Hibernate

Do Birds Hibernate?

There is one bird that is known to hibernate. The Common Poorwill is a true hibernator. src
... a lot of posts recently about the different terms for animals entering a state of rest over winter. Some of these include hibernation, torpor, and brumation. Certain terms refer mainly to certain kinds of animals, such as brumation for reptiles. src

Although birds in the UK do not hibernate, one species uses a similar strategy to cope with periods of cold weather and low food availability. Young swifts go into a state of 'torpor' which is where the body temperature and metabolism is greatly reduced in order to save energy. The energy saved by reducing activity in this way allows these birds to survive for short periods of time until conditions improve. Torpor is a commonly used survival strategy in a number of bird families across the world such as the hummingbirds, nightjars and swifts. Read more at https://ww2.rspb.org.uk/birds-and-wildlife/bird-and-wildlife-guides/ask-an-expert/previous/hibernation.aspx#fmLffgCddLEfycxJ.99&

白胸苦惡鳥:
苦恶鸟羽绒薄，翼翅短，不耐寒，也无法远走高飞。冬季时它们往往钻进干燥的洞穴冬眠，不吃不动，凭借体内脂肪维持生命，直到来年春暖花香。

初冬季节就要到了，天气逐渐变冷，胖乎乎的秧鸡个个急于选择干燥的石洞或泥洞，钻到里面冬眠了。秧鸡在洞里不吃不动，或很少活动，呼吸次数减少，血液循环减慢，新陈代谢减弱，尽可能减少消耗体内的营养物质，凭借贮存的脂肪来维持生命。 src

鸟类冬眠这个事情，最早是由古希腊先哲亚里士多德提出的。当时，亚里士多德观察到家燕每年秋天都会“集体失踪”，于是他断定这些燕子是去冰下冬眠了，待到来年春天，气候回暖，冰河融化，苏醒的燕子就从河里飞出，开始繁衍。

现在人们都知道亚里士多德的推断是荒谬的。家燕是候鸟，冬天会离开繁殖地，迁徙至温暖的南方过冬，并非躲起来冬眠。不过直到19世纪20年代，生物学家才通过野外环志的数据证实了鸟类迁徙现象。如今很多人都知道，鸟类有候鸟和留鸟之分。那么问题来了——候鸟能够通过迁徙躲避寒冬，那么留鸟是否像蟾蜍、黑熊那样要靠冬眠来保存体力以挨过漫漫冬日呢？

其实，大部分留鸟虽不迁徙，但由于食源丰富、绒羽厚实，它们仍能在寒冷的季节维持生存，因此不必冬眠。冬眠的鸟儿仅占极少数。不过更准确地来说，这些鸟儿是在冬天进入一种特殊的休眠状态，可算作睡眠深度较浅的“迷你版”冬眠。因为鸟类的基础代谢率很高，如果像黑熊那样进入真正的冬眠状态，必须要储存相当多的脂肪量才行，这个量实在太大了，以至于根本不可能实现。

人们最早发现的冬眠鸟类是猫头鹰。1916年12月，[Not owl but it is Common Poorwill in 1946] 人们在美国加利福尼亚州的一个深山峡谷里，居然发现了一只正在冬眠的猫头鹰。它蜷缩在峭壁裂缝中，一动也不动，看起来像只死鸟，偶尔才会动一下眼球，表明还活着。科学家一连观察了四年，发现每到冬季这只猫头鹰就会像死鸟一样，睡得昏昏沉沉。有一年冬天，它竟一连88天都纹丝不动。冬眠时，它的体温由平时的40摄氏度降到20摄氏度左右，用听筒也听不到心脏的跳动声。只要春天一到，它的体温便自行升高，随后就苏醒并展翅高飞了。

北美小夜鹰和白胸苦恶鸟，也跟猫头鹰一样有冬眠的习惯。每到冬天，各类昆虫销声匿迹，它们的食物来源也就断绝了。在这种情况下，钻进避风而隐蔽的洞中休眠，将代谢降低至最低水平，不失为一种保存体力和生命力的好方法。它们在洞里不吃不动或很少活动，体温仅比环境温度稍高几度，呼吸频次降低，血液循环变慢，基础代谢率仅为平日里的几十分之一……这一切都是为了尽可能减少消耗体内的营养物质，凭借贮存的脂肪来维持生命。

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Do owls hibernate?
There is only one bird that is known to hibernate in the normal sense (i.e., for periods of weeks at a time), the Common Poorwill. The Common Poorwill ranges over most of the western U.S. and northern Mexico. src

So some owls hibernate or not? No!

