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

MERMOZ, ME; JUAN C. REBOREDA

AUK

Año: 1998 vol. 115 p. 871 - 878

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 its nesting success throughout the breeding season (late September to mid-December) and analyzed the effect of brood parasitism and nest-site selection (i.e. nesting substrate) on daily survival rates (DSR) 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.  With DSR data, we estimated that only 13.3% of the nests are successful.  Predation accounted for 77% of nest losses while the other failures were the result of 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 the egg laying and incubation stages to the brood-rearing stage.  Nesting success was three times higher in nests built in native black rushes or pampa grasses than in exotic thistles.  Eighty-nine percent of the nests were built on thistles which were the most abundant plant in the study area.  However, many black rushes or pampa grasses remained unoccupied. Received 6 June 1997, accepted 18 February 1998.