An Experimental Study of Co-Evolution Between the Cuckoo, Cuculus canorus, and its Hosts. I. Host Egg Discrimination

SUMMARY (1) The nests (n=711) of twenty-four species of passerine birds were parasitized experimentally with model cuckoo eggs. (2) Among current favourite hosts, species for which the cuckoo lays a mimetic egg discriminated against model eggs unlike their own (reed warbler, meadow pipit, pied wagtail), while the dunnock, which is not mimicked, did not discriminate. (3) Other species, suitable as hosts (invertebrate diet, accessible nests) but rarely used by cuckoos, showed just as much rejection of model eggs unlike their own (chaffinch, blackbird, song thrush), or stronger rejection (spotted flycatcher, reed bunting) than the most strongly rejecting of current favourite hosts. (4) Two results suggest that the egg discrimination by suitable hosts has evolved in response to cuckoo parasitism. (a) Species unsuitable as hosts (thus with no history of interaction with cuckoos) mainly showed little if any rejection of model eggs unlike their own (seed-eaters-linnet, greenfinch, bullfinch; hole-nesters-great tit, blue tit, pied flycatcher, wheatear). (b) Meadow pipits and white wagtails in Iceland, where they are isolated from cuckoos, showed significantly less discrimination against eggs unlike their own than in Britain, where they are parasitized. (5) Species with smaller bills suffered greater rejection costs (own eggs damaged) and were more likely to reject model eggs by desertion than species with larger bills, which tended to reject by ejection. (6) Among species suitable as hosts, there was no tendency for smaller-billed species to reject less. Therefore rejection costs influenced the method of rejection but not rejection frequency.