Behavioral Ecology Vol. 10 No. 1: 48-53
© 1999 International Society for Behavioral Ecology
Pairing success relative to male plumage redness and pigment symmetry in the house finch: temporal and geographic constancy
Department of Zoology and Wildlife Science and Alabama Agricultural Experiment Station, 331 Funchess Hall, Auburn University, Auburn, AL 36849-5414, USA
Address correspondence to G. Hill. E-mail: ghill{at}acesag.auburn.edu.
Received 6 April 1998; accepted 11 June 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Male house finches (Carpodacus mexicanus) have carotenoid-based ornamental plumage coloration. In previous research it was shown that for a single population of house finches in a single year, males that paired were on average redder in plumage coloration than males that did not pair, and males with redder plumage tended to nest earlier than males with less red plumage. Here we show that these patterns continued over 6 years and at two widely separated locations. We also tested whether the symmetry of carotenoid-based crown pigmentation differed between paired and unpaired males and found that paired males have, on average, more perfect symmetry of crown pigmentation than males not paired. These observations support the idea that expression of carotenoid-based plumage coloration by males is a persistent and widespread criterion in female mate choice in the house finch.
Key words: carotenoids, Carpodacus mexicanus, fluctuating asymmetry, mate choice, sexual selection.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Over the past few decades, field studies of wild birds have greatly increased our understanding of the evolution and maintenance of ornamental traits (reviewed in Andersson, 1994
;
Møller, 1994). By necessity, such
studies are generally conducted on a single population of birds, at a single
location, over 1 or a few years. What is observed in the study population is
taken as representative of the entire population of animals or even
representative of an entire class of ornaments across a diverse array of
species. However, every field biologist is aware of annual and geographic
variation in the morphological and behavioral traits of animals, and one must
wonder about the universality of some conclusions about the function of
ornamental traits drawn from localized, shortterm studies.
In only a few species of birds has the evolution of an ornamental trait
been studied for more than 5 years and at widely separated locations. In the
barn swallow (Hirundo rustica), female mate choice for males with
long and/or symmetrical tails has been demonstrated over 15 years
(Møller, 1993) and at study sites
across Europe (Denmark: Møller,
1993; Spain: Møller et
al., 1995; Italy: Saino and
Møller, 1994) and North America
(Smith and Montgomerie, 1991). The
function of melanin-based plumage coloration in the pied flycatcher
(Ficedula hypoleuca) and in the closely related collared flycatcher
(Ficedula albicollis) has also been studied across Europe for more
than a decade (e.g., Lundberg and Alatalo,
1992; Part and Quvarnstrom,
1997; Potti and Montalvo,
1991; Saetre and Slagsvold,
1996). In the case of these Ficedula flycatchers,
however, it has been shown that there is substantial geographic variation in
expression of both plumage blackness and the size of the white forehead patch
(Lundberg and Alatalo, 1992;
Saetre et al., 1997). In some studies
females show a mating preference relative to these plumage traits, whereas in
other studies they do not (e.g., Alatalo et al.,
1986; Lundberg and Alatalo,
1992; Saetre and Slagsvold,
1996). These studies of Ficedula flycatchers underscore
the importance of testing the function of ornamental traits over as many
seasons and at as many geographic locations as possible.
One of the most widespread ornamental traits among birds is
carotenoid-based red, orange, and yellow plumage coloration. Carotenoid-based
coloration is particularly interesting to students of sexual selection because
it is a condition-dependent trait (Gray,
1996; Hill, 1996).
Carotenoid pigments cannot be synthesized de novo by birds or by any
vertebrates; they must be ingested (Goodwin,
1984). Because carotenoid pigments must be derived from food,
expression of carotenoid pigmentation is dependent on access to sufficient
amounts of foods that provide the right type of carotenoid pigments
(Brush and Power, 1976;
Fox and McBeth, 1970;
Fox et al., 1970;
Hill, 1992,
1996). Moreover, after carotenoids are
ingested, various parasites can inhibit uptake, transport, or deposition of
carotenoid pigments and therefore can alter expression of plumage coloration
independent of diet (Brawner, 1997;
Hill and Brawner, 1998;
Thompson et al., 1997). The physiological
condition of a bird at the time of molt, independent of parasitic infection,
has also been invoked as a factor in the efficiency with which ingested
carotenoids might be used to pigment feathers
(Bortolotti et al., 1996;
Negro et al., 1998). Models of sexual
selection predict that condition-dependent ornamental traits such as
carotenoid-based plumage coloration should be of particular interest to
females when they choose a mate (see Andersson,
1994; Møller,
1994).
