The physiology of tropical birds is poorly understood, particularly in how it relates to local climate and changes between seasons. This is particularly true of tropical montane species, which may have sensitive thermal tolerances to local microclimates. We studied metabolic rates (using open flow respirometry), body mass and haemoglobin concentrations of five sedentary Mesoamerican songbirds between the summer and winter at two elevations (1550 m and 1950 m, respectively). We asked whether there were uniform seasonal shifts in physiological traits across species, and whether higher elevation species displayed evidence for cold tolerance. Seasonal shifts in metabolic rates differed between the three species for which data were collected. Basal metabolic rates in one species - black-headed nightingale-thrushes Catharus mexicanus - were up-regulated in summer (~19% increase of winter metabolism), however two other species displayed no seasonal regulation. No species exhibited shifts in haemoglobin concentrations across season or across elevation, whereas body mass in two species was significantly higher in the summer. One species restricted to higher elevations - ruddy-capped nightingale-thrushes C. frantzii - displayed physiological traits indicative of cold-tolerance. Although only summer data were available for this species (C. frantzii), metabolic rates were constant across temperatures tested (5-34oC) and haemoglobin concentrations were significantly higher compared to the other four species. Our results suggest that seasonal acclimatisation in physiological traits is variable between species and appear unrelated to changes in local climate. As such, the distinct physiological traits observed in ruddy-capped nightingale-thrushes likely relate to historic isolation and conserved physiological traits rather than contemporary climatic adaption.

中文翻译：

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五个热带山地鸣禽的比较生理学揭示了不同的季节适应和冷适应

人们对热带鸟类的生理学知之甚少，尤其是在它与当地气候以及季节之间的变化之间的关系上。对于热带山地物种尤其如此，其对局部微气候可能具有敏感的热耐受性。我们研究了夏季和冬季两个海拔高度（分别为1550 m和1950 m）之间五只久坐的中美洲鸣禽的代谢率（使用开放式呼吸测定法），体重和血红蛋白浓度。我们询问了跨物种的生理特征是否存在统一的季节性变化，以及海拔较高的物种是否显示出耐寒性的证据。在收集数据的三个物种之间，代谢率的季节性变化有所不同。夏季，黑头夜莺-鹅口疮Cat（Catharus mexicanus）一类的基础代谢率上调（冬季代谢增加约19％），而另外两个物种则没有季节性调控。没有物种在整个季节或整个海拔范围内都表现出血红蛋白浓度的变化，而两个物种的体重在夏季明显更高。限制在较高海拔的一种物种-红顶c夜莺-鹅口疮C. frantzii-表现出指示耐寒性的生理特征。尽管只有该物种（C. frantzii）的夏季数据可用，但在其他测试温度下（5-34oC）代谢速率是恒定的，并且血红蛋白浓度明显高于其他四个物种。我们的研究结果表明，季节性生理适应性在物种之间是可变的，并且与当地气候的变化无关。因此，在红润的夜莺画眉中观察到的独特生理特征可能与历史隔离和保守的生理特征有关，而不是与当代气候适应有关。