Variation in body size, especially mass, is a function of local environmental conditions for any given species. Recent recorded decreases in body size of endotherms have been attributed to climate change in some cases. This prediction is based on the trend of smaller body size of endotherms in warmer climates (Bergmann’s rule) and it implies genetic responses rather than phenotypic flexibility. Alternatively, selection for smaller body size or lower mass could be explained by the starvation-predation hypothesis, where lighter individuals have a higher probability of escaping pursuing predators, such as raptors. Evidence that climate warming is driving patterns of size selection in birds in recent times has been mixed. We inspected data on 40 bird species contributed by bird ringers to the South African Ringing Scheme (SAFRING) for changes in body mass and condition as a function of time (year), minimum temperature of the day of capture, maximum temperature of the previous day, and rainfall data in the south-western Cape Floristic Region (fynbos) around Cape Town, South Africa, for the period 1988–2015. The region shows a warming trend over the study period (0.035 °C yr⁻¹). Interannual body mass and condition change were poorly explained by year or temperature. High daily minimum temperature explained loss of body condition for four species, whereas evidence from recaptured birds indicated negative effects of increasing maximum daily temperature, as well as rain. For the alternative hypothesis, because raptor abundance is stable or only weakly declining, there is little evidence to suggest these as a driver influencing mass trends. Any decrease in body mass over the study period that we observed for birds appear more likely to be plastic responses to stress associated with temperature or rainfall at this time, rather than systematic selection for smaller body size, as predicted by Bergmann’s Rule.

对于任何给定的物种，体型的变化，特别是质量的变化，都是当地环境条件的函数。在某些情况下，最近记录的吸热体大小减少归因于气候变化。该预测是基于在温暖的气候中吸热的体型变小的趋势（Bergmann法则），它暗示了遗传反应而不是表型灵活性。或者，可以选择饥饿或捕食假说来解释选择较小的体重或较小的体重，在这种假设下，较轻的人更有逃避追捕者（例如猛禽）的可能性。近年来，气候变暖是鸟类选择大小的驱动方式的证据好坏参半。我们检查了由鸟铃虫向南非铃响计划（SAFRING）贡献的40种鸟类的数据，以了解体重和状况随时间（年），捕获日的最低温度，前一天的最高温度的变化，以及南非开普敦附近西南开普植物区（fynbos）1988-2015年期间的降雨数据。该区域在研究期内呈变暖趋势（0.035°C年^-1）。年际体重和状况变化很难通过年份或温度来解释。每天的最高最低温度高解释了四种物种的身体状况的丧失，而被重新捕获的鸟类的证据表明，增加最高每日温度以及下雨会带来负面影响。对于另一种假设，由于猛禽的丰度稳定或仅微弱下降，因此几乎没有证据表明它们是影响群体趋势的驱动因素。我们观察到的在研究期内体重的任何下降似乎都更可能是对此时与温度或降雨相关的压力的塑性反应，而不是像伯格曼定律所预测的那样，为较小的体型进行系统选择。