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.晚成雏鸟类（Altricial birds）在孵化后的最初几日，若暴露于低环境温度中，无法维持自身体温。随着产后发育推进，其体温调节能力逐步形成，雏鸟最终将具备恒温能力。诸多因素可在个体与整窝雏鸟两个层面影响该发育过程，但据我们所知，目前尚无研究聚焦于窝雏数本身对单个雏鸟内温性（endothermy）发育的影响。我们针对野外环境中的青山雀（Cyanistes caeruleus）雏鸟开展降温实验，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，以及巢内整窝雏鸟所经历的热环境。所有实验窝雏组的雏鸟，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，减小窝雏组的雏鸟内温性出现更早。该差异在发育早期显现，并随雏鸟生长逐渐消失。雏鸟发育期间，不同实验组的巢内热环境存在差异：在实验周期末尾的所有日间与夜间，减小窝雏组的巢温均低于增大窝雏组。我们据此提出，青山雀雏鸟的内温性发育并非由发育程序固定不变，而是可随窝雏数决定的发育、营养及热环境条件差异发生改变。