Behavioral Ecology Vol. 11 No. 2: 202-209
© 2000 International Society for Behavioral Ecology
Structurally based plumage coloration is an honest signal of quality in male blue grosbeaks
a Department of Genetics, Life Sciences Building, University of Georgia, Athens, GA 30602, USA b Department of Zoology and Wildlife Science, 331 Funchess Hall, Auburn University, AL 36849, USA
Address correspondence to A. J. Keyser. E-mail: keyser{at}arches.uga.edu .
Received 10 June 1999; accepted 16 August 1999.
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ABSTRACT |
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We investigated the signaling function of blue plumage in male blue grosbeaks (Guiraca caerulea) to determine if structurally based coloration may act as a reliable signal of quality to conspecifics. Blue plumage results from the microstructure of feather barbules rather than from pigment granules, and thus it is possible that structurally based plumage ornaments may function differently from sexually selected ornamental coloration that is pigment based. The plumage of male blue grosbeaks reflects maximally in the blue-ultraviolet range, so most variation in plumage coloration among males is invisible to human observers. In previous research, we showed that increased area of blue plumage on the body is associated with a shift in the wavelength of maximum feather reflectance toward the ultraviolet and with high intensity of light reflected at that maximum, and that extreme expression of the male ornament is condition dependent. These observations suggest that blue plumage may be an honest advertisement of male quality. We tested this hypothesis in a wild population of blue grosbeaks. We quantified male quality in three broad categories. (1) Physical condition was assessed from subcutaneous fat deposits, ectoparasite load, and body size. (2) Territory quality was assessed from territory area, prey abundance, and predation risk. (3) Paternal investment was assessed from male feeding rate. We found that the bluest males have the largest body size, maintain the largest territories with the greatest prey abundance, and feed nestlings in the first nest of the season at the highest rates. We conclude that structurally based plumage coloration functions as an honest, intraspecific signal of quality.
Key words: honest advertisement, mate choice, reliable signaling, sexual selection, structural color, ultraviolet reflectance.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Conspicuous secondary sexual characteristics are a prominent feature of males in many species of animals. Enormous body size in elephant seals (Mirounga sp.), elaborate tail fans in male peacocks (Pavo cristatus), and brilliant colors in guppies (Poecilia reticulata) are familiar examples. The honest advertisement model of sexual selection proposes that variation in these secondary sexual characteristics is maintained through intraspecific interactions. First, the model proposes that there is competition for access to mates either through intrasexual interactions or intersexual mate choice (Darwin, 1871
). Second, these
interactions are mitigated by ornamental traits such that individuals bearing
the most extreme expression of the traits have increased mating success
(Darwin, 1871;
Kirkpatrick, 1982). Third,
expression of the ornament depends on the condition of the bearer and covaries
with other measures of individual quality
(Andersson, 1986;
Hamilton and Zuk, 1982;
Kodric-Brown and Brown, 1984;
Zahavi, 1975). Fourth, this
covariance allows conspecifics to assess mate quality accurately during mating
interactions (Hill, 1996;
Kodric-Brown and Brown,
1984).
To make a strong inference about how an ornament functions in a given
species, it is important to document all elements of the reliable signaling
model. One needs to known how the ornament varies, what maintains this
variation, what is the information content of the signal, and whether it is
used in intraspecific interactions. In several well-studied systems (see
general review by Andersson,
1994), all parts of the model have been carefully addressed. For
example, in house finches (Carpodacus mexicanus), male plumage varies
from yellow to red, and this variation depends on nutritional condition and
endoparasite load (Brawner,
1997; Brush and Power,
1976; Hill, 1992;
Hill and Montgomerie, 1994).
Females preferentially mate with the reddest males (Hill,
1990,
1991,
1994;
Hill et al., 1999), which have
the highest overwinter survival rate
(Hill, 1991) and feed
incubating females more than duller males
(Hill, 1991). In another
example, female ring-necked pheasants (Phasianus colchicus) choose
mates based on male spur length, and mating with long-spurred males increases
female reproductive success (von Schantz
et al., 1989). Spur length is associated with major
histocompatibility complex genotype, which suggests an association between
male immune response and display of ornamental spurs
(von Schantz et al., 1996).