Not only is carotenoid-based plumage coloration condition dependent, but
apparently so is the bilateral symmetry of carotenoid pigmentation. Oranger
male house finches (Carpodacus mexicanus) have less symmetrical
carotenoid-based crown pigmentation than do redder males
(Hill, 1998). The degree of this sort of
fluctuating asymmetry has been proposed as a measure of developmental
stability and hence as an indicator of individual quality
(Møller, 1990;
Thornhill, 1992). In the case of feathers
that are replaced once per year, such as the crown feathers of house finches,
pigment symmetry potentially reflects individual condition at the time of
molt. In a feeding experiment with European starlings (Sturnus
vulgaris), it was shown that nutritional stress during feather growth
affects the symmetry of non-carotenoid pigmentation
(Swaddle and Witter, 1994). No equivalent
experiments have been conducted on carotenoid-based coloration.
In both laboratory and field experiments, female house finches have been
shown to prefer mates with redder and more intensely pigmented plumage
(Hill, 1990,
1991,
1994). One result of female preference
for redder males is that the mean redness of paired male house finches is
greater than the mean redness of unpaired males
(Hill, 1990). In addition, among male
house finches that pair, redder males tend to initiate nesting earlier (i.e.,
their mates lay eggs earlier) than less red males
(Hill et al., 1994). The effects of
symmetry of carotenoid-based plumage pigmentation on female mate choice have
not been tested in the house finch or in any bird species.
The above results come from studies made in a single population of house finches and over one or two seasons. We examined the effect of male plumage coloration on pairing success and date of nest initiation over six breeding seasons and for two breeding populations of house finches separated by 1200 km. We also looked at the relationship of pigment symmetry to pairing success and nest initiation date.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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This study was conducted on the main campus of the University of Michigan in southeastern Michigan, USA over four breeding seasons from 1988 to 1991 and on the campus of Auburn University in east-central Alabama, USA for the breeding seasons of 1996 and 1997. At both locations, we trapped house finches at feeding stations and marked them with a unique combination of one aluminum and three colored plastic bands. Red bands were not used on male finches at any location in any year.
The ornamental plumage coloration of males was recorded in two ways. During
the 4 years of study in Michigan, the coloration of each of seven plumage
regions (four areas on the underside, plus the crown, eyestripe, and rump) was
recorded with a separate hue, intensity, and tone score based on comparison
with color chips in the Methuen Handbook of Colour
(Kornerup and Wanscher, 1983). We then
summed the 21 scores that were generated to derive a single index value, which
was used as an estimate of overall plumage brightness (see
Hill, 1992, for details). For the Alabama
portion of this study in 1996 and 1997, we scored male plumage coloration
using a Colortron (Light Source, San Rafael, California), a
Macintosh-compatible reflectance spectrophotometer (see
Hill, 1998, for details). The Colortron
measures the light reflection from a 3 mm x 3 mm area, and three
Colortron measurements were taken at each of three body regions—crown,
breast, and rump. For each body region, we used the Colortron software to
calculate the mean hue, saturation (intensity), and brightness (tone,
blackness) for the three measurements taken, and, for this study, we focused
on plumage hue. Color scores attained through visual assessment and scores
attained from the Colortron were highly correlated
(Hill, 1998). It should be noted that for
scores generated by comparison to the Methuen Handbook of Colour,
higher scores correspond to redder plumage, whereas for scores generated by
the Colortron, lower scores correspond to redder plumage.
In 1996 and 1997 we scored the right versus left symmetry of
carotenoid-based pigmentation of crown feathers on a 1-5 scale: (5)
perfect symmetry; (4) 1-5% of feathers different; (3)
5-50% of feathers different; (2) 50-95% of feathers
different; (1) 95-100% of feathers different. Feathers were
counted as asymmetrical when the equivalent feather on the opposite side of
the crown was obviously a different hue (see Hill,
1998, Hill, 1998, for
details). Symmetry scores were estimated for most but not all males banded in
1996 and 1997, so sample sizes for comparisons of symmetry are consistently
smaller than sample sizes for comparisons of plumage coloration. For 142
males, crown symmetry score was estimated by two or more observers, and we
calculated repeatability (r) using the interclass correlation
coefficient (Lessells and Boag, 1987).
Crown symmetry scores were repeatable between observers (r =.48,
F142, 241 = 3.49, p <.0001).
In Michigan, we began banding each year in January and continued through the breeding season. About 90% of the breeding population was banded in each year of the Michigan study. In Alabama, banding was conducted throughout the year, and about 95% of the breeding population of house finches was banded each year. Male song and the first pair associations become evident in early January in Alabama and early February in Michigan. At these times, daily ad libitum sampling of the populations began.
