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.

育幼收养行为在鸟类中已被广泛报道，但现有收养研究大多聚焦于鸟类的雏鸟阶段，而离巢幼鸟的收养现象则鲜少被深入探讨，尤其在雀形目鸟类中。本研究历经17年的野外观察，收集了238窝欧乌鸫（Turdus merula）的育幼结局数据：其中171窝的离巢幼鸟仅由亲生父母抚育，24窝的幼鸟发生了收养行为，另有43窝至少有1只幼鸟获得养父母的抚育。本研究的分析结果表明，欧乌鸫的离巢幼鸟收养行为发生的条件，契合两类解释鸟类收养现象的经典假说的预测。其一，收养行为多发生在被收养幼鸟年龄与养父母自身后代大致相近，且亲生父母与养父母的巢间距较近的情境中——此类场景下养父母难以准确识别亲生幼鸟，契合繁殖错误假说（Reproductive Error Hypothesis）。其二，部分收养案例的巢间距与被收养幼鸟和养父母自有幼鸟的年龄差，是前述易引发识别错误的收养条件的两倍；此类更大的巢间距与年龄差，则契合代际冲突假说（Intergeneration Conflict Hypothesis）。