Host discrimination between own and parasitic young explain low frequency of shiny cowbird parasitism in bay-winged cowbird nests

DE MÁRSICO, M.C.; REBOREDA, J.C.

Cornell University, USA

Congreso; 12th Biennial Congress of the International Behavioral Ecology Society; 2008

The shiny cowbird, Molothrus bonariensis, is an extreme generalist avian brood parasite that rarely parasitizes the bay-winged cowbird, Agelaioides badius, a species that closely resemble the parasite in diet and body size. We tested the following hypotheses for explaining the low frequency of shiny cowbird parasitism observed in this host: 1) bay-winged cowbirds have effective antiparasitic defenses (i.e. rejection of parasitic females or eggs) against shiny cowbirds, and 2) shiny cowbirds reproductive success in bay-winged cowbird nests is lower than that in frequently parasitized hosts. The study was carried out in the Province of Buenos Aires (Argentina) during 2004-2006 breeding seasons. We conducted two experiments: 1) presentation of dummy models of parasitic females and control species to nesting bay-winged cowbirds, and 2) artificial parasitism of bay-winged cowbird nests with freshly laid shiny cowbird eggs. We found that: 1) bay-winged cowbirds only rarely responded aggressively against female shiny cowbirds and did not reject parasitic eggs, and 2) egg survival, hatching success, and chick survival of shiny cowbirds in bay-winged cowbird nests were as high as in nests of frequently parasitized hosts. However, bay-winged cowbirds stopped providing care to parasite´s fledglings once they left the nest, showing the capacity to discriminate among their own and shiny cowbird youngs. We discuss the mechanism by which host discrimination may have lead to the pattern of parasitism observed today.