Spur length is also positively correlated with male viability and offspring
survival (von Schantz et al.,
1994). Similar examples of female choice for reliable signals of
quality can be found for guppies (P. reticulata; e.g.,
Houde, 1997;
Houde and Torio, 1992;
Kodric-Brown, 1985) and barn
swallows (Hirundo rustica;
Møller, 1994).
In the examples presented above, the ornamental trait under consideration
is either a physical structure such as a long tail or a pigment-based color
patch. Another class of ornamental traits consists of patches of color that
are derived from the microscopic structure of the colored region rather than
endogenous pigments. Such traits, referred to as structurally based colors,
are found in a wide variety of taxonomic groups including arthropods, fish,
and birds (Auber, 1957;
Fox, 1976). Structural
ornaments have been little studied, particularly in birds, perhaps because
human observers perceive little variation in these ornaments
(Borgia and Collis, 1990;
Hunt et al., 1998). However,
growing interest in the function of ultraviolet vision in birds has led
researchers to look for "cryptic" ultraviolet signals in plumage
(Andersson, 1996;
Bennett and Cuthill, 1994;
Burkhardt, 1989). Several
species show variation in short-wavelength plumage reflectance, and females of
some of these species prefer males with the most extreme ultraviolet
coloration (Andersson and Amundsen,
1997; Hunt et al.,
1998).
Our recent work with a wild population of blue grosbeaks (Guiraca
caerulea) documented extensive variation among males in expression of a
structural, blue-ultraviolet ornamental trait
(Keyser and Hill, 1999). In
addition, we found a positive correlation between phenotypic expression of
blue plumage and nutritional condition during molt
(Keyser and Hill, 1999). Thus,
a mechanism is in place whereby the signal is kept "honest" and
suggests that ornamental plumage may function as an honest advertisement in
this species. If male plumage acts as a signal to conspecifics, it is
important to identify what information the ornament conveys.
In this study, we measured male quality in the field and calculated the
correlation between several quality measures and male plumage characteristics.
We quantified body size, current physical condition, territory size, prey
availability, nest predation risk per territory, and male parental investment.
We predicted that male plumage ornamentation would be positively correlated
with quality measures. All of these have the potential to influence, directly
or indirectly, female fitness and thus make female choice of the bluest males
adaptive (Searcy, 1979).
Additionally, significant correlations of body size, current physical
condition, or territory size with ornamentation could suggest a potential role
for plumage ornamentation in male-male competition. If fine-scale variation in
a structural ornament correlates with male quality measures, it would provide
further support for the universal applicability of honest signaling theory to
ornamental traits in animals.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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We studied a color-banded population of blue grosbeaks in Lee County, Alabama, USA, during the breeding seasons (April-August) of 1997 and 1998. The study site consisted of a series of agricultural fields (either planted in cotton, wheat, or used as pasture) surrounded by secondary growth forest and patches of recently clear-cut forest. Our efforts centered on two large (111 ha and 73 ha) and two small (11 ha and 30 ha) fields in 1997. In 1998, the two small fields were dropped from the study due to low density of blue grosbeaks, and a third large field (94 ha) was added. All of these fields were within a few kilometers of one another. However, blue grosbeaks are exclusively early-successional species, and territories established by males included clear-cuts, pastures, planted fields, and especially shrubby hedgerows and field edges. Birds were never heard or observed more than about 75 m into the forest around the fields. Once birds settled onto breeding territories, we did not observe them moving from one field to another.
Birds were captured in mist nets and Potter live-traps baited with sunflower seeds early in the season. Each individual was marked with a unique combination of colored leg bands and an aluminum U.S. Fish and Wildlife service band. In 1997, we captured 31 males, and of these, 20 remained in the study area and established breeding territories. In 1998, we captured 34 males, and 24 established breeding territories. Out of these 24, 7 males had also held territories during 1997.
