In the winter birds can grow twice as many feathers but they still have to shiver almost constantly to increase their body temperature in cold weather. This shivering process is called thermogenosis. The constant shivering produces heat five times that of their normal rate, helping them to maintain an amazingly high body temperature.
It also burns a lot of calories. Birds store the needed calories as fat, but they can only store enough for 16 to 24 hours. This is why you’ll see birds in a panic at your feeders right before it gets dark and at first light.
Scientists have found that some birds like chickadees go one step further to survive the cold winters. The birds go into a nocturnal torpor to conserve energy. Torpor is a kind of deep sleep accompanied by drastically lowered body temperature, heart rate, and breathing. The result is a controlled hypothermia that can save a bird up to 20% of its energy. (Hibernation is defined as a sustained state of torpor.)
When it’s especially cold, birds flock to feeders to build up their energy reserves. A seed blend with black oil sunflower seeds and peanuts is great to offer in the winter. It has a high calorie/ounce ratio due to its high fat and protein content. At Wild Birds Unlimited that would be our most popular WBU No-Mess Blend or WBU Choice blend.
Suet or seed blocks are great foods to offer many of the birds that will visit backyards in the winter. Suet is a high energy, pure fat substance which is invaluable in winter when insects are harder to find and birds need many more calories to keep their bodies warm. I would recommend our peanut butter suet.
Typically, your feeders serve only as a supplemental source of food for birds in your yard. However, during cold, long, severe winter weather, your birds may switch to utilizing them as the critical source of food that enables them to survive from day to day.