Behavioral Ecology Vol. 11 No. 5: 520-527
© 2000 International Society for Behavioral Ecology
Carotenoid-based ornamentation and status signaling in the house finch
Department of Biological Sciences and Alabama Agricultural Experiment Station, 331 Funchess Hall, Auburn University, Auburn, AL 36849-5414, USA
Address correspondence to K. J. McGraw at the Department of Neurobiology and Behavior. Cornell University, Ithaca, NY 14853, USA. E-mail: kjm22{at}cornell.edu .
Received 30 March 1999; revised 18 October 1999; accepted 28 January 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The status signaling hypothesis (SSH) was devised primarily to explain the adaptive significance of avian ornamental coloration during the nonbreeding season. It proposes that conspicuous male plumage serves as an honest signal of social status within a population of birds. However, to date this hypothesis has been well tested and supported for only one type of plumage coloration, melanin-based coloration. Carotenoid-based pigmentation is known to positively reveal male health and condition during molt in a variety of species, but it is poorly understood whether this ornament type can also function as a status signal during the winter. We tested the SSH in male house finches (Carpodacus mexicanus) by manipulating the carotenoid-based plumage brightness of first-year males and then pairing unfamiliar birds of differing coloration in a series of dominance trials in captivity. Manipulated plumage color was unrelated to win/loss outcome in these trials. Similarly, the natural pigmentation of males was a poor predictor of winter dominance; as in other studies with this species, we found only a weak tendency for naturally drab males to dominate naturally bright males. These results suggest that carotenoid-based coloration is not a reliable indicator of social status in male house finches during the nonbreeding season. In fact, carotenoid-based coloration may function only in mate choice in this species, and it may be retained throughout the year either because time constraints preclude a second plumage molt or because it aids in pair formation that begins in late winter.
Key words: aggression, dominance, Carpodacus mexicanus, carotenoids, house finches, plumage coloration.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The evolutionary significance of plumage variability in male birds is a topic of widespread interest to behavioral ecologists. In species in which males complete two annual molts and exhibit ornamental plumage only in the breeding season, it is clear that the ornament is maintained by sexual selection (reviewed in Andersson, 1994
; Hill, 1999).
However, in species that molt only once annually, with males displaying
ornamental plumage throughout the year, it is possible that ornamental plumage
has a function during the nonbreeding season, and this nonbreeding-season
function may or may not be the same as that during the breeding season
(reviewed in Butcher and Rohwer,
1989; Senar,
1999).
Rohwer (1975) proposed the
status signaling hypothesis (SSH) to explain the function of avian plumage
ornamentation during the nonbreeding season. For species that form unstable
social flocks during the winter in which birds regularly interact with new
individuals, Rohwer suggested that aggressive encounters between unfamiliar
individuals over limited food resources could be mediated by ornamental traits
that signal the competitive ability of individuals. As with sexually selected
traits, the honesty of the trait is maintained by a condition-dependent
signaling system, where only the individuals that can afford to pay the high
costs associated with the trait can also bear the most exaggerated ornament
(Rohwer, 1982). Thus, males
displaying the most extravagant form of the ornamental trait are expected to
be more aggressive and win significantly more contests than poorly ornamented
individuals.
Since its inception, the SSH has been tested on a variety of species that
retain their ornamental plumage throughout the winter. To conduct an
appropriate test of the SSH, one must ensure that male ornaments function
directly as signals of social status rather than merely being correlated with
aggression (Slotow et al.,
1993; Whitfield,
1987). As a result, emphasis has been placed on experimental tests
that employ plumage manipulations, control for extraneous variables such as
age and body size, and allow unfamiliar birds to compete in short-term
encounters. Even under such strict conditions, it has been shown in a number
of species that males with experimentally enhanced ornamentation dominate
poorly ornamented males over access to winter food resources (studies reviewed
in Senar, 1999).
