Content uploaded by Geoffrey E. Hill
Author content
All content in this area was uploaded by Geoffrey E. Hill on Aug 05, 2015
Content may be subject to copyright.
2095
It is widely assumed that parasites affect expression of
ornamental coloration in animals and that color displays serve
as honest indicators of parasite resistance (Hamilton and Zuk,
1982). This assumption is supported primarily by correlations
between ornament expression and parasite load measured in
wild animals (for a review, see Hamilton and Poulin, 1997). In
only relatively few studies have the effects of parasites on
ornament expression been studied through carefully controlled
infection experiments. Zuk et al. (1990) conducted infection
experiments on red-jungle fowl Gallus gallus with intestinal
roundworms Ascaridia galli, and showed that roundworm
infection reduced comb size in males. Moreover, in a field
experiment with barn swallows Hirundo rustica, Møller (1990)
showed that males infected with blood-sucking mites grew
shorter tails than males that were not infected. These
experimental studies show that parasites can affect expression
of morphological traits. The effects of parasites on animal
coloration, and especially the brilliant plumage coloration of
birds, is of special interest because it was plumage coloration
that was the focus of the original statement of the
Hamilton–Zuk hypothesis (Hamilton and Zuk, 1982) that
ornamental traits evolve as indicators of parasite resistance.
Carotenoid pigmentation is one of the most widespread
mechanisms for ornamental coloration, particularly in fish and
birds (Goodwin, 1984), and carotenoid pigmentation has
become a text-book example of a condition-dependent display
trait (e.g. Gill, 1995; Alcock, 2001). Carotenoids are a class of
molecules that cannot be synthesized by vertebrates – they
must be ingested to be used as integumentary pigments
(Völker, 1934, 1938). Expression of carotenoid-based
ornamental coloration is thus partly a function of the type and
quantity of pigments that are ingested (Hill, 2002). Once
carotenoids are ingested, however, they still have to be
properly utilized to create ornamental coloration, and parasites
can disrupt this utilization process. Previous experimental
studies have shown that monogenean parasites and the
protozoan parasite, Ichthyophthirius multifillis, can affect
expression of carotenoid pigmentation in guppies Poecilia
reticulata and three-spined stickleback Gasterosteus
aculeatus, respectively (Milinski and Bakker, 1990; Houde and
Torio, 1992). Field correlational studies of yellowhammers
Emberiza citrinella (Sundberg, 1995) and greenfinches
Carduelis chloris (Merila et al., 1999) suggest that parasites
can affect expression of carotenoid-based plumage coloration
in birds. Controlled aviary experiments with American
goldfinches Carduelis tristis also showed that coccidiosis
(McGraw and Hill, 2000) and mycoplasmosis (Navara and
Hill, 2003) can affect expression of ornamental carotenoid
coloration.
In this study we tested the effects of the bacterium
Mycoplasma gallicepticum (MG) on expression of plumage
coloration in the house finch Carpodacus mexicanus, a species
in which males have carotenoid-based ornamental coloration
that varies from pale yellow to bright red (Hill, 1993). Several
The Journal of Experimental Biology 207, 2095-2099
Published by The Company of Biologists 2004
doi:10.1242/jeb.00998
Parasites are widely assumed to cause reduced
expression of ornamental plumage coloration, but few
experimental studies have tested this hypothesis. We
captured young male house finches Carpodacus mexicanus
in Alabama before fall molt and randomly divided them
into two groups. One group was infected with the bacterial
pathogen Mycoplasma gallicepticum (MG) and the other
group was maintained free of MG infection. All birds were
maintained through molt on a diet of seeds with tangerine
juice added to their water as a source of β-cryptoxanthin,
the natural precursor to the primary red carotenoid
pigment in house finch plumage. All males grew drab
plumage, but males with MG infection grew feathers that
were significantly less red (more yellow), less saturated,
and less bright than males that were not infected. MG
targets upper respiratory and ocular tissue. Our
observations show that a pathogen that does not directly
disrupt carotenoid absorption or transportation can still
have a significant effect on carotenoid utilization.
Key words: sexual selection, plumage coloration, carotenoid,
parasite, house finch, Carpodacus mexicanus,Mycoplasma
gallicepticum.
