Data from: Brood size constrains the development of endothermy in blue tits

Altricial birds are unable to maintain body temperature when exposed to low ambient temperatures during the first days after hatching. Thermoregulatory capacity begins to form as postnatal development progresses, and eventually nestlings become homeothermic. Several factors may influence this development at both the level of the individual and the level of the whole brood, but to our knowledge no studies have focused on the effect of brood size per se on the development of endothermy in individual nestlings. We performed cooling experiments on blue tit (Cyanistes caeruleus) nestlings in the field, to study how different experimental brood sizes affected the development of endothermy in individual nestlings and the thermal environment experienced by the whole brood in the nest. Nestlings from all experimental brood sizes showed a decrease in cooling rate as they grew older, but birds from reduced broods showed an earlier onset of endothermy compared with nestlings from enlarged and control broods. This difference manifested during early development and gradually disappeared as nestlings grew older. The thermal environment in the nests differed between treatments during nestling development, such that nest temperature in reduced broods was lower than that in enlarged broods during all days and during nights at the end of the experimental period. We suggest that the development of endothermy in blue tit nestlings is not ontogenetically fixed, but instead may vary according to differences in developmental, nutritional and thermal conditions as determined by brood size.晚成鸟（Altricial birds）在孵化后的最初几日，无法在低环境温度下维持体温。随着胚后发育（postnatal development）推进，其体温调节能力（thermoregulatory capacity）逐步形成，最终雏鸟将具备恒温能力。多种因素可在个体乃至整个窝雏群层面影响这一发育进程，但据我们所知，目前尚无研究聚焦于窝雏数（brood size）本身对单个雏鸟内温性（endothermy）发育的影响。我们针对野外生境中的青山雀（Cyanistes caeruleus）雏鸟开展了降温实验，旨在探究不同实验调控的窝雏数如何影响单个雏鸟的内温性发育，以及巢内整个窝雏群所经历的热环境。所有实验窝雏数组的雏鸟，其降温速率均随日龄增长而降低；但相较于扩群组（enlarged broods）与对照组（control broods），减群组（reduced broods）的雏鸟更早启动内温性发育。该差异在发育早期显现，并随雏鸟生长逐渐消失。在雏鸟发育阶段，不同处理组的巢内热环境存在显著差异：在实验周期末尾的所有日间与夜间，减群组的巢温均低于扩群组。我们认为，青山雀雏鸟的内温性发育并非由个体发育的固有程序固定决定，而是可根据窝雏数所决定的发育、营养与热环境条件的差异发生改变。