Antiparasitic behaviors of Red-winged Blackbirds (Agelaius phoeniceus) in response to simulated Brown-headed Cowbirds (Molothrus ater): further tests of the frontloaded parasite-defense hypothesis

Abstract

ABSTRACT The Red-winged Blackbird (Agelaius phoeniceus; redwing) is a commonly used accepter host species that incubates eggs and cares for nestlings and fledglings of the obligate brood parasitic Brown-headed Cowbird (Molothrus ater; cowbird). This host species, however, may reduce the risk of parasitism with a frontloaded antiparasite strategy in which it attacks parasites that approach active host nests. To test this frontloaded parasite-defense hypothesis (FPDH), we presented taxidermic models of a female Northern Cardinal (Cardinalis cardinalis), which represents no threat to redwings, a male cowbird, which cannot lay a parasitic egg, and a female cowbird, together with species- and sex-specific vocalization playbacks for 5 min. We conducted these presentations at 25 active redwing nests at Newark Road Prairie in south-central Rock County, Wisconsin, USA, where 18% of redwing nests were parasitized by cowbirds in 2015. As predicted by the FPDH, the female cowbird mount elicited the most aggressive re...