Female common cuckoos imitate hawks to fool reed warblers, reports a study published this week in Nature Ecology & Evolution. The study finds that, after laying their eggs in reed warbler nests, female cuckoos give a hawk-like call, distracting the reed warblers and reducing the chance that the parasitic cuckoo eggs will be noticed.

Although male cuckoo calls are thought to play a role in territorial defence - only males make the famous 'cuck-oo' call that gives the bird its name - the role of the female call, known as a 'chuckle' or 'bubble' call, due to its laughter-like sound, remains a mystery. However, the sound is similar to the call of a sparrowhawk, a reed warbler predator.

Jennifer York and Nicholas Davies investigated why female cuckoos call conspicuously after they have planted their eggs in a host nest, which risks drawing attention to their crime. They played recordings of the calls of male and female cuckoos, a sparrowhawk, and a non-threatening control - a collared dove - to reed warblers, and then observed the reed warblers’ behaviour.

They find that the reed warblers respond with the same raised vigilance to female cuckoo calls as to hawk calls by shifting their attention from the clutch to their own safety, but they ignore male cuckoo and collared dove calls. The authors then played the different calls of another species of bird - tits - that are predated by sparrowhawks but not parasitized by cuckoos. The tits showed the same response to female cuckoo calls as to sparrowhawks, suggesting that it is the similarity of the female cuckoo call to that of sparrowhawks that elicits the vigilant behaviour.

This study, combined with the fact that parasitic species across the Cuculinae cuckoo subfamily are more likely to have different female and male calls, suggests that sex-specific calls may form a key part of cuckoos’ parasitic lifestyle.