However, such tests have focused almost exclusively on one type of plumage
ornament, melanin-based coloration. Largely ignored in tests of the SSH are
carotenoid-based ornaments. Melanin- and carotenoid-based colors in birds
respond differently to environmental stress
(Hill and Brawner, 1998;
Mc-Graw and Hill, in press),
and as a result these two signal types may have completely different
information content (Badyaev and Hill,
2000; Gray, 1996).
To our knowledge, the SSH has been tested only once for a carotenoid-based
plumage ornament—the red coloration of the northern cardinal
(Cardinalis cardinalis)
(Wolfenbarger, 1999). Despite
the fact that the natural plumage color of male cardinals was significantly
related to aggressive outcomes in this study, the manipulated plumage
brightness of males was not a reliable predictor of social dominance. Thus,
carotenoid pigmentation in cardinals was apparently not used as a visual
signal of competitive ability but simply was a correlate of social
dominance.
The aim of our study was to test the SSH in another species in which males
display a carotenoid-based plumage ornament. We chose the house finch
(Carpodacus mexicanus) as our study species because males acquire
their ornamental plumage in the late summer/early fall through a complete
prebasic molt and display bright coloration throughout the year
(Hill, 1993b). Additionally,
this ornament has been well studied within a breeding context
(Hill et al., 1999).
Carotenoid pigmentation in male house finches varies in color from red to
orange to yellow, and sexual selection by female mate choice maintains the
trait during the breeding season (Hill,
1990,
1991,
1994;
Hill et al., 1999). Bright red
males pair more often (Hill,
1991; Hill et al.,
1999) and earlier in the season (Hill et al.,
1994,
1999) than do drab orange and
yellow males, which allows the more brightly colored males to fledge more
offspring in a season (McGraw et al., in
press). However, the significance of ornamental plumage coloration
during the nonbreeding season remains poorly understood in this species. Three
studies of house finches in the nonbreeding season suggest a potential
relationship between male ornamentation and social dominance
(Belthoff and Gauthreaux, 1991;
Belthoff et al., 1994;
Brown and Brown, 1988), but to
date no study has performed an experimental test of the SSH in this
species.
To test this hypothesis in male house finches, we conducted two experiments
using captive birds. We manipulated the plumage color of first-year males and
conducted paired trials that pitted two unfamiliar birds of contrasting
plumage brightness in competitions over access to food. We also conducted an
experiment in which we paired unfamiliar males of the same age and of
contrasting natural plumage brightness. Belthoff et al.
(1994) performed a similar
study and detected a trend in which naturally drab male house finches were
dominant to more brightly colored males. However, they did not control for the
competitive ability of males in their staged contests. Recent work suggests
that ornamental plumage may have little impact on the aggressive interactions
of birds in stable flocks and that previous social interactions may affect the
behavior of birds in subsequent dominance trials
(Senar, 1999). Thus, in both
experiments, we observed dominance among males in the flocks in which they
were housed and subsequently matched unfamiliar males by dominance rank in the
paired trials.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Manipulated-plumage experiment
On 22 July 1998, we captured 40 male house finches at feeders from two sites in Lee County, Alabama, USA. These sites were separated by approximately 5 km, and we assumed that males from the two sites had little prior experience with one another. As age-related dominance is common among birds (Enoksson, 1988
;
Hogstad, 1989), we included
only hatch-year males in this study. Age was easily determined at the time of
capture because hatch-year males had not yet molted their juvenal plumage.
Birds were fitted with an aluminum U.S. Fish and Wildlife Service band and a
unique combination of three colored bands to allow for individual
identification.
At capture, we took standard measurements of flattened wing chord length,
bill length (exposed culmen), and tail length (after
Pyle et al., 1987). We
measured male body mass just before each experimental trial (see below for
details). From these four measures, we used a principal component analysis
(PCA) to calculate a single body size index. The first principal component
(PCI) accounted for 50% of the variation in body size measurements, and
eigenvectors for PC1 ranged from 0.31 to 0.62, indicating positive
correlations among the four body size measures. At this time, we also visually
estimated ectoparasite loads (feather mites) on the right wing using an
integer scale from 0 (no mites) to 5 (hundreds of mites on every feather;
after Thompson et al., 1997).
