Out on their own: a test of adult-assisted dispersal in fledgling brood parasites reveals solitary departures from hosts

Brood-parasitic offspring sexually (mis)imprinting on the foster parents is considered one of the greatest constraints to the evolution of interspecific avian brood parasitism. While most nonparasitic juvenile birds learn the behaviours and mate choice preferences from their own parents, social parasites must avoid misimprinting on their host species' phenotype in order to accurately recognize conspecifics. One possible mechanism to assure accurate species recognition by juvenile parasites is to begin to associate with adult parasitic conspecifics, known as the ‘first contact’ scenario, whereby adult female parasites facilitate the dispersal of their offspring away from hosts, thus providing accurate referents for conspecific recognition. Using an automated radiotelemetry system, we determined the presence or absence (every 1–2min during three breeding seasons; 516 315 search occasions) of radiotagged parasitic adult female brown-headed cowbirds, Molothrus ater, and compared their departures from a forest study site with those of genetic offspring or experimentally transplanted (nonrelated) juvenile cowbirds within the female's egg-laying range. Contrary to our predictions, we found no support for the facilitation of juvenile cowbird dispersal by adult female cowbirds. Juvenile cowbirds typically were not located within their mother's home range when departing the forest and, likewise, departure events for natal and experimentally transplanted juveniles (<2%) did not overlap temporally with the departure of the genetically assigned mother or with the departure of other local radiotagged female cowbirds. Surprisingly, we found that juvenile cowbirds primarily departed from the host's territory at sunset, when adult female cowbirds are infrequently present within the forest. Our results suggest that the solitary nocturnal roosting behaviour of juvenile cowbirds may facilitate independence from their hosts, thus minimizing the risk of misimprinting on heterospecific phenotypes. This strategy may also indirectly promote conspecific interactions, providing further evidence for the importance of independent spatial and social preferences of hosts and brood parasites in the evolution of avian brood parasitism.