Nesting success in brown-and-yellow Marshbirds: Effects of timing, nest site, and brood parasitism

The Brown-and-yellow Marshbird (Pseudoleistes virescens) is a nonterritoria l blackbird with helpers at the nest that inhabits temperate marshy areas a nd humid grasslands of Argentina, Uruguay, and Brazil. We estimated nesting success of marshbirds throughout the breeding season (late September to mi d-December) and analyzed the effects of brood parasitism and nest-site sele ction (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 Decembe r: Only 13.3% of the nests fledged chicks. Predation accounted for 77% of n est losses, whereas the other failures resulted from nest desertion, in mos t cases after the loss of several eggs. Sixty-five percent of the nests wer e parasitized by Shiny Cowbirds (Molothrus bonariensis), but brood parasiti sm did not reduce nest survival. Daily survival rates increased from egg la ying and incubation to the brood-rearing stage. Nesting success was three t imes 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 n ests were built in thistles, which were the most abundant plants in the stu dy area. However, many black rushes and pampa grasses remained unoccupied.