Birds that were afflicted with avian pox or mycoplasmal conjunctivitis were
excluded from this study.
We held males in captivity in four separate outdoor cages of 10 birds each.
The cage arrangement visually isolated the flocks to maintain unfamiliarity
between males caught from separate sites. Birds were fed ad libitum diets of
sunflower seeds, millet, water, and vitamins. We minimized the potential for
dominant males to control food resources in flocks by placing multiple food
and water dishes in each cage. Diets were supplemented with a small quantity
of the carotenoid pigment canthaxanthin (125 mg/1 of water; Roxanthin Red 10
WS canthaxanthin beadlets, Roche Vitamins Inc., Parsippany, New Jersey) during
molt to give all males a drab orange plumage. Males did not have identical
postmolt plumage colors, however, so we quantified plumage brightness using a
ColortronTM reflectance spectrophotometer (LightSource Inc., San Rafael,
California; Hill, 1998). We
used mean hue as our plumage brightness index
(Hill et al., 1999), which we
calculated as the arithmetic average of three hue scores for each of the
regions of plumage pigmentation in males (crown, breast, and rump). The
Colortron assigns unitless hue values based on a 360° color wheel with
values increasing from red to orange to yellow.
We allowed birds to acclimate to their new social environment for a minimum of 7 days before making observations of dominance behavior. After this time, we accumulated 4 h of morning observations for each of the four flocks to determine the dominance status of each bird from aggressive and submissive interactions. For each interaction, winners and losers were determined based on success in supplanting other males at perches and food sources. We summed the wins and losses for each bird to construct dominance hierarchies for each flock. Those birds having more wins over others were assigned higher dominance ranks, with a rank of 1 being the most dominant. The resulting hierarchies were linear with no reversals, and this allowed us to assign each bird a relative dominance position in his flock.
We conducted 20 dominance trials from 21 November-10 December 1998. All trials were conducted in the first 2 h of morning daylight. Each male participated in only one trial, and each trial consisted of two unfamiliar males. To prevent previous social interactions with familiar birds in the housing flocks from impacting the performance of individuals during the dominance trials, males were matched for dominance ability. Thus, we pitted the top-ranked male from one cage against the top-ranked male from another, and so forth.
Each trial also pitted males of contrasting manipulated plumage brightness. We used Berol PrismacolorTM non-toxic art markers to color over the pigmented feather patches and in each trial colored one male red and the other yellow. We selected those marker colors that provided the best visual match with the natural variation of house finch plumage pigmentation (red = PM-6 Carmine Red; yellow = PM-17 Sun-burst Yellow). Red markers successfully covered the orange plumage of males, and yellow markers gave males an orange/yellow appearance. We collected spectral reflectance data for these males and found that our color manipulations created close matches with the natural variation in male house finch plumage coloration (Figure 1).
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We also noted that coloring feathers with a market had a small effect on
feather reflectance in the ultraviolet (UV;
Figure 1). However, our plumage
manipulations altered UV reflectance equally for the two experimental groups.
Furthermore, the carotenoid-based pigment patches of male house finches
reflect minimally in the UV (Figure
1), and house finches possess the poorest UV vision among
songbirds tested to date (Chen et al.,
1984). Thus, we assume that UV plumage reflectance plays a minimal
role in the social interactions of male house finches.
Premanipulated plumage hues did not differ significantly between treatment groups, but after being colored with art markers brightened males had significantly redder plumage than did lightened males, with no overlap between the two groups (Table 1). Males for this experiment were all captured on the same day, so the amount of time spent in captivity did not affect trial outcomes. Plumage manipulations were alternated among males in a cage according to dominance rank (e.g., cage 1 = top-ranked colored red, second-ranked yellow, etc., versus cage 2 = top-ranked yellow, second-ranked red, etc.) to prevent one cage from receiving more of one type of manipulation and thus to avoid potential flock/cage/capture-site effects. Brightened and lightened males that were paired in trials did not differ significantly in body size (paired t test, t = 0.91, n = 17, p =.38) or wing mite load (Wilcoxon signed-rank test, Z = -1.60, n = 17, p =.11).
