Spending the winter in northern climes with short days and cold ambient temperatures ( T a ) can be energetically challenging for small birds that have high metabolic and heat loss rates. Hence, maintaining body temperature ( T b ) in T a below thermoneutrality can be energetically costly for a small bird. We still know little about how increased heat production below thermoneutrality affects the level at which T b is maintained, and if these patterns are age specific. To test this, we measured subcutaneous body temperature ( T s ) and resting metabolic rate (RMR) simultaneously in blue tits ( Cyanistes caeruleus ) during winter nights in T a ’s ranging from 25 to − 15 °C. RMR increased below the lower critical temperature (LCT, estimated at 14 °C) and was 6% higher in young (birds in their first winter) compared to old birds (birds in their second winter or older). The higher RMR was also mirrored in higher T s and thermal conductance ( C ) in young birds, which we suggest could be caused by age differences in plumage quality, likely driven by time constraints during moult. Reduction in nightly predicted T b was modest and increased again at the coldest ambient temperatures, suggesting that either heat retention or heat production (or both) improved when T a reached levels which are cold by the standards of birds in our population. Our results show that levels of heat production and T b can be age specific. Further studies should address age-specific differences on quality, structure, and thermal conductivity of plumage more explicitly, to investigate the role of variation in insulation in age-linked metabolic phenotypes.

中文翻译：

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小型雀形目夜间代谢率和体温的年龄差异

在白天短、环境温度 (T a ) 寒冷的北方气候中过冬，对于代谢率和热量损失率高的小型鸟类来说可能是充满活力的挑战。因此，将体温 (T b ) 保持在低于热中性的 T a 中，对于一只小鸟来说可能在能量上是昂贵的。我们对低于热中性的产热增加如何影响 Tb 的维持水平，以及这些模式是否因年龄而异，我们仍然知之甚少。为了测试这一点，我们在 25 到 − 15 °C 的 T a 的冬夜同时测量了蓝山雀 (Cyanistes caeruleus) 的皮下体温 (Ts) 和静息代谢率 (RMR)。RMR 增加到低于下临界温度 (LCT, 估计为 14 °C），与老鸟（第二个冬天或更老的鸟）相比，幼鸟（第一个冬天的鸟）高 6%。较高的 RMR 也反映在幼鸟较高的 Ts 和热导率 (C) 中，我们认为这可能是由羽毛质量的年龄差异引起的，这可能是由于换羽期间的时间限制所致。夜间预测 T b 的减少是适度的，并且在最冷的环境温度下再次增加，这表明当 T a 达到我们种群中鸟类标准的寒冷水平时，保温或产热（或两者）都会有所改善。我们的结果表明，产热水平和 T b 可能因年龄而异。进一步的研究应该更明确地解决特定年龄的羽毛质量、结构和导热性差异，