Responses of Some Rare Cuckoo Hosts to Mimetic Model Cuckoo Eggs and to Foreign Conspecific Eggs

Abstract

Reactions of Willow Warblers Phylloscopus trochilus, Reed Buntings Emberiza schoeniclus, Blackcaps Sylvia atricapilla and Bluethroats Luscinia svecica, towards mimetic model Cuckoo Cuculus canorus eggs were tested by exchanging one of their eggs for a model plastic egg. Reactions of Willow Warblers, Reed Buntings and Bluethroats towards a foreign conspecific egg were also tested. Some Willow Warblers were also tested with Great Tit Parus major eggs. The Willow Warblers, Reed Buntings and Blackcaps rejected mimetic model eggs at a high rate (90-100%), while the Bluethroats showed a lower (but not statistically significant) rejection rate (63%). The Willow Warblers and the Bluethroats accepted conspecific eggs, but the Reed Buntings rejected some of these eggs (3 of 8 trials). Great Tit eggs, which resemble Willow Warbler eggs in colour and pattern but are bigger (although considerably smaller than Cuckoo eggs), were rejected in 5 of 6 trials. The responses of the above species towards mimetic model eggs were similar to those previously reported for non-mimetic eggs. This behaviour pattern is important in providing an explanation for their present status as only rare hosts of the Cuckoo.