Nesting Success in Brown-and-Yellow Marshbirds: Effects of Timing, Nest Site, and Brood Parasitism

Abstract

The Brown-and-yellow Marshbird (Pseudoleistes virescens) is a nonterritorial blackbird with helpers at the nest that inhabits temperate marshy areas and humid grasslands of Argentina, Uruguay, and Brazil. We estimated nesting success of marshbirds throughout the breeding season (late September to mid-December) and analyzed the effects of brood parasitism and nest-site selection (i.e. nesting substrate) on daily survival rates at different stages of the nesting cycle. Most nests were started in November, and these nests had a higher daily survival rate than those initiated in October or December. Only 13.3% of the nests fledged chicks. Predation accounted for 77% of nest losses, whereas the other failures resulted from nest desertion, in most cases after the loss of several eggs. Sixty-five percent of the nests were parasitized by Shiny Cowbirds (Molothrus bonariensis), but brood parasitism did not reduce nest survival. Daily survival rates increased from egg laying and incubation to the brood-rearing stage. Nesting success was three times higher in nests built in native black rushes and pampa grasses than in nests built in two exotic species of thistle. Eighty-nine percent of the nests were built in thistles, which were the most abundant plants in the study area. However, many black rushes and pampa grasses remained unoccupied.