Effect of Molothrus bonariensis parasitism on nests daily survival rate of Polioptila dumicola

Brood parasitism is one of birds’ main causes of nest failure. Molothrus bonariensis parasitism is relevant for Polioptila dumicola, as the parasitism event usually implies nest abandonment due to the host egg’s breakage. Taking into account that modified environments support larger populations of M. bonariensis, the aim of the study was to evaluate if there is a higher rate of nest failure due to M. bonariensis parasitism in isolated forest zones during laying and incubation stages. Nests were systematically searched and monitored for 2 breeding seasons in northeastern of Buenos Aires province (Argentina), recording the encounter date, initiation date (first egg layed) and fate (success or failure). A nest was considered successful if a host egg hatched. The study area was divided into 2 plots: isolated forest fragments immersed in a pasture matrix («isolated») and forest fragments with greater continuity and connectivity («continuous»). We monitored 33 nests in isolated forests and 58 in continuous forests. We built models with the variables of interest using MARK software. The best model included the additive effect of forest type and the occurrence of parasitism, indicating that parasitism had a negative effect on nests daily survival rate, which also decreased in isolated forests. These results are relevant for the conservation of P. dumicola populations and for taking action against the increasing degradation of Talares native forests.