Birds and Cold Weather

Black-capped Chickadees, Poecile atricapillus


            We know that climate has shaped avian anatomy, physiology, and behavior. Body sizes, length of wings, legs, and beaks, fat deposition, feather color, salt retention, respiration rates, red blood cell count, and other such characteristics are evolutionary results of climatic pressures. Birds lose water from their skin and respiratory system, pant, shiver, flutter their throats or tongues, dilate or constrict capillaries, seek shade, adjust their body position, spread their wings, or otherwise respond physiologically or behaviorally to mitigate an environment that is at the edge of their physical comfort zone. People in the developed world can seek relief offered by modern conveniences like air conditioning and furnaces, but birds are at the mercy of the meteorological environment.

            We say that birds and mammals are warm-blooded, but that’s a misleading term. A desert iguana sitting on a rock in the sun can tolerate a body temperature of 1150 F (460C). That would certainly generate warm blood, but we consider lizards cold-blooded, so what we really mean by warm-blooded is that the bird’s temperature is moderated by the body’s physiology to a narrow range. They are “homeothermic,” meaning stable temperature.  Homeothermy has allowed birds to exploit habitats with extremes of temperatures. Cold-blooded animals are those whose body temperature varies with the environment; they are” poikilothermic,” meaning variable temperature. Poikilotherms either cannot inhabit a very cold or hot environment or do so in low numbers. But the distribution of most bird species, even though homeothermic, is still limited by the climate. The population of birds in a particular geographical area is constrained by such variables as food, predators, nesting sites, and competitors, but the entire species’ distribution is fundamentally controlled by temperature.

             There are temperatures at which we humans are comfortable and those at which we shiver or sweat. Humans are able to acclimate to different ranges of temperatures. If you live in North Dakota, a temperature slightly above freezing might seem mild. In California a temperature anywhere near freezing brings out down parkas and earmuffs. People adjust to their climatic environment and become comfortable in a particular range of temperatures. This range of temperatures in which we don’t sweat or shiver is our thermoneutral zone (TNZ). All birds and mammals have a TNZ, the range of ambient temperatures at which their basal metabolic rate is stable and biochemical processes are optimized. Above and below the TNZ birds have to expend additional energy to cool or warm themselves. Since birds can acclimate somewhat, the thermoneutral zone changes with the season, latitude, local conditions, and species of bird. In general the TNZ spans a greater range of temperatures in colder environments than in warmer ones and in larger birds compared to smaller ones. For example, the thermoneutral zone of the tiny Green-backed Firecrown (hummingbird) is 55-820 F (13-280 C) and the Ostrich 50-1220 F (10-500 C.) The TNZ of the desert-dwelling Gambel’s Quail 86-1110 F (30-440 C), and the Adelie Penguin 14-680 F (-10-200 C).