Nestling barn swallows do not discriminate between nestling begging calls and nestling-like male courtship calls

Abstract Intraspecific mimicry is possibly widespread but is difficult to demonstrate, because the standard procedure for testing mimicry, i.e., testing responses of signal receivers with little prior exposure to mimics, is hard to accomplish. To study the effect of signal design of mimics without being confounded by signal content, test subjects with limited exposure to mimics both in the past generations and in an individual experience would be required. Here, we focused on nestlings, in which experience on mimics and selection on discriminating mimics from models are limited. By using playback experiments, we tested whether or not nestling-like male courtship calls (“mimic”) and nestling food-begging calls (“model”) elicited similar positive responses from nestlings of the barn swallow Hirundo rustica . We found that nestling barn swallows exhibit similar positive responses, i.e., showing begging behaviour to the two vocalizations, and these responses were significantly higher scores than nestling responses to male songs. Although both nestling-like male courtship calls and song are used in male pairing attempts, nestlings responded negatively to the latter, but not the former, by hiding in the nest. The current experiment, together with preceding studies, supports the hypothesis that nestling-like male courtship calls evolved as intraspecific mimicry to elicit signal receivers’ positive, rather than negative, reaction to nestlings. Most previous studies focused solely on matured individuals, which is reasonable because mimetic signals are mainly used among them, but immature individuals, which have limited exposure to mimetic signals, would be a suitable subject for studying intraspecific mimicry.