来源：《新科学家》
原文刊登日期：2021年12月6日
本文适合2023考生
A study of nearly all of the 10,000 or so bird species has found that the further birds migrate, the paler their feathers tend to be. This is probably because it helps birds flying in sunlight to keep cool, says Kaspar Delhey at the Max Planck Institute for Ornithology in Germany.
一项对大约10000种鸟类的研究发现，鸟类迁移得越远，它们的羽毛往往越淡。德国马克斯·普朗克鸟类学研究所的卡斯帕·德尔雷说，这可能是因为浅色能帮助在阳光下飞行的鸟类保持凉爽。
“The difference is small, but this is common when it comes to explaining variation in colouration across an entire class of vertebrates,” says Delhey. “Feather colouration is extremely diverse and many factors can drive the evolution of colours, of which migration is but one.”
德尔雷说：“这种颜色差别很小，但这在解释整个脊椎动物类的颜色变化时是很常见的。羽毛的颜色极其多样，许多因素可以推动颜色的演变，迁徙只是其中之一。”
Several recent studies suggest that overheating is more of a problem for birds flying long distances than previously thought. For instance, sensors have shown that the body heat of migrating eider ducks can rise so high that they have to stop flying.
最近的几项研究表明，鸟类在长途飞行时过热的问题比之前认为的要严重得多。例如，传感器显示，迁徙的羽绒鸭的体温可以上升到很高，以至于它们不得不停止飞行。
Another study found that some migrating birds fly much higher during the day than they do at night despite having to expend more energy to stay aloft in thinner air. This suggests that the heat of the sun makes them much more likely to overheat, and made Delhey and his colleagues wonder whether migrating birds have lighter feather to help them cope.
另一项研究发现，一些候鸟白天飞得比晚上高得多，尽管在稀薄的空气中需要消耗更多的能量。这表明，太阳的热量使候鸟更容易过热，这使得德尔雷和他的同事们怀疑候鸟是否有更浅色的羽毛来帮助它们应对。
The researchers used photos from the Handbook of the Birds of the World to rate the lightness of birds’ feather from 0 to 100, treating males and females as separate species because coloration is so often different between the sexes. They found that short-distance migrants were significantly lighter than resident birds, and long-distance migrants were significantly lighter than short-distance migrants, even without controlling for factors such as body size or the environmental conditions where birds live.
研究人员使用《世界鸟类手册》中的照片，从0到100对鸟类羽毛的颜色深浅进行评分，因为雌雄之间的颜色往往不同，所以他们将雄性和雌性视为不同的物种。他们发现，即使不考虑身体大小或鸟类生活的环境条件等因素，短距离迁徙的鸟类明显比常住鸟类颜色更浅，而长距离迁徙的鸟类也明显比短距离迁徙的鸟类颜色更浅。
One implication of these findings is that migrating birds are going to find it even harder to stay cool as the planet heats up. Rising temperatures may force darker species to fly only during the night while migrating, says Delhey, or to fly even higher during the day.
这些发现的一个推论是，随着地球变暖，候鸟会发现更难保持凉爽。德尔雷说，气温上升可能迫使深色羽毛的候鸟在迁徙时只能在夜间飞行，或者在白天飞得更高。
“These responses will probably entail costs, which may reduce individual survival, with potential negative consequences at the population level,” he says.
他说：“候鸟的这些应对措施可能会带来成本，这可能会降低个体的生存率，并在种群层面产生潜在的负面后果。”