Mechanisms of Avian Egg Recognition: Possible Learned and Innate Factors

THE belief that at least some passerine birds reject nonmimetic eggs placed in their nests was confirmed long ago by the experiments of Swynnerton (1916, 1918) and Rensch (1924). The latter worker (Rensch 1925) went on to question whether such birds actually recognize their own eggs (rejection by true egg recognition) or whether they simply reject any egg that differs from the majority (rejection by discordancy). Rensch was interested in egg recognition because he believed it to be vital to the evolutionary interactions between brood parasites and their hosts. Whether birds rejected eggs on the basis of true recognition or discordancy, the behavior would still function as an efficient antiparasite adaptation because brood parasites generally deposit only one egg in each host nest. Although Rensch's (1925) experiments have been widely interpreted as demonstrating rejection by discordancy (e.g. Welty 1963), the bulk of his results actually indicate true egg recognition (Rothstein 1970). Numerous recent experiments and a literature review suggest that most or all passerines that reject foreign eggs practice true egg recognition (Rothstein 1974). Nearly all these recent experiments, though, were conducted on birds that had completed their clutch. The fact that birds that reject foreign eggs practice true egg recognition after completing their clutch prompts two significant questions: (1) Is this recognition as highly developed at earlier stages of the breeding cycle? (2) Which components of rejection by true egg recognition are primarily learned and which are primarily innate? This paper reports on experiments that were designed to deal with these two questions. The new experiments reported here were done on naturally breeding Gray Catbirds (Dumetella carolinensis). Previous experiments showed that catbirds are extremely intolerant of foreign eggs placed in their nests. Single artificial or real cowbird eggs experimentally added to 30 catbird nests in eastern North America (Connecticut, Maryland, and Michigan) were all removed by the catbirds. Catbirds in western North America (Manitoba and Nebraska) may be slightly more tolerant, with 22 of 25 nests yielding ejections of single cowbird eggs. In 17 additional catbird nests (from Connecticut and Maryland), where experiments resulted in clutches consisting of only cowbird eggs or of only one catbird egg and two or more cowbird or other type of foreign eggs, the catbirds ejected the foreign eggs in every instance (see Rothstein 1974 for details).