Data from: Sibling cooperation influences the age of nest leaving in an altricial bird

In altricial birds, siblings raised within a nest usually leave the nest within hours of each other, despite often differing considerably in age. The youngest members of the brood are typically underdeveloped at this time and less likely than their older siblings to survive outside the nest, yet they risk abandonment if they do not fledge with their older siblings. Nest leaving is usually initiated by the older offspring, which may delay this process to provide more time for their younger siblings to mature, increasing the younger siblings’ postfledging survival and their own inclusive fitness. We tested this hypothesis in a population of house wrens Troglodytes aedon and found that broods with broad age spans among siblings had longer nestling periods than broods with narrow age spans and that delayed fledging improves the survival and reproductive prospects of younger siblings, although at a potential cost to future siblings. We also manipulated age spans through cross-fostering and found that older foster nestlings postponed fledging when raised with younger broodmates, as predicted if the age of younger nestlings determines the time of fledging. Our results support kin-selection theory and demonstrate that the exact time of fledging is attributable, in part, to sib-sib interactions.晚成雏鸟类（altricial birds）中，同一巢穴抚育的同胞雏鸟通常会在数小时内先后离巢，尽管彼此间年龄往往存在显著差异。此时，同窝雏鸟中的最年幼个体通常发育尚未完全，相较年长同胞更难在巢外存活；但若未能与年长同胞一同离巢，它们便会面临被遗弃的风险。离巢行为通常由年长幼鸟发起，它们会推迟离巢进程，为年幼同胞争取更多发育时间，以此提升年幼雏鸟的离后存活率，同时提高自身的广义适合度（inclusive fitness）。我们在一个家鹪鹩（Troglodytes aedon）种群中对该假说进行了检验，结果发现：同胞间年龄跨度较大的窝雏，其留巢期长于年龄跨度较小的窝；且推迟离巢能够改善年幼同胞的存活与繁殖前景，尽管这可能会对后续繁殖的后代造成潜在代价。我们还通过交叉抚育（cross-fostering）操作调整了雏鸟的年龄跨度，结果发现：当年长的寄养幼鸟与年幼的同巢伙伴共同抚育时，它们会推迟离巢时间——这与“年幼雏鸟的年龄决定离巢时间”的理论预测相符。本研究结果支持亲缘选择理论（kin-selection theory），并证明：离巢的确切时间在一定程度上源于同胞间的互动行为。