Summary
Introduction
The effect of mycoplasmosis on carotenoid plumage coloration in male house
finches
Geoffrey E. Hill*, Kristy L. Farmer and Michelle L. Beck
Department of Biological Sciences, 331 Funchess Hall, Auburn University, Auburn, Alabama 36849, USA
*Author for correspondence (e-mail: ghill@acesag.auburn.edu)
Accepted 23 March 2004
2096
studies have been conducted on the effects of parasites on
expression of plumage coloration in the house finch.
Thompson et al. (1997) found that males that that were infected
with avian pox prior to molt grew feathers that were less
brightly pigmented compared to males that did not have pox
infection prior to molt. They found a similar relationship
between feather mites and coloration (Thompson et al., 1997).
Brawner et al. (2000) conducted an experiment to test the effect
of Isospora coccidia on plumage coloration. They found that
males that they infected with Isospora during molt grew
drabber plumage than males that were maintained free of
coccidial infection. In this experiment, some birds contracted
mycoplasmal conjunctivitis and birds with MG grew drabber
plumage than birds that were free of MG.
Although these studies represent a substantial body of
work on the effects of parasites on carotenoid-based plumage
coloration, and the house finch has been the focus of more
research than any other passerine bird, fundamental questions
remain regarding the effects of parasites on carotenoid-based
plumage coloration in the house finch. First, the only
carefully controlled experiment looking at the effects of
parasites on plumage coloration was conducted with
isosporan coccidia. Coccidia are parasites of the gastro-
intestinal tract. They are known to directly inhibit carotenoid
absorption and the production of carotenoid carrier proteins
(Allen, 1987a,b). Thus, coccidiosis is a disease that is
expected to have direct effects on expression of carotenoid-
based plumage coloration. The effects on plumage coloration
of diseases that are more systemic and that are not known to
directly inhibit carotenoid uptake have not been tested
experimentally. In the Brawner et al. (2000) study, MG broke
in cages of birds – individuals were not assigned to treatment
groups. Whether or not individual birds became infected was
likely to have been related to their ability to resist the
pathogen. Thus, the effect of parasites is confounded by the
overall health and condition of individual birds. Finally, in
previous aviary studies looking at the effects of parasites on
plumage coloration in house finches, males were maintained
on diets supplemented with the red carotenoid canthaxanthin
(Brawner et al., 2000). Canthaxanthin is used by male house
finches directly as a plumage pigment without being modified
(Inouye et al., 2001; Hill, 2002). The dominant red pigment
in the plumage of wild male house finches, however, is 3-
hydroxy-echinenone, which is the metabolic derivative of the
dietary pigment β-cryptoxanthin (Inouye et al., 2001). By
feeding males a red pigment and bypassing metabolic
pathways, previous feeding experiments may have
underestimated the effects of parasitism on plumage
coloration.
In the present study we tested the effects of Mycoplasma
gallicepticum on expression of plumage coloration in male
house finches. Two groups of males were maintained on a
diet supplemented with β-cryptoxanthin. One group was
experimentally infected with MG while the other group was
maintained free of MG through the molt period. This study was
designed as an experimental test of the effects of a systemic
infection on expression of carotenoid pigmentation when birds
are utilizing a natural dietary precursor for plumage pigments.
Materials and methods
We captured hatch year house finches Carpodacus
mexicanus Müller in Auburn, Alabama, USA from early July
through August. Upon capture, we took 100·µl of blood from
each bird. Blood was spun to separate plasma and red blood
cells, which were stored in TNE buffer at –80°C. Plasma was
stored at 4°C for serological analysis (see below). In juvenile
plumage, male and female house finches cannot be
distinguished morphologically, so we used a molecular-sexing
technique to determine the sex of the birds that we captured.
Briefly, the DNA was extracted from stored red blood cells
using a standard phenol/chloroform technique (Quinn and
White, 1987; Westneat, 1993). Extracted DNA was
resuspended in TE buffer and stored at –20°C. We identified
the sex of the hatch year birds using P2 and P8 microsatellite
primers and the PCR protocol outlined by Griffiths et al.
(1998). These primers amplify introns on the CHD1-W and
CHD1-Z genes. Two bands are present in females, which are
the heterogametic sex, whereas males as the homogametic sex
have a single band (Griffiths et al., 1998). PCR products were
separated on a 1.5% agarose gel laced with ethidium bromide
by electrophoresis at 150·V for 2·h.
