Nestling responses to adult food and alarm calls: 2. Cowbirds and red-winged blackbirds reared by eastern phoebe hosts

We tested whether nestlings of a generalist brood parasite, the brown-headed cowbird, Molothrus ater, had special adaptations for tuning into host parent food and alarm calls. We compared the responses of 7-day-old cowbird nestlings reared by eastern phoebes, Sayornis phoebe, with those of a related, nonparasitic icterid, the red-winged blackbird, Agelaius phoeniceus, reared experimentally in phoebe nests. Cowbirds begged readily to manual stimulation but not to playbacks of either phoebe or red-winged blackbird food calls, irrespective of whether they had been reared alone, or together with phoebe nestlings (and so had experience of nestmates which responded strongly to phoebe food calls). Red-winged blackbirds reared by phoebes did not differ from those reared by conspecifics. They responded as readily as cowbirds to manual stimulation, but also (surprisingly) begged most strongly to phoebe food calls, whose rapid chatter might act as super-normal red-wing food calls. Both cowbirds and red-winged blackbirds reared alone by phoebes reduced begging most to red-winged blackbird alarms, suggesting that they share an innate icterid response, although this selective response was less marked in cowbirds reared alongside phoebe young. We conclude that cowbirds do not tune into phoebe host parent vocalizations. Given the problem of attending to such a variety of host species food and alarm calls, it may be most adaptive for generalist brood parasites simply to respond to general vibrational or visual cues likely to be encountered across all hosts.