COLOR VARIATION AMONG NESTLING BROWN-HEADED COWBIRDS (MOLOTHRUS ATER) DOES NOT REFLECT DIFFERENTIAL SUCCESS WITH HOSTS IN TEXAS

Auk, The, Apr 2007 by Ellison, Kevin, Sealy, Spencer G, McGaha, Hope R

Alternatively, as a potentially sex-influenced trait, color may be affected by androgen levels that can affect expression of xanthophyll pigments (Stoehr and Hill 2001, Faivre et al. 2003). Testosterone levels do not appear to vary such that geographic variation in flange color is likely to be affected (Hahn et al. 2005); nonetheless, the potential for variation exists in maternal provisioning of pigments (i.e., carotenoids in eggs; Bortolotti et al. 2003).

TRAIT MAINTENANCE

Our results support Rothstein's (1978) suggestion that there are no major differences in cowbird fledging success, regardless of whether the young have white or yellow flanges. Alternatively, historical conditions may have favored one flange color or a polymorphism in flange color if cowbirds evolved with entirely different suites of host species, particularly species that now reject parasitism but may once have been important hosts. Thus, the present polymorphism would reflect evolutionary lag and be subject only to genetic drift. Interestingly, only two New World genera, Molothnis and Geospiza, are known to exhibit polymorphisms for nestling flange color, despite the potential for direct selection (host matching in Molothrns) or indirect selection (song types and bill size in Medium Ground-Finches). Because of the rarity of color polymorphisms among nonparasitic species, it seems plausible that selection has maintained this variation in cowbirds. Our data, however, support the lack of nestling discrimination among cowbird hosts, in line with the long-term pattern of generalist host use by individual Brown-headed Cowbirds (Gibbs et al. 1997). Further study of flange color may clarify our understanding of communication between young and adults.

ACKNOWLEDGMENTS

We thank the residents of Fort Clark Springs for allowing us to conduct our research in their community. We also thank W. E. Jensen, G. L. Norman, C. P. and J. C. Ortega, ). N. M. Smith, and S. G. Summers for unpublished data. M. Boyd, S. Coles, M. Cottam, S. Cyr, N. Marino, P. Sullivan, and A. Walleyn assisted in the field; H. L. Gibbs, L. DeSousa and L. Blanchard assisted in the lab; and D. Willard and curatorial staff at the Field Museum of Natural History prepared specimens. The manuscript benefited from comments by R. C. Fleischer, P. R. Grant, S. I. Rothstein, and an anonymous reviewer. This research was funded by a grant from the Natural Sciences and Engineering Research Council of Canada to S.G.S., University of Manitoba Graduate fellowships awarded to K.E. and H.R.M., Frank M. Chapman Memorial awards (American Museum of Natural History) to K.E., and an American Ornithologists' Union Graduate Student Award to H.R.M.

LITERATURE CITED

ALDERSON, G. W., H. L. GIBBS, AND S. G. SEALY. 1999. Parentage and kinship studies in an obligate brood parasitic bird, the Brownheaded Cowbird (Molothnts ater), using microsatellite DNA markers. Journal of Heredity 90:182-190.

AMERICAN ORNITHOLOGISTS' UNION. 2000. Forty-second supplement to the American Ornithologists' Union Check-list of North American Birds. Auk 117:847-858.