Males were housed in small cages with 2 birds per cage for
the duration of the experiment. Birds within a cage received
the same experimental treatment – either infected or not
infected (see below). They were held near windows so they
experienced a natural light cycle. All birds had ad libitum
access to a canary pellet diet (canary maintenance, Avi-Sci
Inc., St Johns, Michigan, USA). Through the molt period, we
added one part tangerine juice (100% pure juice, not from
concentrate, never frozen) to one part drinking water for all
birds. Tangerine juice tended to spoil at room temperature so
tangerine juice/water was changed every 24·h.
To ensure that none of the birds in our experiments had been
previously exposed to MG, we tested the serum of each bird
for antibodies to MG using a serum plate agglutination assay
as described in Roberts et al. (2001). We also tested birds for
the presence of MG by polymerase chain reaction (PCR;
Roberts et al., 2001). We collected samples for analysis by
PCR by swabbing the choanal cleft using a micro-tip swab
(Becton Dickinson and Co. Maryland, USA). Any birds that
were found to have antibodies to MG or that were PCR-
positive were excluded from the study. Coccidiosis is another
widespread disease of house finches that is known to affect
expression of carotenoid coloration (Brawner et al., 2000). To
be certain that coccidiosis did not confound the effects of
mycoplasmosis in this experiment, we added sulfadimethoxine
to the water of all birds to ensure that they remained free of
coccidiosis (Brawner et al., 2000)
We cultured MG from symptomatic wild house finches caught
in Auburn, Alabama. We infected the birds in the MG treatment
group by dropping 10·µl of SP4 medium containing 1×106color-
G. E. Hill, K. L. Farmer and M. L. Beck
2097Mycoplasmosis and plumage color
changing units·ml–1 into each eye for a total dose of 2×104color-
changing units. This dose of MG has been effective in previous
studies in inducing a modest infection among captive house
finches (Roberts et al., 2001). Birds in the uninfected treatment
group were sham infected with the same amount of sterile SP4.
We monitored the birds daily for onset of disease. Disease was
measured for each eye on a five-point scale, where 0=normal eye
and 4=blindness caused by swelling (Roberts et al., 2001). We
captured all birds three weeks post-inoculation to collect blood
for serology and swabs for MG detection by PCR.
Following molt, we measured plumage coloration of all
males using a Colortron reflectance spectrophotometer (Hill,
1998). Male house finches display carotenoid-based plumage
coloration on their crown, breast and rump, and a technician
with no knowledge of the experiment scored plumage color by
taking three measurements in each of these areas. We averaged
these measurements to obtain an overall hue, saturation and
brightness for each male. We photographed the breast patch of
each male along with a size standard and used Sigma Scan 5.0
to measure breast patch size. We calibrated each picture using
the size standard and then traced the breast patch three times
and used the average size in all analyses.
All infection protocols carried out in this study were
approved by the Institutional Animal Care and Use Committee
(IACUC) at Auburn University. We used the smallest sample
of birds that would give us reasonable power to detect
differences among groups.
Results
Birds in the uninfected group remained free of MG
throughout the study. No birds in the uninfected group showed
symptoms of mycoplasmosis nor did any birds show a positive
antibody response to MG. All PCR tests of birds in the
uninfected group were negative.
All birds that were inoculated with MG developed
conjunctivitis in both eyes within 10 days of inoculation. All
birds in this group developed antibodies to MG and all tested
positive for MG by PCR. For most birds the infection lasted
throughout the 8-week molt period, with the most severe
clinical symptoms occurring 2–6 weeks after infection.
After molt on the pellet diet supplemented with tangerine
juice (as a source of β-cryptoxanthin; Hill, 2000), all males
grew pale orange plumage, much drabber than the average wild
male in the Auburn, Alabama population. There was a
significant effect of MG infection on ornamental plumage
coloration. Males that were infected with MG during molt
grew breast feathers that were more yellow/less red (Z=–1.90,
N=15,16, P=0.03), less saturated (Z=–1.86, N=15,16, P=0.03),
and less bright (Z=–1.78, N=15,16, P=0.04) than males that
had no MG infection (Mann–Whitney U Tests) (Fig.·1). There
was no significant effect of the cage in which males held on
any component of plumage coloration (hue: F-ratio=1.62,
P>0.17; saturation: F-ratio=1.84, P>0.12; brightness: F-
ratio=1.33, P=0.29).