At the time of capture, we collected feather samples for spectrophotometric
analysis and took detailed plumage information from each male (n = 65
for both years combined). We measured four plumage variables on the breast and
rump region of each individual: percent blue, peak wavelength, intensity, and
contrast (for detailed methods, see Keyser
and Hill, 1999). Percent blue refers to the percentage of blue
plumage on each body region and was estimated visually. This measure is highly
repeatable among observers (r =.96). The other three variables were
obtained from reflectance spectra taken from the two body regions (see
Figure 1 for representative
spectra and plumage variable explanations). Spectral data provide an objective
way to quantify color (Cuthill et al.,
1999; Endler,
1990). Peak wavelength is the wavelength at which the plumage is
maximally reflective and is an estimate of the hue or color principally
reflected by the feathers. Intensity is the amount of light reflected at the
peak wavelength and is a measure of the brightness of the color. Contrast is
measured as the difference in intensity between the wavelength of maximum
reflectance and the wavelength of minimum reflectance. A thorough
investigation of plumage variation in male blue grosbeaks is presented
elsewhere (Keyser and Hill,
1999); however, the relationship between various elements of
plumage variation is summarized in Figure
2. In general, the bluest male grosbeaks have (1) a high
proportion of blue feathers in the body plumage, (2) short peak wavelengths
(sometimes into the ultraviolet range), (3) high intensity, and (4) high
contrast.
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and measured in nm),
intensity (reflectance at peak as a percentage of a 100% reflective white
standard), and contrast (difference between maximum and minimum
intensity).
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To simplify our analyses, three plumage variables were summarized in a
blueness score for each male (for rationale, see
Keyser and Hill, 1999).
Percent blue and intensity were ranked such that the male with the highest
value received the highest rank. Peak wavelength was ranked in the opposite
manner, the lowest peak wavelength yielding the highest rank. We excluded the
contrast value from the blueness score because it is directly dependent on
intensity. We calculated average rank for each male. Males captured in 1997
(n = 31) were ranked separately from those captured in 1998
(n = 34). Use of this blueness score dramatically reduced the number
of statistical comparisons we made and equally weighted the information
content of the individual plumage variables. If, and only if, a significant
association was found between blueness and a quality variable, then we
reanalyzed the data using the four plumage variables separately to determine
if any single component of blue plumage was contributing disproportionately to
the statistical pattern.
Previous work showed that the expression of plumage blueness is partially
condition dependent (Keyser and Hill,
1999), and our intent was to investigate the potential information
content of ornamental plumage in this species. To that end, we measured
several male attributes (quality measures) that could be important to females
selecting mates or to other males assessing the competition. These were body
size, current physical condition, territory size, territory quality, and male
parental care. Detailed methods are described below.
Data analysis
Many of the variables we measured were non-normally distributed, so
nonparametric statistical tests were used in all cases unless otherwise
stated. In the first set of analyses, we calculated the Spearman rank
correlation between blueness score and the quality measure. This was done for
the breast and rump blueness scores separately. If a quality measure was
significantly correlated with blueness, we calculated a second set of
correlations between that variable and the four individual plumage variables
(percent blue, peak wavelength, intensity, and contrast).
Body size and current physical condition
At the time of capture, we made standard morphological measurements of
tarsus length, wing chord length, and mass. Although individuals were only
measured once per year, they were all measured within the same window of time.
In 1997, all but five males were captured between April 22 and May 24, and in
1998, all males were captured between April 30 and May 29.
We scored several measures of physical condition in the field. Subcutaneous
body fat was scored on an eight-point scale
(Ralph et al., 1993). Feather
lice abundance on primary feathers of the right wing was scored on a
five-point scale: 0-no lice, 1-occasional lice, 2-many lice along rachis of
some feathers, 3-nearly every feather with hundreds of lice, 4-all feathers
with hundreds to thousands of lice. Intensity of avian pox infection was
scored on a four-point scale: 0-no lesions, 1-one lesion, 2-two lesions,
3-three or more lesions.
