Behavioral Ecology Vol. 14 No. 4: 503–509

Multiple sexual ornaments in satin bowerbirds:
ultraviolet plumage and bowers signal different
aspects of male quality
Stéphanie M. Doucet and Robert Montgomerie
Department of Biology, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Much attention has been devoted to understanding the evolution of elaborate male ornaments and how they may signal malequality. However, the evolution of multicomponent sexual signals remains poorly understood, and past research on this type ofsignaling has been largely theoretical. Satin bowerbirds, Ptilonorhynchus violaceus, are polygynous, are sexually dichromatic, andconstruct sexually selected display structures (bowers): a model system for investigating the evolution and signal function ofmultiple sexual signals. We studied the interrelationship between bower features, plumage coloration, and indicators of malequality in this species. To do this, we located the bowers of male satin bowerbirds in rainforest in Queensland, Australia, andquantified bower quality. We captured the male bower owners and used reflectance spectrometry to objectively measure theplumage coloration of several body regions. We measured various indicators of male health and condition, including theintensity of infection from ectoparasites and blood parasites. Bower quality and male ultraviolet plumage coloration weresignificantly correlated. By using multiple regression analyses, we show that bower quality predicts ectoparasite load and bodysize, whereas ultraviolet plumage coloration predicts the intensity of infection from blood parasites, feather growth rate, andbody size. Our findings support the multiple messages hypothesis of multicomponent signals: Female satin bowerbirds should
assess both male and bower features to choose the highest quality mates. Key words: bowerbirds, bowers, honest advertisement,
multiple ornaments, parasites, plumage color, sexual selection, structural colors. [Behav Ecol 14:503–509 (2003)]
laborate, secondary sex traits are often presumed tocommunicate aspects of male quality to discriminating
females (Andersson, 1994). In many species, elaborate sexual
displays involve several distinct signals such as plumage
ornamentation, song, and display behavior. Some theoreticalmodels have shown that, under certain circumstances, species
could evolve multiple quality-revealing sexual ornaments( Johnstone, 1995a), but other models propose that honest
advertising should favor a single most-revealing signal at theexpense of others ( Johnstone, 1996; Schluter and Price, 1993).
Despite some theoretical consideration ( Johnstone 1995a,
1996; Schluter and Price, 1993), the evolution of multiple
quality-indicating sexual ornaments has received little empirical support (for exceptions, see Johnstone, 1995b). In many
species, the evolution of more than one sexual signal can be
more accurately attributed to Fisherian arbitrary selection(Fisher, 1958; Møller and Pomiankowski, 1993; Pomiankowski and Iwasa, 1993), signal amplification (Hasson,1989), enhanced signal detection (Rowe, 1999), or to distinct
signals intended for different receivers (Pryke et al., 2001a,b).
In satin bowerbirds, Ptilonorhynchus violaceus, males clear
display courts, build bowers (elaborate structures constructedfrom twigs), and decorate the bower platform with a variety ofcolorful natural and human-manufactured objects (seeMarshall, 1954). Females of this species observe male courtship from within the bower avenue, and the sexual display ofcourting males incorporates song mimicry, ritualized prancing, and brilliant plumage coloration. Because different
components of the male satin bowerbird’s display will varyEon different time scales (daily for bowers and yearly forplumage color), these multicomponent sexual signals may
reveal different aspects of male quality (Johnstone, 1995b;
Møller and Pomiankowski, 1993), and females may thus
obtain useful information from evaluating all components ofmale display.
In satin bowerbirds, individual variation in the quality ofbower construction, the numbers and types of decorations
adorning the bower platform, and age-correlated features ofcourtship song are known to influence female choice of
copulation partners, as measured by male mating success(Borgia, 1985b; Lofredo and Borgia, 1986). In contrast toother features of the sexual display of male satin bowerbirds,
the structurally based iridescent plumage of males hasreceived less attention, even though this species is highly
sexually dichromatic and plumage ornamentation is a prominent feature of male courtship display. Recent studies ofother bird species have shown that structural plumage
coloration can honestly signal condition (Doucet, 2002;
Keyser and Hill, 1999), male quality (Keyser and Hill, 2000),and viability (Sheldon et al., 1999). Thus plumage coloration
may be a quality-indicating mechanism in male satin bowerbirds and may function as a key component of a complex,
multimodal sexual signal of male quality. In the present study,
we investigated how multicomponent sexual signals canpotentially reveal male quality in this species by studying therelationship between male plumage coloration, bower features, and various indicators of male health and condition.
METHODS