Patch size was measured on males several months after molt
and by that time several males had died or been used in other
experiments. Therefore we had only 12 infected and 13 control
males for patch size comparison. We found no significant
difference in the patch sizes of male that were infected with
MG during molt and the patch sizes of males that were not
infected (Z=–0.65, P=0.51).
Discussion
Understanding the effects of parasites on production of
Hue
28
30
32
34
36
38
30
35
40
45
50
55
60
65
30
35
40
45
Saturation (%)
Brightness (%)
Infected Not infected
Red Yellow
Treatment
N=15N=16
Fig.·1. The effects of Mycoplasma gallicepticum (MG) infection on
expression of carotenoid plumage coloration in male house finches.
Plotted as horizontal lines are the median and 10th, 25th, 75th and
90th percentiles of the hue, saturation and brightness. Comparisons
were made with the mean color of the crown, breast and rump
feathers for each male following molt within experimental treatment
group. Infected males all had conjunctivitis during the molt period.
Uninfected males tested negative for MG both in antibody tests and
PCR tests at capture and were held in a quarantine facility away from
sources of MG during molt. All birds were fed a low-carotenoid
pellet diet and water supplemented with tangerine juice as a source
of carotenoid pigments.
2098
ornamental traits is central to a general understanding of the
signal function of these displays. Signaling parasite resistance
has been proposed to be a key function of color displays and
to have been the driving force in the evolution of such traits
(Hamilton and Zuk, 1982). This theory relies crucially on
parasitic infection reducing the magnitude of display traits.
Despite the importance of the hypothesis that parasites reduce
expression of display traits, few experimental studies have
been conducted to test this idea.
Previous research had shown that in controlled infection
experiments, coccidiosis caused male house finches to grow
less red and less saturated plumage coloration (Brawner et al.,
2000). In these infection experiments that controlled the
exposure of males to coccidiosis, some males contracted
mycoplasmal conjunctivitis, and it was found that males with
mycoplasmal conjunctivitis grew less red, less saturated, and
less bright plumage than males that were not infected (Brawner
et al., 2000). Because males in the Brawner et al. (2000) study
were not infected with MG by researchers, but rather either
contracted or resisted contracting the disease, there is the
chance than male health and condition affected both plumage
coloration and disease state.
In the present study we eliminated the uncertainty of
previous studies and show definitively that mycoplasmal
conjunctivitis during molt depressed expression of ornamental
plumage coloration in male house finches. In our experiment,
we randomly assigned males to treatment groups eliminating
any confounding effects of condition or health. We also fed
males the precursor to the red pigment that is the most
abundant red pigment in the plumage of wild house finches.
By feeding the metabolic precursors to red feather pigments,
we forced birds to include more steps in the utilization of
pigments. Interestingly, even though we forced birds to
metabolically modify dietary pigments to produce red
ornamental coloration, and thus added at least one step to the
process of carotenoid utilization, we saw no greater effect of
MG on plumage coloration in this study, compared to the
previous study in which males were fed red feather pigments
directly (Brawner et al., 2000).
We found a significant negative effect of mycoplasmosis on
plumage coloration, but no significant effect of mycoplasmosis
on the size of carotenoid breast patches. This observation is
consistent with observations from feeding experiments in
which access to carotenoid pigments had a large effect on
plumage coloration but a small effect on patch size (Hill, 1992,
1993). This finding is also consistent with the idea that the
quality of ornament pigmentation (coloration) and the area of
the body with pigment (patch size) are under distinct
developmental control, with largely independent responses to
environmental challenges and different signaling function
(Badyaev et al., 2001).
As a source of β-cryptoxanthin, we fed males in this
experiment tangerine juice (Hill, 2000). β-cryptoxanthin is
the predominant carotenoid in tangerine juice (Mangels et
al., 1993), but the amount of β-cryptoxanthin ingested by
male house finches in this study was probably still small
compared to the amount of the pigment that is likely to be
ingested by wild finches feeding on fruits. Males in both
treatment groups appeared to have fewer pigments available
than they needed for maximum expression of ornamental
coloration, and all males were drab at the end of the feeding
experiment. Experimentally infected males were simply
drabber on average than uninfected males. We assume that
we may have seen larger differences in plumage coloration
between control and infected groups if males had been fed
larger doses of β-cryptoxanthin, but it is also conceivable
that access to more β-cryptoxanthin may have masked the
effects of mycoplasmosis.