Territory size and quality
To map territories, field technicians searched daily (approximately
0500-1100 h from April through August) for each banded male. Usually, every
bird was located every day. When a bird was sighted, we marked its location
and movements on a map of the study area. We also noted consistently used song
perches and delineated the boundary areas where male-male conflicts occurred.
We used a minimum polygon method to connect areas of major activity for each
male territory (Mohr, 1947).
Males occupied a discrete area throughout the breeding season with little or
no shifting of boundaries. Territory area was calculated from polygons plotted
on maps traced from aerial photos.
During 1998 we assessed the availability of suitable prey by sweep-net sampling 100-m transects weekly from mid-May to mid-June. Each male's territory contained from one to three transects. These transects were located within grassy strips along the edges of agricultural fields and pastures. Each transect was sampled by pacing out 100 m and swinging the net 100 times such that it came into contact with the vegetation. Focal animal observations conducted during 1997 confirmed that adult grosbeaks preferentially forage along field edges when feeding nestlings, and they do not appear to leave the territory to forage. Males spent 55% of foraging time in field edges and pasture, and females spent 60% of time in these habitats (foraging time based on ~15 h of direct observations). Since orthopterans were the primary prey items fed to nestlings (as identified from video cameras at nests; Keyser and Hill, unpublished data), orthopterans were separated from sweep samples, counted, and weighed.
Nest predation can cause wide variation in reproductive success, thereby
acting as a strong selective pressure
(Hill, 1988;
Martin, 1995;
Ricklefs, 1969). If predation
risk varies deterministically from territory to territory, it is possible that
plumage ornamentation could signal the ability of a male to hold a territory
with relatively low predation risk. Alternatively, bright males may attract
the attention of predators and suffer increased predation risk. To test these
hypotheses, we fit a logistic regression model to predict the probability of
nest predation from male plumage blueness:
 | (1) |
Paternal investment
Video cameras set up at nests were used to assess whether bright males
provided superior parental care. These miniature cameras (Fuhrman Diversified,
Inc., Seabrook, Texas) can be placed close to active songbird nests using a
camouflaged, articulated arm attached to nearby branches. They are capable of
recording for 24 h on a single T120 VHS videocassette, and an infrared
illuminator within the camera housing allows taping through the night (for
description of the camera system, see
Thompson et al., 1999). We
placed cameras at active nests after nestlings had hatched; battery and
videocassette were changed daily. The camera was removed when nestlings were
9-12 days old. These data were collected during 1998 only. From the video
tapes we calculated male feeding rate per hour (from 0600 to 2000 h).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Body size and current condition
When analyzing the association between blueness and body size, data were collected from all males captured in 1997 and 1998. We included the seven males that were captured in both years only once in the analysis, and we used 1997 measurements for these males. Thus, our sample size for this test was 58. The same data set was used to investigate the association between male blueness and fat score; however, three males were released before fat was measured, and thus sample size was 55 for this comparison.
Male body size as measured by wing chord length was significantly correlated with blueness score on both the breast and the rump region (Table 1 and Figure 3). To assess which components of the blueness score were driving this pattern, wing chord length was compared to each plumage variable separately (Table 2). On the breast, percent blue was positively correlated with wing chord length, and peak wavelength was negatively correlated with wing chord length as expected. On the rump, all plumage variables were significantly positively correlated with wing chord length with the exception of peak wavelength, which was significantly negatively correlated. Fat score, a measure of current physical condition, was not significantly associated with blueness. Feather lice load and avian pox measures proved to be nearly invariant in the population, and no analyses were performed.
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Territory size and quality
During 1997 and 1998, 20 and 24 males, respectively, held breeding
territories. Territories ranged in size from 0.80 ha to 8.73 ha. Seven of the
44 males bred in both years, and only 1997 data for these males were included
in the analysis. An additional male was excluded from our analysis of the
correlation between blueness and territory size due to lack of plumage data
(thus, n = 36). Territory size was strongly positively correlated
with male blueness on both body regions
(Table 1 and
Figure 4). Separate analyses of
the components of the blueness score showed that territory size was
significantly correlated with percent blue on the breast but not on the rump
region. Intensity was positively correlated with territory size, and peak
wavelength was negatively correlated with territory size on both body regions
(Table 2).