Address correspondence to S.M. Doucet, who is now at the
Department of Biological Sciences, 331 Funchess Hall, Auburn
University, Auburn, AL 36849, USA. E-mail: doucets@auburn.edu.
Received 3 April 2002; revised 3 September 2002; accepted 21
September 2002.
 2003 International Society for Behavioral Ecology

This study was conducted from September–December 2000 in
a 700-ha study area in Mount Baldy State Forest, Queensland,
Australia (17309 S, 145309 E). The dominant habitat was
rainforest with a sharp transition to wet sclerophyll forest at
the northeastern edge of our study area. We located 12 satinbowerbird bowers in our study area by listening for male

Behavioral Ecology Vol. 14 No. 4

504

Figure 1
Representative reflectance spectrum (average of rump, mantle,
breast, and wing coverts) for one adult male satin bowerbird. This
individual was chosen because its plumage color was closest to themean for the color variables we calculated across all individuals
sampled.

advertisement calls, usually given near bowers, and systematically searching for active display sites. We assessed bower
quality by evaluating bower construction and quantifying thenumber of decorations used. Two observers independently
evaluated four features of bower construction—overall symmetry of the structure, stick size, stick density, and overall
quality of construction—each on a scale from one (poor) tofour (excellent). We calculated a bower quality score for eachbower as the sum of these measures averaged between the two
observers. Similar indices of bower quality have been shown tocorrelate with mating success in several species of bowerbird(Borgia, 1985b; Borgia and Mueller, 1992; Lenz, 1994; Uy andBorgia, 2000). To assess the extent of bower decoration, we
regularly visited bowers from mid October to late November(n ¼ 3–7 visits per bower) and recorded the number of eachtype of decoration present on the bower. For our analyses, weused the mean number of decorations recorded at eachbower.
We caught 11 adult males (10 of which held bowers) near
their display sites by using mist nets baited with blue objects,
and fitted them with a unique combination of color bands. We
measured tarsus length (to nearest 0.1 mm) as an index ofbody size that is independent of age and condition. We alsomeasured the mass (to the nearest gram) of each bird andused the residuals of a regression of body mass on tarsus
length as an index of body condition. We noted whetherfeathers were predominantly old, new, or molting, and weremoved the right outer rectrix to assess daily feather growthrate.
Plumage color
We quantified plumage reflectance by using an Ocean Optics
S2000 spectrometer and a pulsed xenon lamp (PX–2; Ocean
Optics, Dunedin, Florida, USA). Measurements were takenwith a single fiber-optic probe that provided illuminationfrom the light source and transferred the reflected light to thespectrometer. The probe was mounted in a hard rubber coverthat excluded external light from the measurement area
(approximately 3 mm2) and maintained the probe perpen-

