Do rufous common cuckoo females indeed mimic a predator? An experimental test

The similarity of common cuckoos Cuculus canorus to raptors is accepted as a classic example of predator mimicry. However, cuckoo females are polymorphic: grey females are similar to sparrowhawks Accipiter nisus , while rufous females were assumed to mimic kestrels Falco tinnunculus . Previous evidence based on dummy experiments with grey females consistently showed that both hosts and non‐hosts recognize this brood parasite by its yellow eye and barred underparts. However, these traits are absent in kestrels. Host responses also do not covary geographically with local abundance of supposed models (sparrowhawks/kestrels). These patterns cast doubts on the kestrel‐mimicry hypothesis. Here, we show experimentally for the first time that small birds that are unsuitable as hosts indeed do not mistake rufous cuckoos for kestrels: both tree sparrows Passer montanus and house sparrows P. domesticus feared grey cuckoos (similarly to sparrowhawks and kestrels) but ignored rufous cuckoos (similarly to innocuous Eurasian collared doves Streptopelia decaocto ). These results provide further support for the hawk‐mimicry hypothesis, but reject the kestrel‐mimicry hypothesis. Colour polymorphism in birds is determined genetically, follows simple Mendelian rules and affects only colour but not patterns. These facts and striking similarities between plumages of rufous females and cuckoo fledglings suggest that the rufous morph is simply a colour alternative to the grey morph, did not evolve to mimic kestrels and might have arisen through paedomorphic retention of juvenile plumage to adulthood (neoteny). Research on the genetic, developmental and mechanistic basis of cuckoo plumage polymorphism will be especially revealing.