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Prey abundance was successfully sampled within the territories of 17 males during 1998 (these data were not collected in 1997). A total of 198 samples were taken over an 8-week period, and each transect was sampled an average of 7.6 times. We calculated the average wet weight of orthopterans per 100-m transect within each male's territory over the course of the sampling period. This gave a single prey abundance value for each territory (i.e., available biomass measured as mean wet weight of orthopterans). Prey availability ranged from 0.16 g to 1.25 g per 100-m transect. Mean prey abundance per 100-m transect was compared to male plumage ornamentation (n = 17). Orthopteran weight was significantly correlated with blueness on the rump but not with blueness on the breast (Table 1 and Figure 5). Further analyses revealed that percentage of blue feathers on the rump region is the only component of the blueness score associated with prey availability (Table 2).
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Using logistic regression, we tested the hypothesis that male plumage blueness could signal the probability of nest predation in the territory (either increased or decreased risk). Thirty-six of the males were included in this analysis (same data set as used in territory size analyses). We monitored 76 nests and scored each based on successful fledging of young (1) or failure due to predation (0). Abandoned nests or nests destroyed due to other causes (e.g., storms) were excluded. When we fit the logistic model in Equation 1 with blueness score on the breast as a predictor variable, there was no significant relationship between male ornamentation and risk of nest predation (test for significance of intercept and predictor variable: X2 = 1.02, p =.31). When blueness score on the rump was the predictor variable, the results were also nonsignificant (X2 = 0.03, p =.87).
Paternal investment
On average, females fed nestlings at a rate more than six times higher than
that of males (mean: 3.3 female feedings/h versus 0.52 feedings/h for males).
Male feeding rate was highly variable ranging from 0-2.9 feedings per hour.
Male feeding rate did not vary with hour of the day (repeated-measures ANOVA,
F = 0.665, df = 13, p =.798) or with nestling age
(repeated-measures ANOVA, F = 1.255, df = 5, p =.305).
However, male feeding rate appeared to be higher at the first nest of the
season than at subsequent nests (repeated-measures ANOVA, F = 5.05,
df = 1, p =.075). For the analyses in this paper we pooled data over
all hours of the day and all chick ages to calculate male feeding rate.
However, we analyzed male feeding rate at first nests and feeding rate at
later nests separately.
When we compared male feeding rate at the first nest of the season to male blueness, we found a positive association between these two variables, as predicted (Table 1 and Figure 6), but this trend was nonsignificant for the breast (p =.398) and only marginally significant for the rump (p =.055). However, sample size was small for this analysis (n = 9), and the near significance of the rump analysis suggests that there may be a pattern in the data. When male feeding rate at first nests was compared to each plumage variable separately, there were no statistically significant associations on the rump (Table 2). However, the near significance of the correlations between feeding rate and peak wavelength, intensity, and contrast suggested that all of these blueness components contributed to the correlation reported above. When male feeding rate at later nests was compared to blueness, there was no trend (Table 1).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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In this study, we found that extreme ornamentation in males, characterized by a high proportion of blue feathers in plumage and intense blue-ultraviolet coloration, was positively correlated with male body size, territory size, and prey abundance. The data also suggested that the bluest males may provide more parental care than less ornamented males; however, this result must be interpreted cautiously due to small sample size. These results are concordant with those of several studies investigating the function of pigment-based plumage ornaments. For example, in black-headed grosbeaks (Pheucticus melanocephalus), northern cardinals (Cardinalis cardinalis), and house sparrows (Passer domesticus), researchers found positive correlations between pigment-based ornaments and territory acquisition or social dominance (Hill, 1988
;
Møller, 1987;
Veiga, 1993;
Wolfenbarger, 1999). Positive
correlations between paternal care of nestlings and male ornamentation have
been documented in several avian species that display pigment-based ornaments
(Hill, 1991;
Norris, 1990;
Sætre et al., 1995).
