Croston el al. • FLANGE COLOR DIFFERENCES OF COWBIRDS 
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ll II11 ll 
SOSP YWAR BHCO in SOSP BHCO in YWAR 
nest nest 
FIG. 1. Differences in flange color saturation values by species and host. SOSP Song Sparrow, YWAR Yellow 
Warbler, and BHCO = Brown-headed Cowbird. Bars represent mean saturation values of red. green, and blue across flange 
sites and replicates. Error bars represent standard error, 
value for i, and Y| = the yellow standard for i. 
Repeated measures at each flange site within color 
group (R, G, and B) were averaged across 
replicates. Data were analyzed using three sepa¬ 
rate univariate analyses of variance (Mixed 
Effects ANOVA), where each color value was 
the dependent variable, nestling species (Yellow 
Warbler, Song Sparrow, or Brown-headed Cow- 
bird) and cowbird nestling host species (Yellow 
Warbler or Song Sparrow) were fixed effects, and 
nestling metal-band ID and flange site were 
random effects. Color variables R, G, and B are 
necessarily correlated and we also used principal 
components analysis (PCA) to recombine color 
variables into uncorrelated scores describing 
brightness and chromaticity (following Endler 
andThery 1996). Mean PC scores were compared 
among host nestling species and cowbird nestling 
species groups using ANOVA. All analyses were 
conducted in JMP Version 8.0 (SAS Institute Inc.. 
Cary, NC, USA). 
RESULTS 
Color values were significantly different among 
nestling species in all three univariate analyses of 
red. green, and blue color components (R: Fz.ax = 
1 *.67, P < 0.0001: G: F 2 .w = 11 -14, /* < 0.0001; 
B: *2.50 = 16.61, P < 0.0001), but not between 
cowbird nestlings of different host species <R: 
*i.ii = 0.84, P = 0.543; G; F U6 = 0.12, P = 
0.737; B; F U7 = 1.06. P = 0.319; Fig. I). 
The first two Principal Components (PC) 
together explained 97% ot the variation in 
nestling flange color (Table 1). The first Principal 
Component (PC I) explained 84% of variance in 
the combined RGB variables and described 
variation in brightness for all three color compo¬ 
nents. as implied by the consistently positive 
eigenvalue loadings ot all three red (0.575), green 
(0.613). and blue (0.541) chromatic values. PC 1 
differed across nestling species (F 2 .47 = 24.96. 
P < 0.0001). but did not differ between cowbird 
nestlings from different host species nests (T|.i6 
= 0.004, P = 0.950). Principal Component II (PC 
II) explained 14% of variance and described the 
color content or chromaticity of the RGB 
variables (i.e., hue and saturation), where blue 
was a positively loaded (0.793) eigenvector and 
red was negatively loaded (—0.591). Green was 
also negatively loaded, although this value was 
low (-0.145). Higher values of PC 11 reflected 
higher values for blue and lower values for red. 
PC 11 did not differ by nestling species (F 2 , 5 6 - 
0.84, P - 0.436), but differed significantly 
between cowbird nestlings from different host 
species nests (F,. w = 5-89. P = 0.026) (Fig. 2). 
DISCUSSION 
Flange colors of two species serving as hosts 
for Brown-headed Cowbird nestlings were differ¬ 
ent in most univariate and PC analyses of physical 
coloration measures, confirming the assumptions 
of species-specificity of these visual traits. 
Univariate measures of flange colors revealed no 
differences between cowbird nestlings from the 
two host species’ nests. However, most univariate 