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The day before males participated in a trial, we removed them from their
housing flocks and colored their plumage, giving their feathers time to dry
and resume a natural appearance before the trial. Males were then isolated in
separate indoor cages and held overnight without access to either food or
water. Short-term food deprivation both standardized and maximized the
motivation of individuals to compete for food resources during dominance
trials (see Andersson and
hlund, 1991;
Lemel and Wallin, 1993). Males
were weighted to the nearest 0.1 g on the evening before and on the morning of
the trial in which they participated, and these two measures were tightly
correlated (r =.97, n = 34, p <.0001), so we
used only the morning weights in our statistical analyses.
In the morning, colored bands were removed from males and birds were
separately transferred to the experimental test cage in which neither of the
individuals had been housed previously. This unfamiliar cage was identical in
size to the housing units, was visually isolated from all other cages, and
contained perches and a central food dish. Males were placed in separate
cardboard boxes on the floor of the test cage, where they remained for 5 min,
after which we simultaneously released the males into the cage by pulling on
ropes to lift the boxes. This technique avoided the possibility of prior
residency effects (after Holberton et al.,
1990; Senar et al.,
1990), and it allowed the birds to interact immediately without
human disturbance.
Because we were interested in determining if males were quickly assessing the aggressive ability of competitors solely on the basis of plumage variation, we ran trials for 20 min or until one male won seven more aggressive interactions than the other. Trials that failed to meet these criteria were not considered in our analyses (n = 3 of 20 for this experiment). The 17 successful trials averaged 15.7 ± 4.5 min in length and yielded a total of 127 interactions, only 3% of which (n = 4) were won by males not considered to be trial winners. Thus, this short-term format seemed to be a fair representation of immediate dominance ability and assessment. After the trials, we put colored bands back on males and returned the birds to their flocks to maintain a standardized social setting for all other males prior to their trials.
Natural-plumage experiment
Methodology for this experiment generally followed that given above for the
manipulated-plumage experiment. We captured 36 males from the two previously
mentioned sites between 1 October and 20 November 1998. Because males were not
all caught on the same day, we recorded the date of entry into captivity for
each bird. Again, to control for age we included only first-year males in this
experiment; however, because all males had completed their prebasic molt, age
had to be determined by the extent of skull ossification
(Pyle et al., 1987). We
captured nine drab males and nine bright males from each site. We defined drab
males as having mean plumage hue values > 8 and bright males as having mean
plumage hue scores < 4 (see "Manipulated-plumage experiment"
for details of hue scoring). All bright males appeared red to the human eye,
and all drab males orange or yellow. Males with mean hue scores between 4 and
8 were excluded from this experiment. There was a statistically significant
difference in plumage color between the two groups
(Table 1), and males paired in
trials did not differ in body size (t = -0.39, n = 15,
p =.48) or mite load (Z = -0.58, n = 15, p
=.56). Bright males were captured and placed into captivity sooner than were
drab males (Z = -2.91, n = 15, p =.004), but this
did not affect trial outcomes, as winners and losers did not differ
significantly in their date of entry into captivity (see Results).
With 36 males, we were able to conduct 18 dominance trials between 12 and 23 January 1999. Clear dominance (see criteria above) was established in 15 of these trials. Males of naturally contrasting plumage brightness were paired in each trial. As in our previous experiment, we paired birds with similar histories of aggressive interactions in captivity, but because we could not assign plumage color to males as we did in the manipulated-plumage experiment, we had to be more flexible and match birds that were within one dominance rank of each other.
Statistical analyses
We tested for normality of all variables using Shapiro-Wilk W
tests and for differences in variance using equality-of-variances F
tests. We used nonparametric statistics whenever the data were not normally
distributed or when variances differed significantly. All tests were two
tailed unless otherwise noted. For each cage in each experiment, we used a
Spearman's rank correlation analysis to examine the relationship between
dominance rank in the housing flocks and plumage color, body size, date of
entry into captivity (natural-plumage experiment only), and ectoparasite load.
