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How effective is preening against mobile ectoparasites? An experimental test
with pigeons and hippoboscid flies
Jessica L. Waite
⇑
, Autumn R. Henry, Dale H. Clayton
Department of Biology, University of Utah, Salt Lake City, UT, USA
article info
Article history:
Received 29 December 2011
Received in revised form 7 March 2012
Accepted 8 March 2012
Available online 2 April 2012
Keywords:
Grooming
Behaviour
Defence
Columba livia
Pseudolynchia canariensis
Vector
abstract
Birds combat ectoparasites with many defences but the first line of defence is grooming behaviour, which
includes preening with the bill and scratching with the feet. Preening has been shown to be very effective
against ectoparasites. However, most tests have been with feather lice, which are relatively slow moving.
Less is known about the effectiveness of preening as a defence against more mobile and evasive ectopar-
asites such as hippoboscid flies. Hippoboscids, which feed on blood, have direct effects on the host such as
anaemia, as well as indirect effects as vectors of pathogens. Hence, effective defence against hippoboscid
flies is important. We used captive Rock Pigeons (Columba livia) to test whether preening behaviour helps
to control pigeon flies (Pseudolynchia canariensis). We found that pigeons responded to fly infestation by
preening twice as much as pigeons without flies. Preening birds killed twice as many flies over the course
of our week-long experiment as birds with impaired preening; however, preening did not kill all of the
flies. We also tested the role of the bill overhang, which is critical for effective preening against feather
lice, by experimentally removing the overhang and re-measuring the effectiveness of preening against
flies. Birds without overhangs were as effective at controlling flies as were birds with overhangs. Overall,
we found that preening is effective against mobile hippoboscid flies, yet it does not eliminate them. We
discuss the potential impact of preening on the transmission dynamics of blood parasites vectored by
hippoboscid flies.
Ó2012 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved.
1. Introduction
Birds are infested with a variety of ectoparasites including lice,
mites, ticks, fleas and flies, all of which have the capacity to de-
crease host fitness (Atkinson et al., 2008; Møller et al., 2009). Birds
combat ectoparasites with defences ranging from anti-parasite
behaviour (Hart, 1992, 1997) to immune defences (Wikel, 1996;
Owen et al., 2010). Grooming behaviour, which includes preening
with the bill and scratching with the feet, is the first line of defence
against ectoparasites (Clayton et al., 2010). Preening is an energet-
ically expensive activity (Goldstein, 1988; Croll and McLaren,
1993); furthermore, the time and energy devoted to preening
detracts from other behaviours such as feeding and vigilance
(Redpath, 1988). Therefore, in order to be effective against ectopar-
asites while limiting its energetic cost, preening should be an
inducible defence (Tollrian and Harvell, 1999). The importance of
preening is illustrated by recent work demonstrating that features
of bill morphology, such as the upper mandibular overhang, appear
to have evolved specifically to enhance the effectiveness of
preening for parasite control (Clayton and Walther, 2001; Clayton
et al., 2005).
Nearly all of the work on the effectiveness of preening has been
done with feather lice (Phthiraptera: Ischnocera), which are slow
moving and therefore relatively easy targets for preening birds
(Marshall, 1981; Atkinson et al., 2008). The effectiveness of preen-
ing for controlling more mobile ectoparasites such as fleas and hip-
poboscid flies has not, to our knowledge, been tested. Preening
may also play a role in shaping vector ecology and the evolution
of pathogens transmitted by ectoparasites.
The goal of our study was to test the effectiveness of preening
against hippoboscid flies, which are mobile parasites of birds and
mammals. Avian hippoboscid flies are dorso-ventrally flattened
and very agile at slipping between the feathers. As described by
Rothschild and Clay (1952): ‘‘They have... an extremely efficient
method of moving among feathers – darting and scuttling about
at a remarkable speed – and are extremely difficult to catch on a
living bird.’’ Hippoboscids may also be capable of avoiding preen-
ing by using ‘‘refugia’’ such as the vent region of the bird or behind
the bases of the legs (Waite, personal observation).
