Brood Parasitism of the Shiny Cowbird, Molothrus- Bonariensis, on the Brown-and-Yellow Marshbird, Pseudoleistes-Virescens

MERMOZ, ME; JUAN C. REBOREDA

The Condor

Año: 1994 vol. 96 p. 716 - 721

We studied the relationship between a generalist brood parasite, the Shiny Cowbird (Molothrus bonariensis) and one of its hosts, the Brown-and-Yellow Marshbird (Pseudoleistes virescens). Seventy-four percent of the nests found were parasitized. Although the parasite lays both white and spotted eggs, most of the cowbird eggs found in the nests of this host were spotted. Artificial parasitism experiments showed that the host rejected the cowbird white eggs. Shiny cowbird parasitism reduced the nesting success of the Brown-and-Yellow Marshbird mainly by punctures or cracks of the host eggs. The reduction of the nesting success of the parasite was due mostly to the loss of eggs in multiple parasitized nests. Shiny cowbird chicks were not outcompeted for food although they are smaller than the host chicks. The Brown-and-Yellow Marshbird appears to be a very good host, capable of rearing up to four cowbird chicks in a nest. We compared the nesting success of the Shiny Cowbird in Brown-and-Yellow Marshbird nests with its nesting success in the other sympatric hosts studied.