Common Poorwills are the only species of bird that go into true hibernation. A handful of other bird species go into torpor (light hibernation). Common Poorwills hibernate when the temperatures get really cold, really hot or when food is scare. When necessary, they can even hibernate while incubating their eggs. That’s one way to pass the time, while waiting for the chicks to hatch! src

This was discovered in 1946, when a Common Poorwill Phalaenoptilus nuttallii (a sort of nightjar) was found in a torpid state in a rock crevice in a California desert (Jaeger 1949). The bird was inert, its respiration and heart rate were barely detectable, and its body temperature was 18–20°C, about half the usual level for birds. The individual was ringed, and in subsequent winters it was found hibernating again in the same crevice. Since then other Poorwills have been found in similar sites in the same condition, and their physiology has been studied in laboratory conditions (Withers 1977). The energy consumption of torpid birds was so low that they could live off their body fat for more than three months. Other kinds of birds can also become torpid but remain so only overnight (hummingbirds) or for at most a few days at a time (swifts and colies [also called Mousebirds]). Evidently, long-term hibernation is at best extremely rare among birds, most escaping difficult conditions by migration instead. src

Not just sleep: all about hibernation

An energy-saving device

While there are physiological changes associated with sleep (such as slowed breathing and heart rate), they are not nearly as dramatic or significant as those which occur during hibernation. When hibernating, an animal’s metabolism slows significantly: its heartbeat slows, it breathes more slowly (some animals even stop breathing for periods of over an hour) and its body temperature drops—in some extreme cases to below the freezing point of water (zero degrees Celsius). And here we get to the reason why animals hibernate: reducing their metabolism allows them to conserve energy.

To understand why they might need to do this, let’s take a step back and look at the difference between endothermic and ectothermic organisms.

Ectothermic animals are those whose body temperature depends on the ambient temperature. Endotherms, by contrast, can regulate their own body temperature by generating internal heat (by combusting fuels). People are endotherms. In everyday language, we tend to make a distinction between ‘cold-blooded’ animals, and think of things like snakes and lizards, and ‘warm-blooded’ animals, like mammals and birds. However, this distinction can be a bit misleading, since some fish, reptiles and insects are actually fully or partially endothermic.

In some ways, being ectothermic is an advantage—it means you don’t waste energy regulating your body temperature, and therefore don’t need so many nutrients. On the other hand, ectotherms are more reliant on environmental conditions: a lizard, for example, can only warm up from a cold spell if heat from an external source, like the sun, is applied. Endotherms, by contrast, can warm themselves up by producing heat metabolically and by shivering, for example.

But for endotherms to be able to regulate their temperature, they need enough fuel to burn—in other words, they need enough food to counter the effects of cold. This can be a challenge when freezing weather comes along or when food is scarce. So, in order to survive, many endothermic animals go into what’s known as a state of ‘torpor’. During torpor, physiological processes, like breathing and heart rate, slow down. The body temperature is set at a new, lower point. Animals that are able to adjust their (base) body temperature and metabolism in this way are known as (take a deep breath) heterothermic endotherms.

Tawny frogmouths are an example. The largest bird known to employ torpor, it goes into this energy-saving state at night or in the early morning, especially on cold winter days, between which it feeds and functions as usual. In fact, many birds enter daily (or nightly) torpor, including kingfishers and owls, as do many small mammals.

So how does this relate to hibernation? Well, hibernation is essentially a series of bouts of torpor that each last for many days. Hibernation differs from daily torpor in that it usually involves much lower body temperatures and metabolic rates, and is often seasonal. In addition, while animals who go into daily torpor wake up and forage or feed in the usual way, hibernating animals either feed off their body fat or on specially stored food.

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