Is it possible to determine host-specificity of shiny cowbirds (Molothrus bonariensis) by egg coloration?

DE LA COLINA, M. A.; MAHLER, B.; REBOREDA, J. C.

Cornell University, Ithaca, EEUU

Congreso; 12th International Behavioral Ecology Congress. Cornell University; 2008

ISBE

Obligate avian brood parasites lay their eggs in nests of other host species, which thereafter provide all parental care. The shiny cowbird (Molothrus bonariensis) is an extreme host generalist at population level, but uses only a few hosts at individual level. The aim of this study was to determine if the genetic differentiation found between shiny cowbird females that parasitize the chalk-browed mockingbird (Mimus saturninus) and the Southern house wren (Troglodytes musculus) is associated with a differentiation in egg morphology and coloration. We took morphometric measures of shiny cowbird eggs found in chalk-browed mockingbird (n=86) and Southern house wren (n=20) nests and analyzed the coloration using both digital photography and reflectance spectrometry. Shiny cowbird eggs found in nests of chalk-browed mockingbirds were wider than those found in southern house wren ones, and also showed increased volume and asymmetry. We also found differences in background color between eggs found in nests of both hosts, although these differences were not associated with host egg color. Our results show that genetic differences between shiny cowbird females that parasitize chalk-browed mockingbirds and Southern house wrens are correlated with morphological and color differences of their eggs, which is consistent with the hypothesis of host-specific races and a maternal inheritance of egg size and color.