We used Kruskal-Wallis H tests to explore the effects of plastic leg
band color on dominance in all flocks. We used binomial tests to determine if
plumage color was a reliable predictor of win/loss outcome in the dominance
trials of both experiments. Based on the tendency for drab males to be
dominant to bright males in three previous studies
(Belthoff and Gauthreaux, 1991;
Belthoff et al., 1994;
Brown and Brown, 1988), we used
a one-tailed test for the results of the trials using naturally pigmented
males. We used a two-tailed test in the manipulated-plumage experiment because
the results of Wolfenbarger
(1999) did not allow us to
devise an a priori directional prediction. We used Wilcoxon signed-rank tests
or paired t tests to explore alternative correlates of win/loss
outcome in the trials, including body size, ectoparasite load,
pre-manipulation plumage color (manipulated-plumage experiment only), date of
entry into captivity (natural-plumage experiment only) and dominance rank
(natural-plumage experiment only). We used binomial tests to determine if
capture site influenced dominance ability in these trials.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Manipulated-plumage experiment
Housing flocks
Neither premanipulated plumage color, feather mite load, nor body size were significantly related to dominance rank in any of the four cages of males (all n = 10, all p >.15). Plastic leg band color also did not have a significant effect on dominance status (Kruskal-Wallis H = 15.37, p =.11).
Paired trials
For the 17 trials in which male dominance was established clearly, neither
premanipulated plumage color, mite load, nor body size were significant
predictors of win/loss outcome (Table
2). Capture site also had no effect on dominance ability
(two-tailed binomial test, p =.33). In these trials, experimentally
brightened males won 11 trials and lightened males won 6. This difference in
wins was not statistically significant (two-tailed binomial test, p
=.33).
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We evaluated the statistical power of this test by considering the
probability of detecting results similar to those for species in which a
significant relationship between manipulated plumage color and dominance had
been previously established. Senar
(1999) included a table
listing effect size (r2) values for seven such species in
his review of the status signaling function of avian plumage variation. Tests
for five of these seven species had sample sizes that were smaller (n
= 7-15) than that used in our experiment, yet in all of these studies the
effect size was >.6 (Fugle et al.,
1984; Grasso et al.,
1996; Hogstad and Kroglund,
1993; Lemel and Wallin,
1993; Møllr,
1987a). With n = 17 in this experiment, we had the
statistical power to detect an effect size of at least.6 at 
=.01 and
effects of at least.53 at =.05
(Cohen, 1988). Thus, although
we may have failed to detect smaller effects of manipulated plumage color on
dominance, we had sufficient power to reveal effects as large as those shown
to be of biological importance in other species.
Natural-plumage experiment
Housing flocks
In none of the four cages of males used in this experiment was dominance
rank significantly related to mite load (all n = 10, all p
>.15). We found a significant negative relationship between dominance rank
and plumage color in two of the four cages (cages 1 and 3;
Figure 2); in the other two
cages the trends were in the same direction (both rs =
-.12). Dominant males were significantly larger in one of the cages (cage 4;
rs = -.68, p =.05) and were brought into
captivity significantly earlier in one of the cages (cage 3;
rs =.86, p =.02) than were subordinate males (all
other p >.15). Plastic band color did not significantly affect
dominance among these birds (Kruskal-Wallis H = 11.81, p
=.16).
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Paired trials
There were no effects of body size, mite load, dominance rank, or entry
date into captivity on win/loss outcome in the 15 trials in which we discerned
unequivocal male dominance (Table
3). Capture site also did not influence trial outcomes (two-tailed
binomial test, p = 1.0). The trend for drab males to dominate bright
males was not statistically significant, with naturally drab males winning 10
of the trials and naturally bright males winning 5 (one-tailed binomial test,
p =.15).