dicular to the feather surface. All spectral measurements are
expressed as the proportion of light reflected relative to thatreflected from a Spectralon white standard, an almost perfectreflector. We measured plumage reflectance on four body
regions for each individual: wing coverts, breast, mantle, andrump, taking five readings from each region and moving theprobe by at least 5 mm between readings. We restricted
spectral analyses to wavelengths from 300–700 nm, as mostbirds are sensitive to ultraviolet (UV) wavelengths (300–400
nm; Cuthill et al., 2000), and 700 nm is likely the upper limitof the vertebrate visual spectrum (Jacobs, 1981).
We summarized reflectance data by calculating five color
variables: total brightness, UVV-brightness, UVV-chroma, contrast, and hue. We calculated total brightness as the mean ofreflectance values, in 1-nm intervals, from 300–700 nm(Andersson, 1999; Endler, 1990); this measurement thus gives
the mean proportion of incident light that is reflected fromthe feather surface in the bird-visible range of wavelengths.
Similarly, we calculated UVV-brightness as the mean ofreflectance values in the UV–violet region of the spectrum(300–420 nm; Andersson et al., 1998), in which peakreflectance occurred in all males. As a measure of spectral saturation, we calculated UVV-chroma as the proportion of totalreflectance occurring in the UV–violet region of the spectrum. We calculated contrast as the difference between the
maximum and minimum reflectance across the 300–700-nm
range, such that higher contrast indicates a more intense
color (Keyser and Hill, 1999, 2000). Given the simple,
unimodal shape of adult male satin bowerbird reflectancespectra (Figure 1), contrast is likely a reliable estimate of color
intensity in this species. Finally, to approximate hue, we used
the wavelength at which maximum reflectance was reached,
a commonly used index of spectral location (see Keyser and
Hill, 1999, 2000; Sheldon et al., 1999).
We also calculated an overall ornamental color score using
principal components analysis (PCA). We first calculated themean of each color variable for each body region on each
male. We then performed a PCA by using the mean total
brightness, UVV-chroma, contrast, and hue averaged over thefour regions of each male studied. The first principal
component (PC1) from this analysis explained 46% of thevariation in color among males, with total brightness, contrast,
and UVV-chroma having the highest positive loadings on thePC1 axis (eigenvectors ¼ 0.70, 0.59, and 0.39, respectively).
Therefore, males with a high ‘‘PC1 color score’’ displayed an
overall bright and highly saturated ornamental plumage
coloration. We did not include UVV-brightness in the PCA
because of all the color variables calculated, this was the mostvariable both within and among males (see below); thus, we
wanted to investigate the influence of UVV-brightness foreach body region separately.
Parasites
We assessed ectoparasite load by counting the number ofMyrsidea ptilonorhynchi lice on the head (especially near theeyes) of each male. This louse belongs to a suborder in which
species are known to consume feathers and to feed on the
blood and skin of their hosts; thus, these lice probably elicit
specific immune responses in their hosts and can havea considerable effect on host fitness (Clayton, 1991a,b). This
louse is the only common ectoparasite on satin bowerbirds,
and it is found mainly around the head and eyes where birds
cannot easily preen (Borgia, 1986a; Borgia and Collis, 1989,1990).
To assess the intensity of infection from blood parasites, wecollected a small sample of blood from the brachial vein ofeach male, drawing blood into a capillary tube and thinlyDoucet and Montgomerie • Multiple sexual ornaments in bowerbirds

505

Table 1
Correlation matrix of measures of male satin bowerbird plumage color and bower structure used in multiple regression analyses
UVV-brightness
Covertsa

Rump

MantleBreastNo. of decorations

Bower quality

PC1 color score

0.26

0.36
0.14

0.37
0.29
0.19

0.35
0.39
0.45
0.49

0.30
0.51
0.41
0.40

0.37
0.58
0.26
0.66a

0.73a

0.83a
0.77a

UVV-brightness
Wing coverts
Rump
MantleBreast

0.46
0.30
0.29

0.70
0.42

0.59

No. of decorations
Bower quality score
PC1 color score

0.32
0.40
0.29

0.27
0.13
0.08

0.19
0.24
0.47

0.16
0.25
0.04

0.02
0.003

0.009

Values above diagonal are correlation coefficients (r); below diagonal are p values.
Correlation coefficients with p , .05 before Bonferroni correction.

