Effect of predation and cowbird parasitism on the nesting success of two sympatric neotropical marshbirds

We compared the effect of nest predation and cowbird parasitism on the bree ding success of two simultaneously nesting ecologically similar blackbird s pecies that differ in their breeding strategies. The Scarlet-headed Blackbi rd (Amblyramphus holosericeus) is a monogamous species that performs territ orial defense. In contrast, the Brown-and-yellow Marshbird (Pseudoleistes v irescens) is a non-territorial monogamous breeder that performs mate guardi ng and has helpers at the nest. Both species suffered similar nest predatio n rates throughout their nesting cycle. However, the Brown-and-yellow Marsh bird suffered higher parasitism from I (62.6% vs 15.4%). Brood parasitism a ccounted for most of the egg losses and hatching failures in Brown-and-yell ow Marshbird. Parasitized nests had lower egg survival and hatching success than non-parasitized ones. Mean clutch size was 1.5 eggs larger in Brown-a nd-yellow Marshbird than in Scarlet-headed Blackbird. However, Scarlet-head ed Blackbird had higher hatching success than Brown-and-yellow Marshbird an d similar fledging success. Consequently, both species produced similar num bers of fledglings. We did not detect any relationship between the reproduc tive success of these species and their breeding strategies. The presence o f helpers at Brown-and-yellow Marshbird nests did not affect nest defense o r chick survival, but helpers might account for reduced parental effort by supplementing food delivery to chicks/fledglings.