Helpers-at-the nest mitigate the costs of cowbird brood parasitism in brown-and-yellow marshbirds

Cooperative breeders have been proposed to be differentially used by brood parasites as hosts. This could be because:1) helpers-at-the nest may facilitate nest location for brood parasites, and/or improve parental care of the brood including that of the brood parasite itself; or 2) helpers may reduce the impact of parasitism on hosts reducing selection pressure for the development of antiparasitic defenses. Using a 9-year database, we analyzed the relationship between the number of helpers-at-the-nest in the brown-and-yellow marshbird, Pseudoleistes virescens , and the probability and intensity of brood parasitism by the generalist shiny cowbird, Molothrus bonariensis . We evaluated the costs of brood parasitism and their relationship with the number of helpers-at-the nest. Neither the probability and intensity of brood parasitism by cowbirds nor the number of cowbird fledglings produced were related to the number of helpers attending the nests. However, the number of helpers increased the probability that parasitized nests produce a shiny cowbird fledgling. On the other hand, the number of helpers reduced many costs of parasitism, particularly decreasing the losses and hatching failures of host eggs. Overall, the number of brown-and-yellow marshbird fledglings produced decreased with a higher intensity of parasitism while it increased with the number of helpers. The number of helpers-at-the-nest in brown-and-yellow marshbirds mainly mitigates the costs of parasitism but does not favor the breeding success of the shiny cowbird. The results obtained indicate that cooperative breeding in brown-and-yellow marshbirds would play a minor role in host selection by this highly generalist brood parasite.