Nestling birds use a conspicuous begging display, which includes loud begging calls, to solicit food from their parents. Although these calls are important in communicating offspring need, predators can use begging calls to locate nests. Parents may, however, counteract nestling vulnerability by giving alarm calls to silence calling nestlings when predators are nearby. This defense has been observed in grass- and reed-nesting species, whose nestlings beg to vibrational cues of a parent's arrival, even in the absence of the parents. It is considered less likely to occur, however, in cavity-nesting species, because the rigid substrate of the nest does not provide vibrational cues, and nestlings instead wait for a parental food call before begging. Cavity-nesting Tree Swallows (Tachycineta bicolor) use food calls to solicit begging, which suggests that parental alarm calls are not required to silence them. Older nestling Tree Swallows, however, beg in the absence of their parents, which suggests that responding to parental alarm calls with silence might be adaptive after all, at least at older ages. The goal of our study was to determine whether nestling Tree Swallows alter their begging behavior in response to parental alarm calls and, if so, whether this response varies with age. We found that older nestlings (15 days posthatch) reduced calling and crouched in the nest in response to playback of parental alarm calls, unlike younger nestlings (5 and 10 days posthatch). Our results suggest that parent Tree Swallows might reduce predation risk caused by calling nestlings, especially older nestlings, by giving alarm calls when predators are near their nests. Counter to our prediction, nestlings of cavity-nesting species may indeed respond to alarm calls, particularly if they beg in the absence of parents.
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Vol. 131 • No. 3