How do birds survive the cold weather, especially during really cold winters like the one we are having now in much of the United States?
One part of this answer has to be, sadly perhaps, that the sometimes don’t. But I’ll get to that later.
You need to know two things as context. First, there are a lot of different kinds of birds, and the adaptations I’ll mention below are not found in all of them, and probably all of these adaptations are not found in very many species. Second, many birds are actually at great risk during cold periods because birds generally live on the edge when it comes to energetics. It takes energy (from food) to keep warm. Birds are endothermic, meaning that they produce their own heat (mostly) via blood flow in their bodies, but unlike mammals, birds change their body temperature a fair amount, so the amount of heat they need to produce to fly is a little bit flexible. The thing is, temperate and sub-Arctic birds have to survive not only the cold of winter, but even the chill of a non-winter night. Since they don’t store a lot of energy in fat (that is hard to do for a flying animal) it is quite possible for a bird to run out of heat-producing energy overnight even when it isn’t winter. That would amount to, essentially, starving to death. So, some of the adaptations for surviving the cold apply year round, depending on conditions and the species.
There are several things birds do (or avoid doing) to help them survive the winter.
First, they can leave. Migration is a great strategy to avoid winter. I highly recommend it. Migration is costly, though. One has to spend a lot of time flying instead of feeding, and a bird is probably more susceptible to predation while flying through the territories of various predators and spending time in areas it is not familiar with. Most migratory birds that die during migration probably do it on their first migration, and thereafter have the advantage of knowing the territory a bit better. So, migration is one way to handle winter. Note, however, that there are birds that live in the Arctic or sub-Arctic that migrate south to regions where it is still winter. (Some of these are referred to as snowbirds because they show up during the snowy season).
See Also: Feeding Wild Birds
Birds wear down coats. All their feathers help them to keep warm, but especially the downy under feathers (called, of course, “down”) act like tiny little North Face down coats. Some birds probably grow extra down during the cold season.
When you are trying to stay warm, water is your enemy. Air makes a good insulator but water transmits heat, so wet feathers are bad. So, birds have oil producing glands that allow them to preen a coating of waterproof onto their feathers to avoid the down coats getting wet.
Birds have legs and parts of their faces that are exposed. But they can sit in a position that covers, or partly covers, their legs and feet with their down coat. They can also hunker down in a protected area, with less wind-chill causing wind, in the canopy of an evergreen or some other place. Gregarious over-wintering birds like Chickadees will roost together in little bird-lumps to give each other protection and warmth.
Birds shiver. That helps get added heat from circulation and muscle movement.
Bird feet are covered with scales and have very little cold-damagable tissue in them. They are mostly bone and sinew.
Some birds have a special adaptation in the circulatory system of their feet (and maybe elsewhere) whereby blood is circulated between colder outer areas and warmer inner areas more efficiently than might otherwise be the case, to avoid frostbite.
Check out: Books on birds and nature.
Birds can not only tuck their feet in under their down, but they may also switch which foot is holding them on a branch. Also, as you probably know, bird feet are generally grabbing at rest, so it takes very little energy to stay attached to a branch. The default is “hang on.” Sort of like the safety feature of an Otis elevator, where the default position for the machinery is “don’t drop” so when the power goes off the elevator gets stuck instead of plummeting to the bottom.
Birds may find a place in the sun and use a bit of solar energy. This, of course, depends on the wind. It also puts them out there for predators to find them, but it can work.
Birds may eat more, or selectively eat higher energy food during the winter. For small birds this may include storing up food during the warmer season. They are adapted to find, store, and remember where the food is so they can find it quickly. This makes winter foraging super efficient.
Some birds store some fat, but really, that is not a great strategy for birds that fly.
One of the most effective strategies for having enough energy from food to stay warm is to not do highly energetic things during the cold season. I already mentioned the increase in foraging efficiency for birds that store food. Another obvious strategy is to not do energetically costly things during this season such as defend territories, spend a lot of time singing (singing is very costly in terms of energy), don’t build or maintain nests, don’t produce eggs or have hungry chicks around. This may seem self evident but it is actually very important. Indeed, the reproductive success of a pair of birds in a given year may be significantly hampered by late cold weather or forage-covering snow storms in the spring.
Finally, birds use another strategy that also works against the cost of predation and other forms of death: Reproduce more. Most pairs of birds produce one or a few offspring that become adults a year for five or more years. If all of those adult or subadult birds survive and reproduce, we’d be covered in birds in a few decades. But lots of things kill birds, including predators, disease, starvation, and cold.
Here is a pretty good video that covers some of these things:
The photo of a Dark Eyed Junco is from HERE, where you will also find a bit more discussion on birds in winter.
I also write about birds here.