Red-winged blackbird aggression but not nest defense success is predicted by exposure to brood parasitism by brown-headed cowbirds

The brown-headed cowbird (Molothrus ater) is an obligate brood parasite known to use over 200 host species. The red-winged blackbird (Agelaius phoeniceus) is a commonly used accepter host that incubates cowbird eggs and cares for cowbird nestlings and fledglings. 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 responses and the female cardinal mount the least aggressive, as measured by number of times more than one male redwing responded and number of times the male host attacked the mount, and by Principal Component analyses yielding redwing aggressive behavior and intimidation scores. Contrary to the predictions of FPDH regarding the success of nest defense behaviors, male redwings responding at naturally parasitized nests were significantly more likely to attack the mount than males with nests that were not parasitized. We also compared our results with those of a study using the same methods and conducted in New York State where cowbird parasitism was rare. Wisconsin redwings were more aggressive toward the female cowbird mount than redwings in New York State. Red-winged blackbirds appear to frontload their antiparasite defenses and the aggressiveness, but the apparent success of those defenses depends on individual and population-level experience with parasites.