Differences in egg morphology and coloration between screaming cowbird eggs laid in nests of two hosts

CHOMNALEZ, M.; DI GIACOMO, A.G.; REBOREDA, J.C.

Campos de Jordao, Brasil

Congreso; XXV International Ornithological Congress; 2010

International Ornithological Committee

The screaming cowbird (Molothrus rufoaxillaris) is one of the most specialized brood parasites. This species uses a single host, the bay-winged cowbird (Agelaioides badius) for most part of its distribution, but in northern Argentina and southeast Brazil they also parasitize the chopi blackbird (Gnorimopsar chopi). Recently, it has been shown that mDNA haplotype frequency distributions differ between screaming cowbird chicks from nests of both hosts. This indicates that nest choice by parasitic females is not random and that individual females preferentially parasitize nests of the same host. In this work we analyzed if females that use different hosts lay eggs that differ in morphology and coloration. We took morphometric measurements of screaming cowbird eggs laid in bay-winged cowbird and chopi blackbird nests and analyzed their coloration using reflectance spectrometry. Parasite eggs laid in bay-winged cowbirds were significantly longer and wider than those laid in chopi blackbirds. We classified parasite eggs in two different morphs, “green” and “brown”, according to background coloration. Green eggs had higher reflectance and green-chroma, and lower red-chroma than those classified as brown. The proportion of green eggs was significantly higher in chopi blackbirds than in bay-winged cowbirds. On average, parasite eggs laid in chopi blackbirds had higher reflectance and UV-blue chroma than those laid in bay-winged cowbirds. Our results indicate that the genetic differences found between screaming cowbird females that use different hosts are associated with differences in the size, color and brightness of their eggs.