Do thermoregulatory costs limit altitude distributions of Andean forest birds?

Along tropical mountains, species often occupy narrow altitude ranges. Numerous biotic and abiotic factors have been proposed as determinants of altitude occupancy.
We measured several aspects of thermal physiology of 215 bird species across a 2·6-km altitude gradient in the Peruvian Andes. We predicted that highland species would show adaptation to the colder high-altitude climate and that energy costs of thermoregulation might limit upslope dispersal of lowland natives.
We found reductions in thermal conductance, body temperature and lower critical temperature in highland birds compared with lowland species. These combine to make highland natives more resistant to heat loss.
We did not find convincing evidence that acute thermal limits or energy costs of thermoregulation constrained altitude distributions. Heat-budget models predicted low-to-moderate long-term costs at native altitudes. Costs increased for lowland natives modelled in the highland climate, but for all but a few species, costs remained within putative expenditure limits.
Although we did not test heat tolerances, we measured all species at temperatures similar to the hottest air temperatures at the lowland site. There was no evidence that high lowland temperatures preclude downslope movements of highland birds.
While thermal tolerances probably do not directly determine altitude occupancy by most species, the additional energy cost of thermoregulation experienced by lowland species moving upslope may trade off against investment in important life-history components such as breeding, and thereby affect altitude range limits.

在热带山地中，物种通常仅占据狭窄的海拔区间。诸多生物与非生物因子被认为是决定物种海拔分布范围的关键因素。
本研究在秘鲁安第斯山脉一条2.6公里的海拔梯度上，对215种鸟类的多项热生理（thermal physiology）指标进行了测定。我们提出两项研究假设：其一，高地物种会演化出适应高海拔寒冷气候的特征；其二，体温调节的能量成本可能会限制低地原生物种向高海拔区域的扩散。
与低地物种相比，高地鸟类的热传导率（thermal conductance）、体温（body temperature）以及最低临界温度（lower critical temperature）均有所降低。这些特征共同使得高地原生物种的热量流失抗性更强。
本研究未发现可信证据表明，急性热极限（acute thermal limits）或体温调节的能量成本会限制物种的海拔分布范围。热量收支模型（heat-budget models）预测，物种在原生海拔区域的长期能量成本处于低至中等水平。在模拟高海拔气候环境时，低地原生物种的能量成本有所上升，但除极少数物种外，其余物种的成本均处于推测的支出阈值范围内。
尽管我们未测试物种的热耐受能力（heat tolerances），但所有实验物种均在与低地观测点最高气温相近的温度条件下完成了测定。未发现证据表明，低地的高温会阻碍高地鸟类向低海拔区域移动。
尽管热耐受能力可能并未直接决定多数物种的海拔分布范围，但低地物种向高海拔迁移时需额外付出的体温调节能量成本，可能会与繁殖等重要生活史特征（life-history components）的资源投入产生权衡，进而影响物种的海拔分布范围上限。