House finches form strong pair associations (Hill,
1993a; Thompson, 1960),
so once a pair is formed, the paired individuals are seldom seen alone
(Hill, 1991,
1993a). Therefore, before females begin
incubating, it is relatively easy to determine the pairing status of
individuals as well as the identity of mates. In addition, females are
conspicuous in their efforts to build nests (Hill,
1993a; Thompson, 1960),
so it was possible to record virtually all nesting activity on the study
sites. Throughout the breeding season at both sites, each time a male was
observed it was described as "paired" if it was associating with a
female and "unpaired" if it was not associating with a female.
Observation of a male simply in proximity to a female was not sufficient to
rank that male as paired. The female with which the male was associating had
to show some indication that she was encouraging the male's presence. Such
encouragement was generally in the form of the female following the male, but
also included the male and female together driving off intruding house finches
and any courtship activities by the male to which the female was receptive
(see Hill, 1993b, for additional details
of field techniques). If a male was observed in such an association with a
female at any time during the breeding season, it was counted as paired for
that season.
We checked nests once every 3-4 days in Michigan and every other day in
Alabama. Nest activity was monitored in Alabama only in 1997; in 1996
only the pair status of males was recorded. House finches lay one egg per day,
early in the morning (Hill, 1993a), so
when a nest had more than one egg when first checked, first-egg date was
determined by backdating. There are two main ways that first nesting date
could be incorrectly assigned to birds: if birds nested and failed
before they were detected or if birds moved onto the study area after a first
nesting attempt outside the study area. In both cases, the incorrect
first-nest dates that would result would tend to obscure patterns of redness
and nesting date. Therefore, we believe that our test of male coloration and
nest initiation is conservative.
We used a one-tailed Mann-Whitney U test to make comparisons of
pairing status to plumage redness and pigment symmetry. We had clear a priori
predictions about the expected direction of differences between males that
paired and males that did not pair based on sexual selection theory and
previous findings from a single year of observation
(Hill, 1990). A Bonferroni correction was
made for simultaneously testing two hypotheses.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Compared to unpaired males, paired males had redder plumage in both Michigan and Alabama and in all 6 years of observation. The probability that by chance the mean redness of paired males would be greater than that of unpaired males in 6 of 6 years of observation is 0.016 (sign test). After Bonferroni correction for testing two hypotheses simultaneously (color and symmetry related to redness), the mean difference in plumage coloration between paired males and unpaired males was significant in 5 of 6 years (Table 1). In 2 years of observation in Alabama, paired males also had significantly less pigment asymmetry than unpaired males (Table 1).
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Because pigment symmetry is highly correlated with plumage redness (1996: rs = -.36, n = 93, p =.0004; 1997: rs = -.39, n = 263, p =.0001), we also compared the effect of plumage redness and pigment symmetry independently. To derive the degree of asymmetry independent of redness, we retained the residual values from the regression of pigment symmetry (dependent variable) on plumage redness (independent variable). Similarly, to derive the degree of plumage redness independent of pigment symmetry, we retained the residual values from the regression of redness (dependent variable) on pigment symmetry (independent variable). Using these residual values as measures of symmetry independent of plumage hue, and plumage hue independent of symmetry, we compared the residual values of paired males versus unpaired males paired. Paired males had lower residual plumage values (redder plumage) than unpaired males, but after Bonferroni correction, the difference was not significant in either year (Table 2). After Bonferroni correction, paired males had significantly greater residual symmetry values (more perfect symmetry) than unpaired males in 1 of 2 years (Table 2).
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In 3 of 4 years in Michigan, there was a significant negative relationship between the plumage brightness of a male and the first date in which an egg appeared in a nest he was attending (1988: rs = -.48, n = 41, p =.003; 1989: rs = -.28, n = 57, p =.03; 1990: rs = -.33, n = 60, p =.01; Figure 1). In the fourth year, the trend was for brighter males to nest earlier, but the result was not significant (1991: rs = -.16, n = 47, p =.27; Figure 1). In the 1 year of data for Alabama, there was no significant relationship between male plumage coloration and nest initiation date when all males were included (rs =.11, n = 26, p =.57; Figure 2). However, although first-nest initiation dates spanned a range of about 60 days in Michigan, some Alabama males were first recorded nesting up to 95 days after the first nest in the population was initiated. We believe that all males in the population had initiated their first nest within 60 days of the first nest initiation and that males first sighted nesting after about 60 days are renesting birds moving into the study area from outlying areas. Thus we repeated the analysis excluding nests that were initiated more than 60 days after the first nest in the population. With this correction, there was again a significant relationship between male plumage brightness and the first date on which an egg appeared in the nest of the attending male (rs =.46, n = 20, p =.04; Figure 2). (Note that the correlation coefficient is now positive because male coloration was measured with a Colortron, for which redder hues are given lower scores.) When the 60-day cutoff was applied, males with more symmetrical plumage tended to nest earlier than males with less symmetrical plumage, but the relationship was not significant (rs = -.36, n = 22, p =.09).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Across 6 years of study at two widely separated sites, male house finches that paired with a female had redder plumage coloration, on average, than unpaired males. This difference was significant in 5 of 6 years. Among males that paired, there was also a significant relationship between plumage redness and first egg date in 4 of 5 years, and a nonsignificant trend in the same direction in the year 5. These data suggest that over a wide geographic area and across at least a decade, female house finches remained consistent in their choice of brightly colored mates. These observations, combined with laboratory experiments on female mate choice in the house finch (Hill, 1990
,
1994), make a convincing case that female
choice based on plumage redness is a general phenomenon in house finches.