Hippoboscid flies are a diverse group of parasites. More than
200 species are recognised, 75% of which parasitise birds belonging
to 18 orders; the rest parasitise mammals (Lloyd, 2002; Lehane,
2005). Most species of bird flies are winged and capable of flight
between individual hosts (Harbison et al., 2009; Harbison and
Clayton, 2011). They spend most of their time on the body of the
0020-7519/$36.00 Ó2012 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijpara.2012.03.005
⇑
Corresponding author. Tel.: +1 801 585 9742; fax: +1 801 581 4668.
E-mail address: jessi.waite@gmail.com (J.L. Waite).
International Journal for Parasitology 42 (2012) 463–467
Contents lists available at SciVerse ScienceDirect
International Journal for Parasitology
journal homepage: www.elsevier.com/locate/ijpara
Author's personal copy
bird, where they feed on blood several times a day (Coatney, 1931).
Hippoboscid feeding can cause anaemia (Jones, 1985), emaciation
(Lloyd, 2002) and slow nestling development (Bishopp, 1929). Par-
ents of hippoboscid-infested nestlings have lower reproductive
success (Bize et al., 2004). Hippoboscid flies also transmit blood
parasites that can have negative effects on birds, including malaria
(Sol et al., 2003), trypanosomes (Baker, 1967) and possibly viruses
such as West Nile (Farajollahi et al., 2005). In short, hippoboscids
pose both direct and indirect threats to the health and fitness of
their hosts.
To test the effectiveness of preening against hippoboscid flies,
we used wild caught Rock Pigeons (Columba livia) that we experi-
mentally infested with the pigeon fly Pseudolynchia canariensis
(Diptera: Hippoboscidae). We conducted two separate experi-
ments. The first experiment addressed two questions: (i) do Rock
Pigeons infested with flies increase the amount of time they spend
preening and (ii) is preening effective in killing flies? The second
experiment addressed a third question: is the bill overhang impor-
tant in the effectiveness of preening for fly control?
2. Materials and methods
2.1. Experiment 1: preening and flies
Twenty-four Rock Pigeons were caught using walk-in traps in
Salt Lake City, Utah, USA. The birds were transported to the Univer-
sity of Utah animal facility, where they were individually housed in
wire mesh cages (30 30 56 cm) suspended over newspaper-
lined trays. Each cage/tray was completely enclosed within a fly-
proof net, which prevented flies from moving between birds in
different cages. Birds were given ad libitum food, water and grit
and kept in a 12-h light/dark cycle. They were maintained in cap-
tivity for at least 6 months at low humidity prior to the experi-
ment, which killed feather lice and their eggs that were present
on the birds when they were captured (Harbison et al., 2008).
Any flies present on pigeons when they were captured would have
died during the 6 month period because the life span of pigeon flies
is only 2–3 months (Fahmy et al., 1977). Since pigeons trapped in
Salt Lake City do not usually have other ectoparasites, the birds
were ectoparasite-free at the start of our experiment. Prior to the
start of the experiment, birds were carefully examined to confirm
that they did not, in fact, have any ectoparasites.
We blocked the 24 birds using two factors: (i) location trapped
and (ii) time in captivity; we then randomly assigned birds to one
of three treatments, with eight birds per treatment. All birds were
sexed and weighed. Birds in the first two treatments were then in-
fested with 20 flies each (10 male flies, 10 female flies), which is
the maximum number recorded from wild pigeons (mean = 5.07
flies; Stekhoven et al., 1954). Flies used to infest birds were cul-
tured from wild caught stock on pigeons kept for this purpose in
another room. The third group of eight birds was not infested with
flies.
Flies were removed from culture birds using CO
2
(Moyer et al.,
2002). They were sexed under a microscope at 25before putting
them on experimental birds. Half of the birds (chosen at random)
in each of the two fly-infested treatments had plastic attachments
fitted to their bill to impair their ability to preen. The attachments
are small C-shaped pieces of plastic that, when fitted in the nares of
a pigeon, create a 1.0–3.0 mm gap between the mandibles. This gap
prevents the full occlusion of the bill needed for effective preening
(Clayton et al., 2005). The attachments are harmless; they do not
impair feeding or alter the amount of time that birds attempt to
preen (Clayton and Tompkins, 1995; Koop et al., 2011).
To address our first question whether pigeons preen more when
they are infested with flies, we compared the behaviour of birds with
normal (unimpaired) preening with and without flies. Preening
behaviour was quantified using instantaneous scan sampling be-
tween 13:00 and 16:00 h (Altmann, 1974). Preening was defined
as touching the plumage with the bill (Clayton and Cotgreave,
1994). Birds were observed at 6 s intervals (Clayton, 1990) for 30
observations per bird per day, for 5 days following infestation. We
calculated the proportion of time that birds spent preening.
