Winter is a tough time for birds. Most insect-eating birds have made their way to the tropics of Central or South America, others have migrated down from higher latitudes or altitudes to winter in the valley, and some birds just stay put. The non-migratory species around northern California such as the Western Scrub Jay, Spotted Towhee, or House Finch not only have to tolerate winter conditions compete for food from the winter visitors such as Dark-eyed Juncos and White-and Golden-crowned Sparrows.
Birds are warm-blooded, or, more accurately, homeothermic. This means that they maintain the same body temperature all the time, no matter what temperature it is in their environment. To do that they have to eat enough to produce the energy to warm the body and perform metabolic and muscular functions. There are exception to this, but I’ll save that for another blog.
Mortality of songbirds over the winter can be 30-50% or more, depending on the bird species and weather conditions. Rainy 40 degree days are certainly challenging, but just think of birds spending the winter in central Illinois or Ontario, Canada, such as the Cedar Waxwing or Black-capped Chickadee. Waxwings have to find berries and Chickadees insects, larvae, and larval cases along with small seeds. Most birds eat only during daylight, so they have less than 12 hours to eat enough food to sustain them overnight. They might put on ten percent extra body weight – that’s like a 150 pound person gaining 15 pounds in one day! But then the extra weight is used up overnight keeping warm.
But our avian friends have several mechanisms to cope. They have feathers, which are great insulators. Fluffing up the feathers produces more layers of air for more insulation. They may find a hole to roost in and may roost in a tight group of several birds, sharing body warmth. One roost of Pygmy Nuthatches was found to have over 100 birds! Ptarmigan and grouse may burrow into the snow. Birds also have a high metabolism but can slow it down at night. Their normal body temperature of 104 to 108 degrees may drop to 90. Hummingbirds go into a state of torpor and their body temperature might go as low as 40 degrees over the ambient temperature; on a 30 degree night, the body temperature of a hummingbird might go as low as 70 degrees!
Birds have little exposed skin which minimizes heat loss. In some waterfowl such as ducks and geese, blood vessels in the legs are arranged so that the cold venous blood is warmed by the arterial blood before returning to the heart, keeping the body from cooling. Shorebirds sleep with one foot tucked into their breast feathers; loons and grebes fold up one or both feet under their wings as they float on the water.
Bigger birds such as waterfowl and hawks lose proportionately less body heat than do smaller birds and can withstand cold much better and their mortality rates are lower. Of course, the winners of the cold tolerance game are the penguins. With their thick skins, layers of fat, and tendency to huddle together, they can withstand 100 mph winds in -50 degrees Fahrenheit temperatures.
Though may be scarce in winter but the lack of water might be a bigger problem. Most sources of water in certain areas of the country may be frozen and although birds can eat snow, it takes away body heat. In addition to metabolic functions, water is important for preening and keeping their feathers in good condition for insulation and flight.
If you feed birds, you might also consider installing a bird bath and if you live in a cold area, you can get a heated bird bath. It’s cold out there.