The most amazing thing about the little birds that visit the feeder is that they can survive at all. Scarcely more than tiny puffs of feathers and a heartbeat, how do they do it? In fact, they have a number of coping mechanisms, both behavioral and physiological, that help them make it through the winter.
The chickadee, for example, changes its diet. In the summer, chickadees prefer insects and other invertebrates, but in the winter, they change to a predominantly seed diet. Seeds may be a more clumped resource, and seeds tend to have a higher fat content than animal matter in winter, thus reducing the physical costs of foraging. Chickadees also cache seeds against periods when food is hard to find. See A Chickadee Never Forgets for more on this.
When it comes to staying warm, size does matter. Conservation of heat can be maximized by minimizing the surface-area-to-volume ratio. Thus, animals in cold climates tend to be bigger than their southern relatives (Bergmann’s rule). An example is the polar bear, which is larger than black bears that live farther south. In this, little birds are at a disadvantage. Species such as chickadees are distinctive for having the highest ratios of feather weight to body mass, averaging 10 to 11% compared to an average of 6 to 8% for other avian species. Also, chickadees have a prolonged molt after the breeding season, which ensures less feather wear and thus greater insulation at the beginning of the winter.
In winter, night may be the most difficult time for most small birds because of its colder temperatures and length, combined with a relatively short daylight foraging period. Physiological adaptations allow winter-acclimatized goldfinches to maintain a constant 40° body temperature for up to eight hours at -70° C. To maintain their body temperature in such extreme cold, they shiver intensely and produce heat at a rate four to five times their basal rate. This high level of thermogenic endurance is seasonal. Summer-acclimatized goldfinches cannot maintain normal body temperature for more than an hour in such frigid temperatures.
Chickadees have a different strategy. They utilize a period of regulated hypothermia at night, letting their body temperature drop by as much as 10° C. This reduction in body temperature lowers the metablolic expenditure for the night by as much as 23%. The benefit of this strategy decreases as the size of the bird increases. Large birds would require too much enegy to reheat their bigger bodies to use this system.
The elevated metabolic rates that small birds must maintain through cold nights place a premium on energy reserves. These reserves consist primarily of the undigested food in the crop and other parts of the digestive tract, and fat. Some birds begin roosting with relatively large amounts of seed in their crops. However, the primary energy reserve for most species is fat. In winter-acclimatized goldfinches, fat stores at the onset of roosting are significantly higher than in summer, and may reach 15% of lean body size.
It’s all pretty amazing, but I’m glad I’m inside, sipping hot chocolate and thinking about those poor goldfinches, shivering in the night.