smearing it onto a glass slide. We prepared the slides by using
the Hema 3 staining procedure (Fisher Scientific). We thenobserved each slide at 31250 magnification under oil
immersion, scanning for haemosporidian parasites until
10,000 red blood cells had been surveyed, and identifying
each parasite to genus (Campbell, 1988). All but one of theblood parasites scored were Haemoproteus; thus, only mature(intra-erythrocytic) Haemoproteus parasites are considered in
the following analyses. All slides were scored by the sameobserver, blind to the identity of the individual bird beingscored.
We assessed feather growth rates by measuring the width ofalternating dark and light bars on the right outer rectrix ofeach male. Each pair of bars represents 1 day’s growth(Michener and Michener, 1938), and the width of these bars
has been associated with nutritional condition in several
species (Grubb, 1989, 1991; Jenkins et al., 2001). We measured
the width of six pairs of bars on either side of the midpoint of
the feather, from which we calculated a 12-day average daily
feather growth rate for each male (see Hill and Montgomerie,1994).

Statistical analysis
None of the variables described in this study deviatedsignificantly from normality (all, p . .10); thus, no transformations were required for parametric analyses. Values arereported as mean 6 SE, and all probabilities are two-tailed. To
determine which plumage and bower variables would best
predict four male traits that may reveal male quality(ectoparasite load, intensity of infection from blood parasites,feather growth rate, body size), we constructed four backward
stepwise multiple regression models. In each of the fourmodels, we used PC1 color score, UVV-brightness (of rump,wing coverts, mantle, and breast, separately), bower quality
score, and number of bower decorations as potential predictor variables. We performed a backward selection procedure so that variables that could significantly predict maletraits in combination would be included in the models even if
they were not significant predictors individually (see Zar,
1999). The models reported here include only variables thatwere significant predictors (p  .05) of each quality trait
under investigation. We also constructed correlation matrices
and examined leverage plots for indications of collinearity inpredictor variables that might make these regression models
difficult to interpret; none of the regression models showed
evidence of serious problems with collinearity. Because of oursmall sample size, we make no attempt to assess the relative

importance of these different signals in predicting aspects ofmale quality, although the standardized regression coefficients may serve as a rough guide. Analyses that could reliably
reveal the relative importance of different predictor variables(e.g., path analysis) would require much larger sample sizes.
RESULTS
Plumage coloration
The rump, mantle, breast, and wing coverts of male satinbowerbirds reflect most strongly in the UV and violet regionsof the bird-visible spectrum (Figure 1). Indeed, the average
male hue across the four body regions measured was 362 6
6.8 nm, well within the UV range (300–400 nm). There wasconsiderable variation in male plumage characteristics, both
among males and among body regions within males. UVVbrightness was the most variable of the plumage characteristics we calculated, with coefficients of variation (CV) among
males ranging from 0.25–0.38 for the wing coverts, mantle,
breast, and rump. In comparison, mean CVs for total
brightness, UVV-chroma, contrast, and hue were 0.29, 0.08,
0.17, and 0.07, respectively. Mean UVV-brightness was alsosignificantly different across the four body regions measured(ANOVA: F3,40 ¼ 5.86, p ¼ .002; mean UVV-brightness of therump, 7.8 6 0.90; mantle, 10.5 6 0.79; breast, 6.3 6 0.66;
wing coverts, 7.0 6 0.64) despite looking identical to us.
Interestingly, correlations of UVV-brightness between regions(within males) were weak and both positive and negative(Table 1).
Plumage coloration and bower characteristics
Measures of bower structure and male plumage coloration
were intricately associated in satin bowerbirds. Male ornamental plumage color (PC1 color score) was significantly
positively related to both bower quality (Figure 2a) and theaverage number of decorations adorning bowers (Figure 2b).
Thus, the satin bowerbird’s bower provides females witha useful index of a male’s appearance, even in his absence,with bower quality score and number of decorations together
explaining about 75% of the variation in this PC1 color score(multiple regression: F2,7 ¼ 10.3, p ¼ .008).
Parasites
To determine whether plumage coloration and bower
features revealed aspects of male quality, we compared these
attributes to the degree of infection from ectoparasites andBehavioral Ecology Vol. 14 No. 4506

