Fledgling adoption in European Blackbirds: an unrecognized phenomenon in a well-known species

Adoption behavior is well-known in birds, but the majority of adoption studies concern the nestling phase of birds' lives, whereas fledgling adoption is a much less well-known phenomenon, especially in passerines. During 17 years of observations, we collected data on the fate of 238 broods of European Blackbirds Turdus merula. In 171 cases fledglings were fed only by their own parents, in 24 cases the fledglings were given to adoption, while in 43 cases at least one fledgling was cared for by foster parents. Our analyses suggest that fledgling adoption in Blackbirds occurred under conditions that are consistent with the predictions of 2 hypotheses that explain the adoption phenomenon in birds. First, adoptions involved young fledglings of roughly the same age as the foster parents' offspring and in the context of a short distance between the biological parents and foster parents' nests: this gave rise to errors in foster parents recognizing their own young-in line with the Reproductive Error Hypothesis. Second, adoptions also occurred in instances where the distance between nests and the age difference between the adopted and the foster parents' own fledglings was twice as great compared to the conditions suggesting erroneous adoption. The longer distance between nests and the bigger age difference are in line with the Intergeneration Conflict Hypothesis.