A password for species recognition in a brood-parasitic bird.

Recognition of conspecifics is an essential precursor of sexual reproduction. Most mammals and birds learn salient features of their parents or siblings early in ontogeny and later recognize individuals whose phenotypes match the mental image (template) of relatives closely enough as conspecifics. However, the young of brood parasites are reared among heterospecifics, so social learning will yield inappropriate species recognition templates. Initially, it was inferred that conspecific recognition in brood parasites depended on genetically determined templates. More recently it was demonstrated that learning plays a critical role in the development of parasites' social preferences. Here we propose a mechanism that accommodates the interaction of learned and genetic components of recognition. We suggest that conspecific recognition is initiated when a young parasite encounters some unique species-specific signal or "password" (e.g. a vocalization, behaviour or other characteristic) that triggers learning of additional aspects of the password-giver's phenotype. We examined the possibility that nestlings of the obligately brood-parasitic brown-headed cowbird (Molothrus ater) could use a species-specific vocalization, the "chatter", as a password. We found that six-day-old nestlings responded (begged) significantly more frequently to playbacks of chatters than to other avian sounds and that two-month-old fledglings approached playbacks of chatters more quickly than vocalizations of heterospecifics. Free-living cowbird fledglings and adults also approached playbacks of chatters more often than control sounds. Passwords may be involved in the ontogeny of species recognition in brood parasites generally.