Sexual imprinting and the origin of obligate brood parasitism in birds

We discuss two pathways along which obligate brood parasitism (OBP) may evo lve and examine some of the critical steps that must be passed by letting g reat tits Parus major be reared by blue tits Parus caeruleus in a field exp eriment. The cross-fostered chicks survived well in blue tit nests, but the ir local recruitment and reproductive success was much lower than that of c ontrols. The effect was strongest when great tits grew up with siblings of the host species rather than with conspecific siblings in blue tit nests. T he low success seemed to be caused by misimprinting because the cross-foste red birds behaved like blue tits in several aspects (species association, a larm calls, and aggressive response by resident females to caged intruders) . Some birds of both sexes were apparently so strongly imprinted that they did not attract or accept a social mate of their own species. We conclude t hat imprinting may be necessary for OBP to evolve in birds because the para site must be attracted to the nests of the host species to add eggs and the reby continue the parasitic life cycle. However, strong imprinting may also prevent OBP from occurring if parasitic offspring seek a mate from the hos t species.