of feather growth rate and together explained an additional
28% of the variation. UVV-brightness of rump and wing
coverts were not correlated within males nor was either ofthese significantly correlated with PC1 color score (Table 1).
Body size
Rump UVV-brightness, bower quality score, and number ofbower decorations were all significant predictors of body size
(Table 2, Figure 3), together explaining more than 80% of thevariation in tarsus length. Rump UVV-brightness was the best
predictor of tarsus length (Table 2), explaining 46% of thevariation, with bower quality score and number of decorations
together explaining an additional 36% of the variation. Bower
quality score and number of decorations were significantly
correlated within males, but neither of these bower variables
was correlated with rump UVV-brightness (Table 1). Rump
UVV-brightness and average number of bower decorations
were positive predictors of tarsus length, whereas bower
quality score was a negative predictor. Note, however, thatbower quality was a negative predictor of tarsus length only inthis multiple regression analysis, in which the number ofbower decorations is statistically controlled. By itself, bower
quality score was positively, although not significantly, related
to tarsus length (r ¼ .10, n ¼ 10, p ¼ .79).
DISCUSSION

Figure 2
Relations between overall male plumage color (PC1 color score)and bower quality score (a; y ¼ 3.8 þ 0.5x; r 2 ¼ .60, n ¼ 10,
p ¼ .009) and average number of decorations (b; y ¼ 2.1 þ 0.1x;
r 2 ¼ .69, n ¼ 10, p ¼ .003) in male satin bowerbird bowers. Males
with high PC1 color scores displayed high total brightness, contrast,
and UVV-chroma in their structural plumage coloration. Trendlines are model II regressions.

endoparasites. In a stepwise multiple regression analysis, thequality of bower construction emerged as the only significant
predictor of ectoparasite load among the variables tested(Table 2, Figure 3). That is, males with high quality bowers
had fewer ectoparasites, and bower quality explained morethan 50% of the variation in ectoparasite load. On the other
hand, variation in the intensity of endoparasite infection was
best explained by the UVV-brightness of the male’s rumpplumage (Table 2, Figure 3); males with the brightest rumps
had the lowest intensity of infection by Haemoproteus bloodparasites, with rump UVV-brightness explaining 50% of thevariation in the intensity of infection.
Feather growth rate
Overall plumage color (PC1 color score), rump UVVbrightness, and wing covert UVV-brightness were all significant predictors of feather growth rate (Table 2, Figure 3),
together explaining 74% of the variation in growth rate. PC1
color score was the best predictor of feather growth rate,
explaining 46% of the variation. Note, however, that PC1
color score varied negatively with feather growth rate, in the
opposite direction to that predicted. Rump and wing covert
UVV-brightness, on the other hand, were positive predictors

Our analyses show that male plumage coloration and bower
quality features are intricately related in satin bowerbirds andthat, together, these elaborate sexual ornaments reveal
important aspects of male quality. This is the first study toidentify an association between bower features and maleplumage ornamentation in bowerbirds, suggesting thatbowers are an extension of the male phenotype that females
can use to assess male quality. It is possible that the observed
positive relations between bower quality score, number ofdecorations, and overall plumage color (PC1 color score)(Figure 2) may result from a general increase in the quality ofbower construction (Borgia, 1986b) and structural plumage
color with male age. However, females seem to consistently
prefer specific individuals in a population (Uy et al., 2000,
2001), thereby suggesting that some aspects of male quality
may be independent of age. More work will be needed todetermine the effects of male age on their plumage colorationand quality of bower construction to determine whether thesesignals also provide females with useful fitness-related information about male longevity.
As we have shown, the signal function of decorated bowers
and bright plumage in satin bowerbirds may be explained inpart by their significant correlation with parasite load, an
important indicator of male quality (see also Doucet andMontgomerie, in press). According to the Hamilton-Zuk
hypothesis of parasite-mediated sexual selection, females
should prefer males with the most elaborate sexual ornaments
because the degree of ornament elaboration may be limited
by a male’s ability to resist disease infection (Hamilton andZuk, 1982). Hence, by mating with highly ornamented males,
females stand to acquire heritable parasite resistance for theiroffspring (Hamilton and Zuk, 1982). Here, we provide
support for the Hamilton-Zuk hypothesis by two means. First,
we show that quality of bower construction is a significant
predictor of ectoparasite load in this population. Thus,
females could potentially assess male ectoparasite load byevaluating bower quality, even in the absence of the bower
owner. In another population of satin bowerbirds, maleectoparasite load was negatively related to mating successbut was unrelated to bower quality (Borgia and Collis, 1989,

