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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124, No. 1. March 2012 
TABLE I. Eigenvalues from the principal components analysis of variation in nestling flange color. The first two 
principal components explained 97.4% of the total variance. 
Paramelcr 
Eigenvalue 
Eigenvector 
Variance explained (%) 
Principal Component I 
2.51 
red 0.58 
green 0.61 
blue 0.54 
83.7 
Principal Component II 
0.41 
red -0.59 
green -0.15 
blue 0.79 
13.7 
Principal Component III 
0.08 
red 0.56 
green —0.78 
blue 0.28 
1.1 
analyses of color traits are confounded with 
variation in brightness (Endler and Thery 1996). 
PC analysis, in contrast, revealed differences in 
PC 11 describing the relative saturation of red and 
blue among cowbird nestlings. Thus, parasitic 
offspring in nests of the two sympatric host 
species differ in physical measures of coloration, 
as recorded on digital photographs of live 
nestlings in the field. These differences may be 
behaviorally relevant for host discrimination, as 
recent studies ol hosts of brood parasitic birds 
have demonstrated the avian-specific role of 
particular chromatic elements influencing egg 
rejection behavior (Honza et al. 2007; Cassey 
et al. 2008. 2009). 
The host-specilicity of the chromatic variation 
of cowbird nestling flange color detected suggests 
a potential signaling function in the context of 
parasitic nestling discrimination. A mechanism 
may exist by which female cowbirds preferential¬ 
ly parasitize hosts whose nestling flange color 
matches that of their own chicks, despite the vast 
diversity of cowbird host species (Friedmann 
1929). This mechanism may he similar to female 
parasites* host-specific egg color matching, doc¬ 
umented in Common Cuckoos (Cuculus caiumts) 
(Cherry et al. 2007, Aviles 2008). 
The ontogenetic basis of host-parasite flange 
color matching requires additional study, as it 
may involve genetic and maternal effects of host- 
specific parasitism by female cowbirds. Altcrna 
tivcly, it may be the result of host-specific 
plasticity of cowbird chicks' coloration during 
development. Experimental manipulation of cow¬ 
bird nestling flange color, and cross-fostering 
studies of parasitic eggs between different host 
species should provide experimental tests o! 
some of these alternatives, as demonstrated in 
