Barn swallows (Hirundo rustica) differentiate between common cuckoo and sparrowhawk in China: alarm calls convey information on threat

Morphological mimicry of sparrowhawks Accipiter spp. by cuckoos acts to deceive hosts and thus promotes parasitism by cuckoos. Recent studies have suggested that common hosts are able to identify parasites and make special alarm calls as the result of co-evolution between hosts and brood parasites. Previous studies showed that barn swallows Hirundo rustica could distinguish between common cuckoo Cuculus canorus and sparrowhawk Accipiter nisus in Denmark, but seemed unable to make this distinction in China. However, these previous studies did not investigate vocal responses in terms of alarm calls. We tested whether barn swallows, which are regular hosts of the common cuckoo in China, could distinguish between the common cuckoo and sparrowhawk in China. In Heilongjiang, in the presence of the cuckoo and, in Hainan, in its absence, we used dummies of common cuckoo, sparrowhawk, and Oriental turtle dove Streptopelia orientalis (neutral control) to induce and record alarm calls that were used to estimate response to playbacks. Barn swallows responded more strongly to playbacks of swallow hawk alarm calls than in response to swallow cuckoo or swallow dove alarm calls. Given that alarm calls conveyed information about the presence of potential threats, our study showed that barn swallows from both study regions in China were able to distinguish between common cuckoo and sparrowhawk and respond accordingly in their vocal behavior.