Data from: Experimentally increased nest temperature affects body temperature, growth and apparent survival in blue tit nestlings

The thermal environment experienced by birds during early postembryonic development may be an important factor shaping growth and survival. However, few studies have directly manipulated nest temperature (Tn) during the nestling phase, and none have measured the consequences of experimental heat stress on nestlings’ body temperature (Tb). It is therefore not known to what extent any fitness consequences of development in a thermally challenging environment arise as a direct, or indirect, effect of heat stress. We, therefore, studied how experimentally increased Tn affected Tb in 8-12 days old blue tit (Cyanistes caeruleus) nestlings, to investigate if increased thermoregulatory demands to maintain normothermic Tb influenced nestling growth and apparent long-term survival. Nestlings in heated nest-boxes had significantly higher Tb compared to unheated nestlings during most of the experimental period. Yet, despite facing Tn > 50 °C (as measured in the bottom of the nest cup below the nestlings), the highest nestling Tb recorded was 43.8 °C with nestlings showing evidence of controlled facultative hyperthermia without any increased nestling mortality in heated nests. However, body mass gain was lower in these nestlings compared to nestlings from control nest-boxes. Contrary to our prediction, a larger proportion of nestlings from heated nest-boxes were recaptured during their first winter, or subsequently recruited into the breeding population as first- or second-year breeders. This result should, however, be treated with caution because of low recapture rates. This study highlights the importance of the thermal environment during nestling development, and its role in shaping both growth patterns and possibly also apparent survival.

鸟类胚胎发育后早期所经历的热环境，可能是影响其生长与存活的重要因素。然而，目前鲜有研究在雏鸟阶段直接操控巢温（nest temperature, Tn），且尚无研究量化实验性热应激对雏鸟体温（body temperature, Tb）的影响。因此，尚不清楚在热胁迫环境下发育所带来的适合度后果，在多大程度上源于热应激的直接或间接作用。
为此，本研究以8至12日龄的蓝山雀（Cyanistes caeruleus）雏鸟为对象，探究实验性升高巢温对其体温的影响，以验证维持正常体温所需的额外体温调节需求，是否会对雏鸟生长及表观长期存活产生影响。
在实验的大部分时段，加热巢箱内的雏鸟体温显著高于对照巢箱的雏鸟。尽管加热巢箱内的巢温超过50℃（测量位点为雏鸟下方的巢杯底部），但记录到的雏鸟最高体温为43.8℃，且雏鸟表现出可控的兼性体温过高（facultative hyperthermia）现象，同时加热巢内未出现雏鸟死亡率升高的情况。
然而，与对照巢箱的雏鸟相比，加热组雏鸟的体重增长速率更低。与我们的预测相反，加热组巢箱中雏鸟在首个冬季被重捕的比例更高，或是在次年作为一龄或二龄繁殖个体被招募进入繁殖种群的比例也更高。不过，由于重捕率较低，该结果需谨慎解读。
本研究凸显了雏鸟发育阶段热环境的重要性，及其对生长模式乃至表观存活的潜在调控作用。