Data from: Fledgling adoption in European Blackbirds: an unrecognised phenomenon in a well-known species

Adoption behaviour 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 two 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.

鸟类幼崽收养行为已广为人知，但现有多数收养研究均聚焦于鸟类的巢内雏鸟阶段，而离巢幼鸟的收养现象则远未得到充分研究，在雀形目鸟类中尤甚。本研究历经17年的野外观测，共收集了238窝乌鸫（Turdus merula）的幼鸟抚育命运数据。其中171窝的离巢幼鸟仅由亲生父母抚育，24窝的幼鸟出现收养行为，另有43窝至少有1只幼鸟由养父母代为抚育。研究分析显示，乌鸫的离巢幼鸟收养行为符合两类解释鸟类收养现象的经典假说的预测条件。其一，收养行为多发生在养父母的自有后代与被收养幼鸟年龄相近，且亲生父母巢穴与养父母巢穴间距较近的场景中：此类情境会导致养父母难以准确识别自身幼鸟，这与**繁殖错误假说（Reproductive Error Hypothesis）**的推论相符。其二，当巢穴间距、被收养幼鸟与养父母自有幼鸟的年龄差均为错误识别场景下的两倍时，同样会出现收养行为。此类更大的巢穴间距与年龄差，符合**代际冲突假说（Intergeneration Conflict Hypothesis）**的预期。