To address our second question whether preening is effective in
killing flies, we compared the number of flies killed by birds with
impaired preening with flies killed by birds with normal preening.
The experiment lasted 1 week, after which one of the authors
(Waite) removed dead flies from the bottom of each cage; food
and water dishes were also checked for dead flies. Another author
(Henry) re-examined all cages to ensure that nothing was over-
looked. Damage to flies was observed and recorded under a micro-
scope at 25. Flies were scored as preening-damaged if the head,
thorax, abdomen or at least one wing was crushed or missing, or
if at least three legs were missing. We calculated the proportion
of flies with preening-damage out of the total number of dead flies
recovered for each host after 1 week.
2.2. Experiment 2: bill overhang
Another 12 wild-caught (individually caged) pigeons were used
for this experiment. Birds were again blocked by location trapped
and time in captivity. Half of the birds, chosen at random, had their
bill overhang trimmed away with a dremel tool. The other half was
sham trimmed, i.e. they were handled but no part of the bill was
removed (Fig. 1). The trimming method, which is fully described
in Clayton et al. (2005), does not harm the birds in any way. One
week after trimming (or sham trimming) all birds were sexed
and weighed, and then each bird was infested with 20 flies (10
males, 10 females). Preening behaviour and fly mortality were
quantified as in Experiment 1.
2.3. Statistical analysis
Statistical analyses were performed in Prism
Ò
v. 5.0b (GraphPad
Software, Inc.). Data were analysed using Mann–Whitney UTests
Fig. 1. Rock Pigeon bill showing upper mandibular overhang before (A) and after
(B) removal of the overhang. The overhang grows back after several weeks. Figure
reproduced from Clayton et al. (2005).
464 J.L. Waite et al./ International Journal for Parasitology 42 (2012) 463–467
Author's personal copy
for comparisons between two groups. ANOVAs were used for com-
parisons among three groups. The sex ratio of pigeon hosts in each
experiment was compared using a Chi-square or Fisher’s Exact test,
as appropriate. Values are presented as mean ± S.E. Results were
considered significant at P60.05.
3. Results
Sex and body mass of hosts did not differ significantly by treat-
ment in either experiment (Experiment 1: sex, Chi-square test,
P=0.77; mass, ANOVA, F
2,21
=1.47, P= 0.25; Experiment 2: sex,
Fisher’s Exact test, P= 1.00; mass, Mann–Whitney U= 12.5,
P=0.42).
3.1. Experiment 1: preening and flies
Birds infested with flies preened more than twice as much as
birds without flies; birds with flies preened 23.49 ± 3.96% of the
time observed, whereas birds without flies preened 11.21 ± 2.11%
of the time observed; (Fig. 2). The difference in preening rates be-
tween the two groups was statistically significant (Mann–Whitney
U= 10.5, P= 0.03).
Birds with normal preening killed twice as many flies as birds
with impaired preening; birds with normal preening killed
43.75 ± 5.41% of flies, compared with 21.88 ± 5.74% of flies killed
by birds with impaired preening (Fig. 3A). The difference in the
number of flies killed was statistically significant (U= 11.0,
P=0.03).
Birds with normal preening also damaged a significantly greater
proportion of dead flies than did birds with impaired preening
(Fig. 3B; Mann–Whitney U=7.0, P=0.01). Of the dead flies recov-
ered from normally preening birds, 44.6 ± 0.06% were damaged,
while only 16.6 ± 0.13% of flies recovered from birds with impaired
preening were damaged.
3.2. Experiment 2: bill overhang
Removal of the bill overhang had no significant effect on preen-
ing time; birds without overhangs preened 12.96 ± 1.08% of the
time observed, while birds with overhangs preened 16.81 ± 3.90%
of the time observed (Mann–Whitney U=13.0, P=0.47). Birds with
overhangs did not kill significantly more flies than birds with no
overhang; birds with overhangs killed 50.83 ± 11.93% of flies, com-
pared with 45.00 ± 11.76% of flies killed by birds with no overhang
(Fig. 4; Mann–Whitney U= 15.0, P=0.69). Thus, the bill overhang
was not a factor in the efficiency with which preening killed flies.