The observation that females remained constant in their choice of brightly
colored mates across all years of study is particularly interesting when one
considers that between 1996 and 1997 in Alabama, an epidemic of mycoplasmosis
caused by Mycoplasma gallicepticum swept through the population
(Nolan et al., 1998). Between 1996 and
1997 approximately 60% of the house finches in our study population in
Alabama developed gross symptoms of the disease, and many of these birds died
as a result (Nolan et al., 1998). The sex
ratio was shifted from male biased to female biased, and the mean plumage
redness of males increased (Nolan et al.,
1998). With a shift in sex ratio from male biased to female
biased, the intensity of sexual selection undoubtedly was lower in 1997 than
in previous years (see Hill, 1994, for a
discussion of the role of sex ratio in sexual selection in the house finch)
and because the males remaining in the population were shifted away from the
previous mean toward redder plumage (presumably because drab males died from
the disease; Nolan et al., 1998),
there was less variation in male plumage coloration on which females could
base their mate choice. Nevertheless, males that paired were redder than
unpaired males.
The tendency for redder males to initiate nesting earlier might be
interpreted in at least two ways. Darwin
(1871) and later Møller
(1988) suggested that one way for
variance in reproductive success to exist in monogamous birds is if there is
variation in the quality of females such that more attractive males pair with
higher quality females. Higher quality females would nest earlier in the
season. Therefore, a relationship is predicted between the speed with which
males pair and nest and the quality of their ornamentation
(Darwin, 1871;
Møller, 1988). It is possible,
however, that bright males might actually instigate earlier nesting by
providing more food to females during courtship. Either way, the observation
that across years bright males tend to begin nesting earlier than drab males
supports the idea that plumage redness is tied to reproductive success in male
house finches because early nesting leads to increased reproductive success in
the house finch (McGraw et al., unpublished data).
This is the first study to look at the effect of symmetry of
carotenoid-based plumage pigmentation on male pairing success. Previous
studies of female mate choice relative to ornament symmetry have focused
primarily on elongated tails (Balmford et al.,
1993). The only studies on pigment symmetry have focused on
melanin-based coloration, and both found a significant female preference for
more symmetrically pigmented males (Swaddle and
Cuthill, 1994; Swaddle and Witter,
1994). In house finches it was previously observed that pigment
symmetry is significantly correlated with plumage redness
(Hill, 1998), so in this study it was
important to tease apart the independent contribution of symmetry from the
independent contribution of redness to the relationship between plumage
characteristics and pairing success. We found that independent of plumage
redness, there was, on average, greater pigment symmetry of crown plumage in
paired males than in males not observed with a female. This suggests that
females assess male pigment symmetry as well as plumage coloration when
choosing mates. Indeed, from these observations alone, it would appear that
females show a stronger preference for pigment symmetry than for redness
because the effect of pigment symmetry on pairing success of males remained
significant in one year when the effects of redness were removed, but the
effect of redness on pairing success was no longer significant when the
effects of symmetry were removed. However, female mate choice for
symmetrically pigmented males in the house finch needs to be corroborated with
an experiment in which pigment symmetry is uncoupled from all potential
correlated effects.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Research in Michigan was supported by Chapman Grants from the American Museum of Natural History, a grant from the Animal Behavior Society, a grant-in-aid of research from Sigma Xi, the Scientific Research Society, Wetmore and Van Tyne Grants from the American Ornithologists' Union, a Hinsdale-Walker Grant from the Museum of Zoology, University of Michigan, and by the Department of Biology and the Rackham Graduate School at the University of Michigan, all to G.E.H. Research in Alabama was supported by grants to G.E.H. by the Department of Zoology and Wildlife Sciences and the College of Science and Mathematics at Auburn University, by the Alabama Experimental Research Station, and by the National Science Foundation (IBN-9722171).
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