Mimicry of a harmless model (aggressive mimicry) is used by egg, chick and fledgling brood parasites that resemble the host's own eggs, chicks and fledglings. However, aggressive mimicry may also evolve in adult brood parasites, to avoid attack from hosts and/or manipulate their perception of parasitism risk. We tested the hypothesis that female cuckoo finches (
Anomalospiza imberbis
) are aggressive mimics of female
Euplectes
weavers, such as the harmless, abundant and sympatric southern red bishop (
Euplectes orix
). We show that female cuckoo finch plumage colour and pattern more closely resembled those of
Euplectes
weavers (putative models) than
Vidua
finches (closest relatives); that their tawny-flanked prinia (
Prinia subflava
) hosts were equally aggressive towards female cuckoo finches and southern red bishops, and more aggressive to both than to their male counterparts; and that prinias were equally likely to reject an egg after seeing a female cuckoo finch or bishop, and more likely to do so than after seeing a male bishop near their nest. This is, to our knowledge, the first quantitative evidence for aggressive mimicry in an adult bird, and suggests that host–parasite coevolution can select for aggressive mimicry by avian brood parasites, and counter-defences by hosts, at all stages of the reproductive cycle.
Nest prospecting brown-headed cowbirds ‘parasitize’ social information when the value of personal information is lacking