and others (2008)
found anthocyanins were the primary phenolic
constituent in black chokeberry, and that supple-
menting rat (Rattus spp.) diets with these fruit
extracts lowered levels of oxidative stress.
Anthocyanins and phenolic compounds are also
known to have antioxidant properties both in vitro
and in vivo (Halliwell and Gutteridge 2007). We
found black chokeberry had relatively moderate
amounts of antioxidants compared to the other
species. The three different antioxidant metrics
we assessed are measured on different scales and
use assays with differing chemical properties.
Thus, we cannot make an exact comparison
between the three antioxidant metrics and it is
difficult to directly assess the contributions of TA
and TPH to DPPH. Our statistical analyses
suggest that each of these three metrics is
significantly correlated with the others. The
antioxidants measured in our study represent only
the hydrophilic antioxidants present in fruits.
Future studies should investigate the lipophilic
antioxidants in fruits, such as vitamin E and
carotenoids, in relation to both hydrophilic
antioxidants and fruit selection of birds.
Numerous environmental and genetic variables
have been studied in relation to within-plant
species variation in antioxidant levels (Connor
et al. 2002) with some evidence this creates large-
scale differences in antioxidant levels in the same
plant species (Latti et al. 2008). We suspect the
relatively small-scale spatial variation in antiox-
idant levels within plant species we documented
were produced by differences in microclimate,
soil type, or soil moisture, although this requires
further study. The lack of significant within-
species variation in fruit antioxidant levels that we
documented suggests birds can effectively in-
crease their antioxidant intake by selecting fruits
from certain plant species without the need to
extensively sample fruits from the same plant
species.
Antioxidants and Avian Consumption of
Fruits.—Antioxidants in fruits may benefit both
fruiting plants as well as migrating birds that
consume these fruits. Migrating birds use fat as a
primary fuel, and fat metabolism results in
production of free radicals (Bairlein and Gwinner
1994, Surai 2002, McWilliams et al. 2004, Pierce
and McWilliams 2005); thus, long-distance flight
likely causes oxidative damage, although this
remains to be directly demonstrated. Plants may
produce antioxidants, in part, to defend against
oxidative stress while also providing a nutritional
reward for consumers. The two fruit species in our
study with the highest antioxidant concentrations
were also the highest in fat content (northern
arrowwood: 41.3 65.8%; Virginia creeper: 23.6
64.8%dry weight 6SE) (Smith et al. 2007b).
These high levels of antioxidants might protect
the plant, as well as the consumer, from the
oxidative stress associated with fat metabolism
(Klasing 1998). Plants with higher concentrations
of antioxidants in their fruits, in addition to other
nutritional benefits of fruit for consumers, may
attract birds and achieve higher rates and
distances of seed dispersal.
Numerous studies have explored patterns in
avian fruit consumption (Parrish 1997, 2000;
Smith et al. 2007b) with varying results. It is
likely birds select certain fruits based upon several
factors, including antioxidants, that work at
different spatial scales and which may vary
seasonally (Parrish 2000, Levey and Martinez
del Rio 2001). Flo¨rchinger and others (2010)
found fruit size and color were more important
than phenolic compounds in affecting which fruits
were consumed by birds. Garden Warblers
(Sylivia borin) presented with a paired choice
between carotenoid enriched and non-enriched
foods did not prefer a diet high in carotenoids
overall, but individual birds consumed consistent
levels of carotenoids throughout the experiment
(Catoni et al. 2011). These results suggest birds
can detect carotenoids in food and consequently
regulate intake of antioxidants. The antioxidants
that we measured included anthocyanins, which
are pigmented (dark purple), and provide a visual
signal of a nutritional reward. Our results are
consistent with recent research showing birds in
captivity prefer foods with anthocyanins over
foods without (Schaefer et al. 2008) and represent
one of the first field experiments to support the
hypothesis that wild birds may select fruits, in
part, based on antioxidant content.
ACKNOWLEDGMENTS
We are grateful to the Block Island Division of The
Nature Conservancy, and especially Scott Comings, for
facilitating our field research. We thank all who assisted in
the field and laboratory including: Jay Winiarski, Tiffany
Heywood, Sean Camilieri, and Hang Ma. Funding was
Bolser et al. NFRUIT ANTIOXIDANTS
105
provided by the National Science Foundation (IBN-
9984920, IOS-0748349), Rhode Island Agricultural Exper-
iment Station, U.S. Department of Agriculture (538748),
and a University of Rhode Island Provost’s Research Grant.
This is contribution #5298 from the University of Rhode
Island Agricultural Experiment Station.
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