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For this test, we also considered the likelihood of detecting results
similar to those published for other species. In this case, we found studies
of four species in which natural male plumage color was significantly related
to winter dominance and in which large effect sizes were obtained (>.5)
using sample sizes smaller (n = 7-13) than that used in our
experiment (Hogstad and Kroglund,
1993; Møller,
1987b; Senar et al.,
1993; Wolfenbarger,
1999). Again, we had ample statistical power (with n =
15) to detect these biologically significant effects (r2
>.5) at =.05 (Cohen,
1988) and thus feel justified in concluding that, in this species,
plumage color is poorly related to a male's success in contests with
unfamiliar rivals.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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We found no conclusive evidence in this study that the carotenoid-based plumage coloration of male house finches acts as a reliable status signal of dominance and aggression in the nonbreeding season. We paired unfamiliar males that differed only in manipulated plumage color (drab versus bright) in dominance trials and detected no significant relationship between aggression and the type of plumage manipulation. Although the number of birds tested was relatively small, we had sufficient power to detect the effect sizes found in those species for which a significant relationship between color and dominance had been established previously. To date, the SSH has been supported only in species in which males display melanin-based ornamentation (e.g., dark-eyed junco, Junco hyemalis: Grasso et al., 1996
, willow tit, Parus montanus:
Hogstad and Kroglund, 1993;
Eurasian siskin, Carduelis spinus:
Senar and Camerino, 1998).
This study represents the second test of this hypothesis for carotenoid
pigmentation, and as of yet there is no indication that this type of plumage
coloration functions as a signal of social status during the winter.
Why should melanin pigmentation serve as a badge of status while
carotenoid-based ornamentation does not? It appears as though the separate
costs associated with producing and/or displaying melanin and carotenoid
ornaments generate differences in the information content of these two
pigment-based ornament types. Few costs seem to be associated with the
production of melanin-based plumage ornaments. Melanin pigments can be
synthesized endogenously from basic dietary (amino acid) precursors
(Brush, 1978;
Fox, 1976), and, although a
link between nutritional condition during molt and ornament expression has
been suggested (Veiga and Puerta,
1996), three recent studies have demonstrated that the deposition
of melanins is unaffected by environmental factors such as food stress or
parasitism (Hill and Brawner,
1998; McGraw and Hill, in
press; Senar JC, unpublished data). Furthermore, melanin-based
coloration is strongly heritable in some species
(Møller, 1989;
Norris, 1993; but see
Griffith et al., 1999) and is
highly canalized during development (Oster
and Murray, 1989). Instead of being nutritionally
condition-dependent, the honesty of melanin ornaments appears to be maintained
by social mediation, where individuals with extreme melanin-based displays are
challenged repeatedly, and only males with superior fighting ability can bear
the cost of elaborate ornamentation
(Rohwer and Rohwer, 1978;
Senar, 1999).
Conversely, the expression of carotenoid-based plumage coloration bears
high production costs and directly reflects the general condition of males
(Hill, 1996,
1999;
Hill and Montgomerie, 1994;
Olson and Owens, 1998).
Carotenoid pigments cannot be synthesized de novo by birds
(Brush and Power, 1976), so
trait exaggeration is directly related to the ability of males to obtain these
pigments in their diet and physiologically transport, process, and deposit
them in their feathers (Brush,
1990; Goodwin,
1984). During molt, pigment access (Hill,
1992,
1993a), nutritional condition
(Hill and Montgomerie, 1994;
Hill, in press), and parasite
burden (Brawner et al., in
press; McGraw and Hill, in
press; Olson,
1996) can all influence carotenoid deposition. Strong sexual
selection through female choice maintains these traits, with females
preferring to mate with the males in best condition displaying the brightest
ornaments (e.g. Hill, 1990;
Johnson et al., 1993;
Sundberg, 1995). However,
depending on the type and value of a resource and seasonal variation in the
structure of the social system, male aggressive behavior may vary
unpredictably with carotenoid-based plumage brightness and condition. Males
differing in condition and mating status may have different motivations and
may obtain different benefits for competing for access to either food or mates
throughout the year (Enquist and Leimar, 1985;
Hammerstein, 1981). Thus, we
might expect selective forces driving the evolution of male-male signaling
systems to be comparatively weak and less directional in species with
carotenoid-based plumage ornaments.