Doucet and Montgomerie • Multiple sexual ornaments in bowerbirds

507

Table 2Significant predictors of male quality in satin bowerbirds
Dependent variables

Significant predictors

b9

F

p

df

I. Ectoparasites
II. Endoparasites

Bower quality score (R 2 ¼ .51)
Rump UVV-brightness (R 2 ¼ .50)

0.72
0.70

8.4
8.9

.02
.02

1,8
1,8

III. Feather growth rate

Whole model (R 2 ¼ .74)
PC1 plumage color score
Wing coverts UVV-brightness
Rump UVV-brightness

1.39
0.66
0.63

6.7
17.8
5.9
5.0

.02
.004
.04
.05

3,9
1,9
1,9
1,9

Whole model (R 2 ¼ .82)
Rump UVV-brightness
Bower quality score
Number of decorations

0.83
0.89
0.77

8.9
16.7
10.4
8.9

.01
.006
.02
.02

3,8
1,8
1,8
1,8

IV. Body size

Each model was constructed using backward stepwise multiple regression analysis with both bower
features (bower quality score, number of decorations) and plumage features (PC1 plumage color score,
UVV-brightness of the wing coverts, mantle, rump, and breast) as independent variables. Standardized
regression coefficients (b9) and significance tests are shown for each significant predictor variable.

