Natural and experimental nest-switching in Barn Owl Tyto alba fledglings

In altricial birds fledglings may be selected for visiting nests of non-bio logical parents when the rearing conditions in the nest of origin are poor or when they have difficulty in finding food just after being independent. When nest-switching is frequent and incurs fitness costs to the foster fami ly, kin-recognition mechanisms may evolve to discriminate against foreign f ledglings. Although nest-switching has been reported in several altricial b irds, so far it has not been experimentally investigated whether nest-switc hers grow as well as their non-switched nestmates. For a test, I performed an experiment in the Barn Owl Tyto alba a non-colonial species with natural ly occurring nest-switching. Single fledglings were cross-fostered between pairs of nests, so that without altering brood size each family was given t he possibility to adopt a foreign fledgling. I also visited unmanipulated c ontrol broods at fledgling time. Three days later I captured the birds pres ent in experimental and control nests. Cross-fostered fledglings were still present in foster nests as often as their non-switched nestmates and contr ol fledglings, and they showed the same body mass change as fledglings of o rigin. Thus, foster families tolerated or were unable to expel foreign fled glings from the nest. This study suggests that in the Barn Owl nest-switchi ng is an available option for fledglings to improve survival prospect when their parents are poor food provisioners or when they have difficulty in fi nding food after independence.