Host parent responses to heterospecific parasite nestling alarm calls are independent of past and current experience with experimental brood parasitism.

Communication between parents and dependent offspring is critical not only during provisioning, but also in antipredator contexts. In altricial birds, a top cause of reproductive failure is nest predation, and alarm calls both by parents and chicks can serve to alert others and increase the likelihood of offspring escaping predation. Understanding the factors that determine the strength of parental antipredator responses to different nestling alarm calls can provide insight into parent-offspring recognition. The prothonotary warbler (Protonotaria citrea), a host of the obligate brood parasite, the brown-headed cowbird (Molothrus ater), never rejects cowbird young and raises the parasite together with its own offspring. To determine whether warbler parents learn cowbird nestling alarm calls, we presented experimentally parasitized or non-parasitized parents with playbacks of conspecific warbler, parasitic cowbird, and a harmless heterospecific control, eastern bluebird (Sialis sialis), nestling alarm calls. We recorded the latency to respond and the number of chips given by members of the resident warbler pair. We found that parents were most likely to respond to warbler nestling alarm calls, least likely to respond to bluebird calls, with a statistically intermediate likelihood of responding to cowbird calls. Critically, current and past parasitism status did not affect the likelihood of response to any playback or the number of chips given, however, currently parasitized parents had greater response latencies to playbacks than non-parasitized parents. These results suggest that warbler parents do not learn cowbird alarm calls from breeding experiences and, in turn, that cowbirds may employ a generalized, bet-hedging alarm call.

Exposure to a mimetic or non-mimetic model avian brood parasite egg does not produce differential glucocorticoid responses in an egg-accepter host species.

Avian obligate brood parasitism, a reproductive strategy where a parasite lays its egg into the nest of another species, imposes significant fitness costs upon host parents and their offspring. To combat brood parasitism, many host species recognize and reject foreign eggs (rejecters), but others are accepters that raise the parasitic progeny. Some accepter hosts may be unable to grasp or pierce parasitic eggs even if they recognize them as foreign eggs in the clutch, whereas other accepters may not have evolved the cognitive skillsets to recognize dissimilar eggs in the nest. Here we assessed the endocrine responses of an accepter host species to model parasitic eggs to address these two alternatives. We experimentally parasitized nests of a locally common host of the brood-parasitic brown-headed cowbird (Molothrus ater), the prothonotary warbler (Protonotaria citrea; a cowbird-egg accepter), with a mimetic or non-mimetic model cowbird-sized egg. Our goal was to determine whether they perceived the non-mimetic egg as a greater stressor by measuring circulating corticosterone levels. We added eggs to nests during the incubation stage and obtained blood plasma samples from females on the nest 2 h later, using females with unmanipulated clutches as controls. Incubating females showed no differences in baseline plasma corticosterone levels between our different treatments. We conclude that exposure to foreign eggs does not activate the hypothalamic-pituitary-adrenal axis of prothonotary warbler hosts in this experimental paradigm.