We compared the effect of nest predation and cowbird parasitism on the breeding success of two simultaneously nesting ecologically similar blackbird species that differ in their breeding strategies. The Scarlet-headed Blackbird (Amblyramphus holosericeus) is a monogamous species that performs territorial defense. In contrast, the Brown-and-yellow Marshbird (Pseudoleistes virescens) is a non-territorial monogamous breeder that performs mate guarding and has helpers at the nest. Both species suffered similar nest predation rates throughout their nesting cycle. However, the Brown-and-yellow Marshbird suffered higher parasitism from Shiny (Molothrus bonariensis) and Screaming cowbirds (M. rufoaxillaris) than the Scarlet-headed Blackbird (62.6% vs 15.4%). Brood parasitism accounted for most of the egg losses and hatching failures in Brown-and-yellow Marshbird. Parasitized nests had lower egg survival and hatching success than non-parasitized ones. Mean clutch size was 1.5 eggs larger in Brown-and-yellow Marshbird than in Scarlet-headed Blackbird. However, Scarlet-headed Blackbird had higher hatching success than Brown-and-yellow Marshbird and similar fledging success. Consequently, both species produced similar numbers of fledglings. We did not detect any relationship between the reproductive success of these species and their breeding strategies. The presence of helpers at Brown-and-yellow Marshbird nests did not affect nest defense or chick survival, but helpers might account for reduced parental effort by supplementing food delivery to chicks/fledglings.