There is also evidence that males displaying extreme plumage ornamentation
obtain more extrapair matings than less ornamented males
(Sundberg and Dixon, 1996;
Yezerinac and Weatherhead,
1997). Our observations of blue grosbeaks suggest that
structurally based blue coloration may have the same sort of intraspecific
signaling properties as pigment-based coloration, despite its different
mechanistic basis.
The discovery of ultraviolet-sensitive retinal cones in birds
(Chen and Goldsmith, 1986)
sparked interest in the possible existence of hidden, ultraviolet signals in
plumage (Bennett and Cuthill,
1994, Finger and Burkhardt,
1994). Researchers who tested for a role of ultraviolet color
display in mate choice found that female starlings (Sturnus vulgaris)
and zebra finches (Taeniopygia guttata) are sensitive to a lack of
ultraviolet reflectance in male plumage (Bennett et al.,
1996,
1997). The throat patch of male
bluethroats (Luscinia s. svecica) has peak reflectance in the
ultraviolet portion of the spectrum, and in aviary and field experiments,
females discriminated against experimental males whose throat patch did not
reflect ultraviolet light (Andersson and
Amundsen, 1997; Johnsen et
al., 1998). Moreover, in the field, male bluethroats with
nonreflective throat patches guarded their mates more intensely, obtained
fewer extrapair fertilizations, and were cuckolded more frequently than
control males (Johnsen et al.,
1998). In blue tits (Parus caeruleus), males and females,
which appear nearly monomorphic to human observers, are dimorphic when
ultraviolet plumage reflectance is measured
(Andersson et al., 1998;
Hunt et al., 1998). Females
prefer males with the brightest ultraviolet ornaments
(Hunt et al., 1998), and
assortative mating based on ultraviolet coloration occurs in the wild
(Andersson et al., 1998).
These studies demonstrate that ultraviolet coloration can be important in
intraspecific interactions. Our previous work on blue grosbeaks demonstrated
that males vary substantially in the wavelength of peak light reflectance and
in the magnitude of reflectance in the ultraviolet portion of the spectrum
(Keyser and Hill, 1999). Males
did not vary randomly with respect to these two measures of color display.
Rather, males with the greatest magnitude of light reflectance (most intensely
colored males) also peaked at shorter wavelengths. Both the magnitude of
maximum reflectance and wavelength of peak light reflectance were
significantly positively correlated with nutritional condition during feather
growth (Keyser and Hill,
1999). These findings set the stage for our current study, in
which we showed that plumage blueness in male blue grosbeaks honestly
advertises male quality.
Because expression of blue plumage depends on the physical condition of a
male during the fall when new feathers are molted
(Keyser and Hill, 1999),
conspecifics assessing male ornamentation in the spring receive information
about a male's past nutritional history. The correlation we found in this
study between male plumage ornamentation and body size suggests that blueness
may also provide information about the conditions in which a male was raised.
The relationship between blueness and territory quality (and perhaps paternal
care) documented here reflects the current status of a male. Thus,
blue-ultraviolet plumage ornamentation is an honest advertisement of quality
that can be assessed by conspecifics during mate choice or during male-male
competition.
The question remains: does plumage blueness of male blue grosbeaks serve
primarily as a signal between rival males, in female mate choice, or for both?
Male body size was significantly positively associated with plumage blueness,
but male size could be important in either female choice or male-male
competition. The strong association between territory size and male blueness,
on the other hand, suggests an intrasexual function for male ornamentation
because territory boundaries are established through direct male contests. It
is interesting that all aspects of plumage blueness showed a significant
association with territory size in the predicted direction, except percent
blue on the rump. This may be due to reduced variance in percent blue on the
rump (mean = 0.828, 
2 = 0.062) relative to the breast (mean
= 0.694, 2 = 0.112). Even birds whose plumage was almost
100% brown showed some proportion of blue feathers on the rump, which is often
the bluest part of the body (Keyser and Hill, personal observations). Because
the rump patch is often displayed during interactions with other males but
rarely at any other time, rump coloration appears to function directly in
male-male competition. Apparently, males prioritize development of blue rump
feathers such that the rump is blue even when other parts of the plumage are
brown, and this suggests that the rump plumage has a particularly important
signaling function in male-male interactions.
