Received 2 February 2015; revised 31 March 2015; accepted for publication 31 March 2015The 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 rufousfemales were assumed to mimic kestrels Falco tinnunculus. Previous evidence based on dummy experiments withgrey females consistently showed that both hosts and non-hosts recognize this brood parasite by its yellow eye andbarred underparts. However, these traits are absent in kestrels. Host responses also do not covary geographicallywith 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 hostsindeed do not mistake rufous cuckoos for kestrels: both tree sparrows Passer montanus and house sparrowsP. domesticus feared grey cuckoos (similarly to sparrowhawks and kestrels) but ignored rufous cuckoos (similarlyto 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 determinedgenetically, follows simple Mendelian rules and affects only colour but not patterns. These facts and strikingsimilarities between plumages of rufous females and cuckoo fledglings suggest that the rufous morph is simply acolour alternative to the grey morph, did not evolve to mimic kestrels and might have arisen throughpaedomorphic retention of juvenile plumage to adulthood (neoteny). Research on the genetic, developmental andmechanistic basis of cuckoo plumage polymorphism will be especially revealing. © 2015 The Linnean Society ofLondon, Biological Journal of the Linnean Society, 2015, 00: 000–000.