We compared the red, green, and blue color values from digital photographs of the rictal flanges of nestling Brown-headed Cowbirds (Molothrus ater), a generalist obligate brood parasite, in sympatric Yellow Warbler (Setophaga petechia) and Song Sparrow (Melospiza melodia) nests at Mono Lake, California, USA. We detected significant differences in all three color components across nestlings of different species (R: P < 0.0001; G: P < 0.0001; B: P < 0.0001), but differences among cowbird nestlings from the nests of these two hosts were not significant (R: P = 0.543; G: P = 0.737; B: P = 0.319). Principal components results were mixed: Principal Component I described brightness and accounted for 84% of the variance. It did not differ among cowbird nestlings from nests of different hosts (P = 0.319). Principal Component II described chromaticity and accounted for 14% of the variance, which differed significantly among cowbird nestlings from the two different hosts' nests (P = 0.026). Color differences between cowbird nestlings from nests of different host species may result from selective parasitism by female parasites based on host nestling flange morphology, or ontogenetic effects on cowbird nestlings reared by different host species.
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