RECOGNITION ERRORS AND PROBABILITY OF PARASITISM DETERMINE WHETHER REED WARBLERS SHOULD ACCEPT OR REJECT MIMETIC CUCKOO EGGS

Reed warblers sometimes make recognition errors when faced with a mime tic cuckoo egg in their nest and reject one or more of their own eggs rather than tile foreign egg. Using the framework of signal detection theory, we analyse responses to model eggs to quantify the costs and b enefits of acceptance versus rejection in parasitized and unparasitize d nests. We show that below a threshold of 19-41% parasitism, the warb lers should accept mimetic cuckoo eggs because the costs of rejection outweigh the benefits, whereas above this threshold they should reject . The warblers behaved as predicted; when they saw a cuckoo at their n est they usually showed rejection, but without the sight of the cuckoo they behaved appropriately for the average parasitism rate in Britain (6%) and tended to accept.