BROOD PARASITISM OF THE SHINY COWBIRD, MOLOTHRUS-BONARIENSIS, ON THE BROWN-AND-YELLOW MARSHBIRD, PSEUDOLEISTES-VIRESCENS

We studied the relationship between a generalist brood parasite, the S hiny Cowbird (Molothrus bonariensis) and one of its hosts, the Brown-a nd-Yellow Marshbird (Pseudoleistes virescens). Seventy-four percent of the nests found were parasitized. Although the parasite lays both whi te and spotted eggs, most of the cowbird eggs found in the nests of th is host were spotted. Artificial parasitism experiments showed that th e host rejected the cowbird white eggs. Shiny cowbird parasitism reduc ed the nesting success of the Brown-and-Yellow Marshbird mainly by pun ctures or cracks of the host eggs. The reduction of the nesting succes s of the parasite was due mostly to the loss of eggs in multiple paras itized nests. Shiny cowbird chicks were not outcompeted for food altho ugh they are smaller than the host chicks. The Brown-and-Yellow Marshb ird appears to be a very good host, capable of rearing up to four cowb ird chicks in a nest. We compared the nesting success of the Shiny Cow bird in Brown-and-Yellow Marshbird nests with its nesting success in t he other sympatric hosts studied.