Why Screaming Cowbirds parasitize almost exclusively Bay-winged Cowbirds and Shiny Cowbirds avoid doing it?

DE MÁRSICO, M.C.; REBOREDA, J.C.

Veracruz, México

Congreso; 4th. North American Ornithological Conference; 2006

Ornithological Societies of North America

Bay-winged Cowbirds, _Agelaioides badius_ are used as hosts by two brood parasites: specialist Screaming Cowbird, _Molothrus rufoaxillaris_ (parasitism: 83-100%), and generalist Shiny Cowbird, _M. bonariensis_ (parasitism: 17-24%). In this work we tested two hypotheses to explain host specialization by Screaming Cowbirds and low use of Bay-winged by Shiny Cowbirds: non-used or rare hosts have better antiparasitic defenses against the less common parasite, and chick survival of the parasite is lower in nests of the non-used or rare hosts. Antiparasitic defenses of two common Shiny Cowbird hosts (Chalk-browed Mockingbirds and House Wrens) were more intense against Shiny than Screaming Cowbirds while antiparasitic defenses of Bay-winged Cowbirds were more intense against Screaming than Shiny Cowbirds. Survival of Screaming Cowbird chicks in nests of mockingbirds and wrens was lower than in nests of Bay-winged Cowbirds. Survival of Shiny and Screaming Cowbird chicks in nests of Bay-winged Cowbirds did not differ, but Shiny Cowbird´s young did not receive parental care after they left the nest. Our results show that host specialization in Screaming Cowbirds is associated to low chick survival in alternative hosts, and low use of Bay-winged Cowbirds by Shiny Cowbirds, to host discrimination of Shiny Cowbird´s fledglings.