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.对于晚成鸟类（altricial birds）而言，同巢抚育的雏鸟通常会在数小时内相继离巢，尽管它们的日龄往往存在显著差异。同窝雏鸟中最年幼的个体在此时通常发育尚未完全，相比年长同胞更难在巢外存活，但如果未能与年长同胞一同离巢，它们便面临被亲鸟遗弃的风险。离巢行为通常由年长的雏鸟发起，它们可能会推迟这一进程，为年幼同胞争取更多发育时间，从而提升年幼同胞的出飞后存活率，同时增加自身的广义适合度（inclusive fitness）。我们在屋鹪鹩（Troglodytes aedon）种群中对这一假说进行了检验，结果发现，同胞间日龄跨度较大的一窝雏鸟，其雏鸟留巢期时长较日龄跨度较小的窝更长；延迟出飞能够提升年幼同胞的存活与繁殖前景，尽管这可能会对未来的同胞产生潜在代价。我们还通过交叉寄养（cross-fostering）操控了同胞间的日龄跨度，结果发现，当与年幼的同巢伙伴一同被抚育时，寄养的年长雏鸟会推迟出飞时间，这与"年幼雏鸟的日龄决定出飞时间"的预测相符。我们的研究结果支持亲缘选择理论（kin-selection theory），并证明雏鸟的确切出飞时间在一定程度上归因于同胞间的互动。