Nestling responses to adult food and alarm calls: 1. Species-specific responses in two cowbird hosts

Begging by nestlings can prove costly, either through energy expenditure if food-bearing parents are not present, or through increased predation risk. Therefore, parents may provide cues to modulate begging. We investigated responses of 7-day old nestlings of eastern phoebes, Sayornis phoebe, and red-winged blackbirds, Agelaius phoeniceus, to various adult calls. Phoebes begged strongly to playback of conspecific food calls but not to other vocal stimuli, and only weakly to manual stimulation. They had no specific response to phoebe alarm calls. We suggest that phoebe alarms, which were given mainly when a partner was nearby, at both egg and chick stages, function primarily to warn mates. Red-winged blackbirds begged most readily to manual stimulation and ceased begging, and crouched, specifically to conspecific alarm calls. Therefore, in phoebes begging is 'switched on', and in red-winged blackbirds it is 'switched off', by parental calls. We suggest that for species like red-winged blackbirds, which nest on flexible substrates, nestlings readily beg to vibrational cues such as nest movement, so parent alarms are important to switch off begging at inappropriate times. For species like phoebes, which nest on rigid substrates, food calls induce begging in the absence of vibrational stimuli, and may replace the need for alarm calls to nestlings. The marked differences seen in these hosts of the brown-headed cowbird, Molothrus ater, raises the question of whether nestlings of this generalist brood parasite can eavesdrop on such diversity in host cues, a problem we address in our companion paper.