The relationship between territory size and plumage blueness and the
apparent directed display of colored plumage patches during male contests
suggests that signaling between males may be a primary role for plumage
blueness. However, the correlation between male ornamentation and prey
abundance and paternal care indicates that female choice for highly ornamented
males could be adaptive. Female mate choice has proven to be difficult to
disentangle from male-male competition in territorial species (e.g., numerous
red-winged blackbird studies summarized in
Beletsky, 1996;
Searcy and Yasukawa, 1995). If
highly ornamented males establish and defend quality territories, then females
may not need to assess males directly but instead may choose mates based on
territory attributes. In this case, female choice and male ornamentation are
indirectly linked, but it would be inaccurate to conclude that female choice
was a selective pressure driving the evolution of male ornamentation. A more
definitive assessment regarding the basis of mate choice in female blue
grosbeaks will require controlled mate choice experiments.
A complementary approach to understanding female mate choice is genetic
paternity analyses. One basic prediction of sexual selection theory is that
more highly ornamented males should experience greater reproductive success
than less ornamented males. It is now well established that in most species of
birds some chicks are fathered outside the pair bond and that such extrapair
paternity must be accounted for if success of individual males is to be
accurately assessed (Birkhead and
Møller, 1992; Gibbs et
al., 1990; Weatherhead and
Boag, 1995; Webster et al.,
1995). In the indigo bunting (Passerina cyanea), a close
relative of the blue grosbeak with a similar social system, 20-40% of
offspring are fathered by males other than the attending male
(Payne, 1992). Genetic
paternity analyses of our marked blue grosbeak population are currently
underway.
In general, males of many species are expected to experience higher
variation in reproductive success than females. Thus competition among males
for access to mates either through monopolization of resources or through
exaggeration of traits that are attractive to females is expected
(Darwin, 1871). From the
female perspective, mate choice decisions can be costly in terms of time and
energy. Female choice can be adaptive when it is based on male ornaments that
provide honest information about male attributes
(Searcy, 1979). The results of
this study indicate that blue-ultraviolet coloration in blue grosbeaks is the
kind of ornamental trait predicted by sexual selection theory
(Kodric-Brown and Brown,
1984). The bluest males are also high-quality males. They are
physically larger, hold larger territories with abundant prey, and contribute
to female provisioning at the nest. These results, in conjunction with other
studies reviewed above, contribute to a more complete understanding of
structurally based, ultraviolet plumage ornamentation in the context of sexual
selection theory.
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ACKNOWLEDGEMENTS |
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We thank Barbara Ballentine for her constant enthusiasm about blue grosbeaks. Our very hard-working field assistants included Lynn Siefferman, Jonathan Ariail, Renée Duckworth, Joe Smith, Scott Lovell, Angela Martin, Anthony Floyd, Michael Barbour, Kelly Volansky, and Stephanie Heller. This manuscript was much improved by comments from the Promislow lab, the Hill lab, Steve Dobson, Gary Hepp, Christine Spencer, and two anonymous reviewers. Birds were handled according to protocols approved by the Auburn University Institutional Animal Care and Use Committee (PRN # 9809-R-1025). This research was supported by the Department of Zoology and Wildlife Science at Auburn University, the Alabama Agricultural Experiment Station, and American Cyanamid Company, Global Agricultural Products Research Division.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Andersson S, 1996. Bright ultraviolet colouration in
the Asian whistling-thrushes (Myiophonus spp.). Proc R Soc
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843-848.
Andersson S, Amundsen T, 1997. Ultraviolet colour
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