4. Discussion
We examined the effectiveness of preening against mobile ecto-
parasitic flies. Pigeons experimentally infested with flies preened
twice as much as pigeons without flies (Fig. 2). Preening also
Fig. 2. Proportion of time that birds with and without flies spent preening.
Fig. 3. Effect of preening and an example of preening damage. (A) Proportion of flies
killed by birds with normal versus impaired preening. (B) Example of intact versus
preening-damaged flies.
Fig. 4. Proportion of flies that were dead in cages of birds with and without bill
overhangs.
J.L. Waite et al./ International Journal for Parasitology 42 (2012) 463–467 465
Author's personal copy
proved to be effective against flies (Fig. 3A); we recovered twice as
many dead flies from the cages of birds that could preen, compared
with those that could not preen. Pigeons were able to catch and
crush flies (Fig. 3B), even though the flies are extremely adept at
moving quickly and evasively through the feathers (Rothschild
and Clay, 1952).
Removal of the bill overhang did not decrease the efficiency of
preening significantly (Fig. 4). Clayton et al. (2005) showed that
lice are crushed when birds preen by the mortar-and-pestle action
of the tip of the lower mandible moving against the upper mandib-
ular overhang. Although the overhang is essential for controlling
feather lice, our results show that it is not needed when preening
flies, presumably because the flies are much larger and softer-bod-
ied than lice. Although preening proved to be an effective defence
against flies, it did not eliminate all of them over the course of our
week-long experiment. Only one of 40 birds in the two experi-
ments cleared itself completely of flies.
Preening may have the added benefit of helping to protect birds
from pathogens for which the flies are vectors. In principle, preen-
ing can prevent transmission of pathogens if it kills infected vec-
tors before they have an opportunity to bite the host. The fly P.
canariensis is a known vector of the blood parasites Haemoproteus
columbae and Trypanosoma hannae (Fahmy et al., 1977; Mandal,
1991). Waite (unpublished data) recently showed that pigeons ex-
posed to just five flies for 3 days can become infected with H. col-
umbae. In our study, only an average of 50% of flies placed on
pigeons were killed during the week-long experiment (Fig. 3A).
Thus, even birds with relatively efficient preening may remain at
risk of acquiring blood parasites. If preening irritates flies, encour-
aging them to move between hosts, then preening might even have
the effect of increasing pathogen transmission (Hodgson et al.,
2001). It would be very interesting to measure the impact of preen-
ing on pathogen transmission by hippoboscid flies among birds in a
population.
We found that pigeons infested with flies doubled the amount
of time that they spent preening compared with controls (without
flies) and compared with the typical rates of preening for other pi-
geons and doves (Clayton, 1990; Koop et al., 2011). One might pre-
dict that experimental birds would spend even more time
preening, given that they did not completely remove their infesta-
tions in most cases. However, research on the cost of preening
shows that it is energetically expensive. When birds preen, their
metabolic rates increase by as much as 200% (Wooley, 1978; Croll
and McLaren, 1993). The energetic cost of preening might explain
why preening is an inducible defence against hippoboscid flies.
Additional indirect costs of preening include the time taken away
from courtship behaviour, foraging and predator surveillance (Red-
path, 1988). Thus, in addition to the direct impact of hippoboscid
flies on host fitness, flies may have indirect effects mediated by
the energetic and time related costs of preening. Indeed, there
may well be a trade-off between the indirect cost of preening
and the more direct costs of fly infestation.
Acknowledgements
All work was performed with the University of Utah IACUC,
USA, approval (Protocol #08-08004). We thank Sung Ki Hong for
assistance with data collection and animal care, and Kari Smith
for assistance in maintaining the fly culture. We thank Jennifer
Koop for help with behavioural data collection methods and Sarah
Bush for discussion and help with graphics. We are grateful to
Franz Goller, Jael Malenke and Lesley Chesson for comments on
the manuscript. We thank three anonymous reviewers, whose
comments improved the manuscript. We thank the Royal Society,
UK for permission to reproduce Fig. 1. Funding was provided by
Sigma Xi, USA and the American Ornithologists Union, USA to
J.L.W., the University of Utah Undergraduate Bioscience Research
Program to A.R.H, and the National Science Foundation, USA
DEB-0816877 to D.H.C.
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