In fact, the relationship between male competitive ability and plumage
brightness is not only weak in this species, but if there is a trend at all it
appears to be paradoxical. Before our experiments, three separate studies
suggested a possible negative relationship between natural plumage brightness
and winter dominance, with drab birds tending to dominate bright birds
(Belthoff and Gauthreaux, 1991;
Belthoff et al., 1994;
Brown and Brown, 1988). Our
study represents a fourth case in which there is a nonsignificant tendency in
this same direction. We found statistically significant negative relationships
between plumage redness and aggression in two of four cages of males
displaying natural variation in carotenoid pigmentation, and in trials in
which we paired males of contrasting plumage brightness we detected only a
marginally significant trend for drab males to be dominant. The accumulation
of studies testing the effect of natural plumage coloration on winter
dominance among male house finches allows us to meta-analyze the significance
of the patterns found to date. Interestingly enough, all 12 of the statistical
comparisons in these 4 studies have detected negative relationships between
plumage brightness and dominance (sign-test, p <.001). If we
combine the actual probabilities of these significance tests
(Sokal and Rohlf, 1995), the
result is also statistically significant (
2 = 48.26, df = 24,
p <.01).
So why is the drab plumage of male house finches only weakly associated
with social dominance in the nonbreeding season? Although we have no
conclusive evidence in support of any particular hypothesis at this time, we
offer a few potential explanations. One possibility is that this aggressive
male behavior is associated not with a form of intermale communication, but
instead represents an aspect of the male-female signaling system in this
species. Although redder males enjoy considerable reproductive advantages over
less red males (Hill, 1991;
Hill et al., 1994,
1999;
McGraw et al., in press), drab
males are actually dominant to bright males in contests over access to food
during the breeding season (McGraw and
Hill, 2000). Whereas sexually preferred and healthy bright males
may not need to invest heavily in behaviors associated with the acquisition of
either food or mates, drab males may allocate more effort to improving their
poor condition and competing more for reliable access to either concentrated
food sources or females. Such a negatively correlated handicap, where signal
intensity is inversely proportional to signaler quality
(Lotem, 1998), may occur in
the nonbreeding season if ornamental display facilitates early mate
acquisition; in fact, house finches begin forming pairs in January
(Hill, 1993b).
Brown and Brown (1988)
offered an alternative explanation for why drab males tend to be dominant
during the nonbreeding season: they suggested that bright males may mistake
drab males for females and avoid them because females are dominant to males in
this species. However, it seems as though sexual differences in plumage
pattern should allow closely interacting birds to discriminate between the
sexes. Clearly this idea needs further testing. If this plumage signal does
not benefit males in any way during the nonbreeding season, then males may
display plumage variation throughout the year simply because there is
insufficient time to complete two annual molts and grow separate breeding and
nonbreeding plumages in a year. Resident species like the house finch often do
not complete their prebasic molt until mid-October and then begin associating
with mates as early as January, which leaves them only 10 weeks to fully
replace their feathers (Hill,
1993b). On this note, it is interesting that male house finches
obscure their colorful winter plumage with buffy feather edges that wear off
in about 2 months, which suggests that protecting and/or hiding ornamental
plumage in the nonbreeding season may be beneficial.
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ACKNOWLEDGEMENTS |
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We thank J. Gurganis and S. Otis for help in maintaining healthy birds and clean aviary facilities, and J. Dale, A. Dervan, G. Hepp, P. Nolan, D. Robinson, A. Stoehr, R. Ydenberg, and two anonymous reviewers for making comments that helped improve the quality of the manuscript. We also thank Roche Vitamins for supplying the carotenoid pigments. Experimental work with the animals in this study was approved by the Institutional Animal Care and Use Committee at Auburn University (PRN 0004-R-2011). This work was supported by the National Science Foundation (grant IBN9722171 to G.E.H.), and by the College of Science and Mathematics, the Alabama Agricultural Experiment Station, and the Department of Biological Sciences at Auburn University.
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