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I live in a region with cold winters (ca. -40 C) and many quite small birds (sparrows, crows, ducks) stay for the winter.

How can the birds survive this?

As I understand it, smaller bodies will be less able to produce and retain heat. Despite this, birds seem to cope much better than humans, sitting in the cold all winter and even swimming in cold water. In comparison, humans lose mobility in their muscles when cold, and can suffer severe frostbite causing them to lose fingers, toes, or limbs from relatively short exposure to the same conditions.

What specific adaptations allow birds to cope so well in these conditions?

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  • $\begingroup$ Are you actually seeing these species where you live during winter? Birds tend to be migratory. Feathers and fats tend to make fairly good insulators though. They would need a food source and some form of shelter. But I would assume if you do not see them in the winter, then they have migrated towards the equator. $\endgroup$ – AMR Jan 10 '16 at 23:43
  • $\begingroup$ Also, if it is actually water, then it is greater than 0°C, so the water may actually be warmer than the air temp. And like I said above, fat and feathers make for good insulation. $\endgroup$ – AMR Jan 10 '16 at 23:45
  • $\begingroup$ @AMR: yes, I actually see them. Maybe there are less birds than in summer, but there definitely are many. I know they should migrate, but for some reason they don't really always do that.. idk, the migration thing seems broken, I see really many birds in winter in the city. Yeah, they can find heat sources in the city, but still they spend quite much time just wandering around in the cold. Good point about the water temperature.. Idk how, but sometimes, when it's -20C, some parts of the city channels are not iced, probably someting warms the water.. $\endgroup$ – noncom Jan 11 '16 at 0:13
  • $\begingroup$ @AMR: also - what about their legs? They are in no way protected... still their muscules and joints do not suffer, they can just as easy grip to their sitting place, like a branch or a wire.. $\endgroup$ – noncom Jan 11 '16 at 0:19
  • $\begingroup$ The coldest water is around 0 Celsius. Beyond that it's ice. $\endgroup$ – ott-- Jan 11 '16 at 19:15
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There are a variety of physiological, anatomical and behavioral adaptations that keep birds warm in winter.

For starters, birds are endothermic ("warm-blooded"), similar to mammals. You suggested that the coats humans wear are superior to feathers as insulation. Yet some of the finest coats and sleeping bags were long filled with feathers (e.g. goose down). Animals that live in cold regions tend to have smaller extremities. For example, you probably won't find many long-legged flamingos or ibises in the Arctic during the winter (though food supply is another reason they couldn't survive there).

As someone else suggested, water temperature is a relative constant, which means water (or better yet, snow) can actually insulate animals from colder temperatures.

Countercurrent heat exchange is a strategy that helps endothermic animals conserve heat. The page I linked to is part of a book titled Animal Adaptations to Cold, which includes a chapter titled "Avian [Bird] Adjustments to Cold."

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During cold weather, birds will go into torpor where they will reduce their metabolic rate to conserve energy over the night. This burns fewer calories and maintains fat reserves that help insulate them from cold temperatures. Behaviorally, many bird species will roost communally to share heat among each other.

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