1990). Second, we show that a male’s rump UVV-brightness
can reveal the intensity of his infection from blood parasites.
Thus, female satin bowerbirds can assess haemosporidianparasite intensities by evaluating male plumage brightness
during courtship display (Figure 3).
We also found that rump plumage coloration, average
number of bower decorations, and quality of bower
construction were significant predictors of male body size
(Figure 3). The association between the number ofdecorations and body size is perhaps best explained in thecontext of male choice of bower decorations. Male satinbowerbirds preferentially decorate their bowers with bluefeathers, flowers, and berries, as well as small mammal skulls
and other bones, all of which are relatively rare in thesurrounding environment (Borgia and Gore, 1986; Doucet
SD and Montgomerie R, unpublished data). As a consequence, theft from other males appears to be the primarymeans of obtaining these decorations (Borgia and Gore,1986). Larger males may have a competitive advantage
during theft-related confrontations, allowing them to steal
more decorations from other males and to defend their own
decorations from thieving neighbors.
The fact that overall plumage color (PC1 color score) was
negatively associated with tail feather growth rate is surprising.
Growth rates have traditionally been used as indicators ofcondition at the time of molt (Grubb, 1989, 1991; Jenkins
et al., 2001), and positive associations between feather growthrate and quality-indicating plumage coloration have beenfound in several species (Doucet, 2002; Hill and Montgomerie, 1994; Keyser and Hill, 1999). We can only suggest thatfeather growth rate may not be a useful index of maleplumage quality in satin bowerbirds, for two reasons. First,
males who built better quality bowers also had a higher PC1
color score (Figure 2), suggesting that this overall plumage
color score is a good measure of male quality despite its
negative association with feather growth rate. Second, males
spend considerable time and effort building, maintaining,and defending their bowers during the breeding season(Borgia, 1985a; Borgia and Gore, 1986), and this may explain
the negative relationship between bower quality and residual
body mass in our study (r ¼ .79, n ¼ 10, p ¼ .009). Ourfindings suggest that males had to work hard to maintain theirhigh-quality bowers, perhaps at the expense of othermaintenance activities such as foraging. Thus, by the end of
the breeding season when molt begins (Vellenga, 1980), high-quality, nutritionally stressed males might be expected to growtheir new feathers more slowly.
In summary, our study shows that bower features and maleplumage coloration together explained considerable variation
in four potential quality indicators: ectoparasite load, intensityof infection from blood parasites, feather growth rate, andbody size. Consequently, female satin bowerbirds could assessmales using both bower features and male plumage coloration (Figure 3) to obtain better information about malequality than is available from either trait alone. It is alreadyknown that male mating success is related to bower quality inthis species (Borgia, 1985b), and our study associates bower
quality with ectoparasite load and body size. Despite this, only
the UVV-brightness of the rump was a significant predictor of
the intensity of infection from blood parasites. Our findings
support recent work on mate searching strategies of thefemale satin bowerbird; Uy et al. (2000, 2001) found thatfemales make courtship visits to only a subset of males ina population, and eventually mate with only a subset of themales they visit. This sequential elimination process iscomplemented by our findings: Bower features can signal
aspects of male quality; however, both bower features andplumage traits must be evaluated if females are to gain an
overall appreciation of male quality.
Given that bowers and plumage can vary on different timescales, and that these various components of the male’s
phenotype reveal different aspects of male quality, our study
supports the multiple messages hypothesis of multicomponent signaling (Johnstone, 1995a; Møller and Pomiankowski,1993). Thus, the evolution of multiple sexual ornaments insatin bowerbirds may be driven by selection pressure to signal
multiple aspects of male quality. However, our findings alsosuggest that sexual signals and the quality information they
reveal follow a complex mosaic in which both signal redundancy and exclusivity may coexist (Figure 3). It seems
likely that further study of multicomponent sexual signaling
will reveal similar patterns in other species. Because oursample of males and bowers was relatively small, our statisticalanalyses may have lacked the power to detect other potentiallyrevealing relationships. Thus, our main findings (Table 2,
Figure 3) are likely a conservative estimate of the extent ofmulticomponent signaling in satin bowerbirds.
Interestingly, female satin bowerbirds appear to haverelatively large brains compared with those of their malecounterparts (Madden, 2001). Although male brain capacity

508

Figure 3
Associations between five male attributes (signals) potentially usedby females in mate choice and four indicators of male quality(information) in the satin bowerbird. Arrows show variables thatsignificantly (p  .05 in multiple regression models) predicted eachtarget quality indicator.

is probably essential to building complex bowers, searchingfor bower decorations, and decorating bowers appropriately,
female brain size may be as important in searching forpotential mates (Uy et al., 2000, 2001) and assessing complex
bowers, male plumage, and elaborate male displays. Thus,
female satin bowerbirds may be particularly well suited toevaluate the impressive array of signals of quality discovered so
far in this species, from bower features and male plumage
coloration to song and perhaps even display complexity.
We are grateful to M. Bhardwaj for excellent field assistance and to D.
Westcott for indispensable help with establishing this project. We
thank D. J. Mennill for assistance in the field and helpful commentson this manuscript, two anonymous reviewers for their helpful
suggestions, the Australian Bird & Bat Banding Scheme and theQueensland Department of Environment for permission to work on
satin bowerbirds, the Queensland Department of Natural Resources
for permission to work in State Forest, and CSIRO Australia and theTropical Forest Research Center in Atherton for logistical support.
Funding was provided by the Natural Sciences and Engineering
Research Council of Canada in the form of a PGS A scholarship to
S.M.D. and both research and equipment grants to R.M.

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