Previous experimental studies of parasites and plumage
coloration in house finches have focused on coccidiosis. In one
sense, these studies are particularly valuable because they
focus on a parasite for which a mechanism for direct inhibition
of carotenoid utilization is known (Brawner et al., 2000). On
the other hand, studies of coccidia leave open the question of
the effects of parasites that do not directly inhibit carotenoid
uptake or transport, but that have more general systemic effects
on the bird and would have indirect effects on pigment
utilization. MG is an upper respiratory disease (Jordan, 1996).
It does not infect gastro-intestinal tissues and thus it is unlikely
to directly affect carotenoid absorption. Nevertheless, we
found that MG depressed expression of ornamental coloration.
The present study, coupled with previous experiments on the
effects of disease on house finches, shows that both parasites
that specifically target gastro-intestinal tissues and directly
disrupt carotenoid utilization as well as parasites that infect
tissues outside the gastro-intestinal track can have a significant
effect on how carotenoid pigments are utilized. The relative
importance of these two types of parasites on expression of
plumage coloration among males in wild populations remains
to be determined.
One intriguing possibility is that, in birds infected with MG,
carotenoids that could have been used for ornamental display
were instead diverted to bolster the immune system against the
infection (Lozano, 1994; Møller et al., 2000). We cannot assess
this hypothesis with the data at hand, but it has yet to be shown
for any species that there is a trade-off between use of
carotenoids for ornamental plumage coloration and use of
carotenoids for immune defense (Hill, 1999). A recent study
on American goldfinches failed to find such a trade-off in a
carefully controlled infection experiment (Navara and Hill
2003). A simpler explanation is that the house finches in this
study that were infected with MG diverted energy away from
pigment utilization to immune defense and this diversion of
energy caused the loss of plumage coloration in infected males.
We thank Lisa Snowberg and Brad Staton for caring for the
birds. This study was funded by NSF grants DEB0077804 and
IBN9722171 to G.E.H.
References
Alcock, J. (2001). Animal Behavior: An Evolutionary Approach. Sinauer
Associates, Inc., Sunderland, Massachusetts.
G. E. Hill, K. L. Farmer and M. L. Beck
2099Mycoplasmosis and plumage color
Allen, P. C. (1987a). Effect of Eimeria acervulina infection on chick (Gallus
domesticus) high density lipoprotein composition. Comp. Biochem. Physiol.
87B, 313-319.
Allen, P. C. (1987b). Physiological response of chicken gut tissue to coccidial
infection: Comparative effects of Eimeria acervulina and Eimeria mitis on
mucosla mass, carotenoid content, and brush border enzyme activity.
Poultry Sci. 66, 1306-1315.
Badyaev, A. V., Hill, G. E., Dunn, P. O. and Glen, J. C. (2001). Plumage
color as a composite trait: Developmental and functional integration of
sexual ornamentation. Am. Nat. 158, 221-235.
Brawner, W. R., III, Hill, G. E. and Sundermann, C. A. (2000). Effects of
coccidial and mycoplasmal infections on carotenoid-based plumage
pigmentation in male house finches. Auk 117, 952-963.
Gill, F. (1995). Ornithology. W. H. Freeman and Co., New York.
Goodwin, T. W. (1984). The Biochemistry of Carotenoids. Chapman and Hall,
New York.
Griffiths, R., Double, M. C., Orr, K. and Dawson, R. J. G. (1998). A DNA
test to sex most birds. Mol. Ecol. 7, 1071-1075.
Hamilton, W. D. and Zuk, M. (1982). Heritable true fitness and bright birds:
a role for parasites? Science 218, 384-386.
Hamilton, W. J. and Poulin, R. (1997). The Hamilton and Zuk hypothesis
revisted: a meta-analytical approach. Behaviour 134, 299-320.
Hill, G. E. (1992). Proximate basis of variation in carotenoid pigmentation in
male house finches. Auk 109, 1-12.
Hill, G. E. (1993). Geographic variation in the carotenoid plumage
pigmentation of male house finches (Carpodacus mexicanus). Biol. J. Linn.
Soc. 49, 63-86.
Hill, G. E. (1998). An easy, inexpensive means to quantify plumage
colouration. J. Field Ornithol. 69, 353-363.
