Parasitism by Shiny Cowbirds of Rufous-Bellied Thrushes

I studied the interaction between a brood parasite, the Shiny Cowbird (Molothrus bonariensis), and a large host, the Rufous-bellied Thrush (Turdus rufiventris), in the Pampas Region of Argentina. Shiny Cowbird eggs were observed in 34 out of 70 (48.6%) thrush nests throughout the host's breeding season. The main damage inflicted by Shiny Cowbirds was the puncture of host eggs, which occurred in 55.9% of the parasitized nests and significantly reduced the host clutch size. Because of the host's short incubation period, cowbird eggs did not hatch more than one day before the host young. This small advantage of earlier hatching was not enough to outweigh the large size difference between the parasitic chicks and the host young. In 68.7% of experimentally-created broods containing one host and one cowbird chick, the cowbird chicks died of starvation. I conclude that the Rufous-bellied Thrush is a particularly poor host for Shiny Cowbirds because of its large size and short incubation period. The high rate of parasitism, even of such a poor host, indicates little selectivity by Shiny Cowbird females.