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自身体温。随着产后发育进程推进，其体温调节能力逐步形成，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过程，但据我们所知，目前尚无研究聚焦于窝雏数（brood size）本身对单只雏鸟内温性（endothermy）发育的影响。我们在野外对青山雀（Cyanistes caeruleus）雏鸟开展了降温实验（cooling experiments），以探究不同实验性窝雏数如何影响单只雏鸟的内温性发育，以及巢内整个窝雏群所经历的热环境。所有实验性窝雏数组的雏鸟，其降温速率均随日龄增长而降低，但相较于增大窝雏组与对照组雏鸟，减小组雏鸟的内温性起始时间更早。这一差异在发育早期显现，并随雏鸟成长逐渐消失。实验期间，不同处理组的巢内热环境存在显著差异：在实验全程的日间以及实验末期的夜间，减小组的巢温均低于增大窝雏组。我们认为，青山雀雏鸟的内温性发育并非由个体发育程序固定不变，而是可根据窝雏数所决定的发育、营养与热环境差异发生变化。