Small vertebrates in winter

the challenges of thermoregulation for the tiny

Our recent spell of single-digit temperatures made me think about our smallest birds and mammals and how they deal with the cold. First, some background information:

Birds and mammals are ‘warm-blooded’, referring to their ability to keep their bodies warm even when temperatures are low. A more technical term is ‘homeothermic’, meaning the ability to maintain a constant body temperature. This thermoregulation is done metabolically, so it requires energy.

Maintaining a warm body temperature in cold weather requires a LOT of energy. This is a bigger problem for small birds and mammals than for big ones. It all comes down to the ratio between surface area (from which body heat is lost) and body volume (the muscles and bones and other organs that metabolize food and produce heat). The bigger the animal, the smaller the surface area relative to volume (given that body shape is similar). That’s because surface increases as the square of linear dimension, whereas volume increases as the cube. So heat production can exceed heat loss more easily in bigger critters. Conversely, the smaller the animal, the greater the surface area in relation to volume, so small critters lose body heat quicker than big ones. That makes it very expensive, metabolically, to be small in the cold.

For this essay, I chose an arbitrary average size limit to call ‘small’, but in reality there is no such cutoff point; there is, of course, a gradation of size from wee to huge. Furthermore, animals of every species vary in size, so using an average just provides an approximation to a range of body sizes. With those caveats in mind, I’ll discuss some local birds and mammals that are reported to have average weights of about half an ounce or roughly 14 grams.

There are several ways for small warm-blooded critters to deal with cold. They can escape it by migrating to warmer climes. They can reduce the metabolic costs by hibernating—going into torpor with lowered metabolic rates. They may conserve energy, by seeking shelter or roosting in groups, while keeping a high metabolism. And they can pay the high metabolic costs by eating more, storing food, and/or putting on body fat for the season.

Among the mammals that live in Southeast, the very smallest ones are shrews and bats (averaging roughly between five and fifteen grams). The common little brown bat, studied by ADFG, hibernates in rock crevices; three less common species are also thought to hibernate. But the uncommon silver-haired bat may be migratory. Shrews, on the other hand, stay active all winter. They burrow under a blanket of snow, if there is one, and that provides some shelter. They also eat prodigiously; nothing beats a shrew for voracity! Even in summer they need to eat roughly their own weight in food every day (imagine, if you can, a 150-pound person eating 150 pounds of food daily!). In winter, their daily energy requirements may double, and it seems likely that they often run out of food.

Our smallest bird, the rufous hummingbird (about four or five grams) has customarily bailed out entirely, migrating down to Mexico. As winters get milder, however, we sometimes see a few around, coming to feeders. They can save some energy by nocturnal torpor. Only slightly larger are the kinglets (around six grams). The ruby-crowned kinglet migrates to the deep south or the California coast, but the golden-crowned species generally stays in our region all year. They forage actively in the conifer canopy, sometimes conserving energy by huddling together in small groups in sheltered spots such as next to tree trunks or (perhaps) in old nests.

There are several bird species that typically weigh in the range of eight to twelve grams, and all of them stay active through the winter. Pacific wrens flit about in brush piles and root wads; they may roost communally in severe weather. Red-breasted nuthatches store seeds in scattered locations in tree bark (although if the cone crops fail they may depart for the winter). They are not known to roost communally, although some related species do so. Chestnut-backed chickadees throng at our bird feeders and forage throughout the woods, often storing seeds in bark crevices. The closely-related black-capped chickadee is reported to enter partial torpor in cold weather, dropping its body temperature a few degrees to save energy, but not becoming totally ‘asleep’; the chestnut-backed species may do this too. Brown creepers sometimes roost in small groups in semi-protected sites. In general, all these birds seek shelter in thick vegetation and cavities and, like many other species, tuck their bills and feet deep into their feathers to conserve heat.

The common redpoll (average weight about thirteen grams) nests in boreal woodlands and has apparently been studied more thoroughly than other small wintering birds. Redpolls have a denser coat of feathers in winter than in summer, reducing heat loss. They store a night-time supply of food in pouches off the esophagus. They also have the energy-saving habit of tunneling down under the snow to escape severe cold, sometimes roosting in small groups. And they forage actively even at low light levels, giving them a longer day for food-finding. If seed crops fail on the nesting grounds, they wander in winter to wherever the seed crops are good, and often come to us in February and March.

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