Hill, G. E. (1999). Is there an immunological cost to carotenoid-based
ornamental coloration? Am. Nat. 154, 589-595.
Hill, G. E. (2000). Energetic constraints on expression of carotenoid-based
plumage coloration. J. Avian Biol. 31, 559-566.
Hill, G. E. (2002). A Red Bird in a Brown Bag: The function and evolution of
ornamental plumage coloration in the House Finch. New York: Oxford
University Press.
Houde, A. E. and Torio, A. J. (1992). Effect of parasitic infection on male
color pattern and female choice in guppies. Behav. Ecol. 3, 346-351.
Inouye, C. Y., Hill, G. E. Montgomerie, R. and Stradi, R. D. (2001).
Carotenoid pigments in male house finch plumage in relation to age,
subspecies, and ornamental coloration. Auk 118, 900-915.
Jordan, F. T. W. (1996). Avian mycoplasmosis. In Poultry Diseases (ed. F.
T. W. Jordan and M. Pattison), pp. 81-93. London, UK: W. B. Saunders Co.
Ltd.
Lozano, G. A. (1994). Carotenoids, parasites, and sexual selection. Oikos 70,
309-311.
Mangels, A. R., Holden, J. M., Beecher, G. R., Forman, M. R. and Lanza,
E. (1993). Carotenoid content of fruits and vegetables: an evaluation of
analytic data. J. Am. Diet. Assn. 93, 284-296.
McGraw, K. J. and Hill, G. E. (2000). Differential effects of endoparasitism
on the expression of carotenoid- and melanin-based ornamental coloration.
Proc. R. Soc. Lond. B 267, 1525-1531.
Merila, J., Sheldon, B. C. and Lindstrom, K. (1999). Plumage brightness in
relation to haematozoan infections in the greenfinch Carduelis chloris:
bright males are a good bet. Ecosci. 6, 12-18.
Milinski, M. and Bakker, T. C. M. (1990). Female sticklebacks use male
coloration in mate choice and hence avoid parasitized males. Nature 344,
330-333.
Møller, A. P. (1990). Effects of a haematophagous mite on the barn swallow
(Hirundo rustica): a test of the Hamilton and Zuk hypothesis. Evolution 44,
771-784.
Møller, A. P., Biard, C., Blount, J. D., Houston, D. C., Ninni, P., Saino, N.
and Surai, P. F. (2000). Carotenoid-dependent signals: Indicators of
foraging efficiency, immunocompetence or detoxification? Poult. Av. Biol.
Rev. 11, 137-159.
Navara, K. J. and Hill, G. E. (2003). Dietary carotenoid pigments and
immune function in a songbird with extensive carotenoid-based plumage
coloration. Behav. Ecol. Sociobiol. 6, 909-916.
Quinn, T. W. and White, B. N. (1987). Identification of restriction-fragment-
length polymorphisms in genomic DNA of the lesser snow goose (Anser
caerulescens caerulescens). Mol. Biol. Evol. 4, 126-143.
Roberts, S. R., Nolan, P. M. and Hill, G. E. (2001). Characterization of
mycoplasmal conjunctivitis in captive house finches (Carpodacus
mexicanus) in 1998. Avian Diseases 45, 70-75.
Sundberg, J. (1995). Parasites, plumage coloration and reproductive success
in the yellowhammer, Emberiza citrinella. Oikos 74, 331-339.
Thompson, C. W., Hillgarth, N., Leu, M. and McClure, H. E. (1997). High
parasite load in house finches (Carpodacus mexicanus) is correlated with
reduced expression of a sexually selected trait. Am. Nat. 149, 270-294.
Völker, O. (1934). Die Abhangigkeit der lipochrombildung bei vögeln von
pflanzlichen carotinoiden. J. Ornithol. 82, 439.
Völker, O. (1938). The dependence of lipochrome-formation in birds on plant
carotenoids. Proc. 8th Int. Orthinol. Congr. 1938, 425-426.
Westneat, D. F. (1993). Polygyny and extrapair fertilizations in eastern red-
winged blackbirds (Agelaius phoeniceus). Behav. Ecol. 4, 49-60.
Zuk, M., Johnson, K., Thornhill, R. and Ligon, J. D. (1990). Parasites and
male ornaments in free-ranging and captive red jungle fowl. Behaviour 114,
232-248.