|During exceptional cold snaps, bark gleaners (such as Brown Creeper, left) may have greater resilience than active insectivores (such as Golden-crowned Kinglet. Photos by Peter LaTourrette / www.birdphotography.com|
Birds have evolved physical and behavioral strategies for dealing with ordinary weather within their range. They are waterproof (well, at least water-resistant), they know when and how to seek shelter, and because they are feathered they can thermoregulate (stay warm in cold and cool in heat). Migratory kinds are able to depart high latitudes before the cold of winter sets in. Many desert species do not need much (if any) water, as they derive enough fluid from animal prey like insects, lizards, and kangaroo rats.
In addition to evolved physiologies and behaviors, most birds are able to adapt to change in the short run by selecting secondary food sources when primary ones are unavailable.
Tree Swallows (like all swallows) are aerial insectivores but, when hit with freezing weather upon early return to northerly latitudes in the eastern U.S., will switch to eating bayberries until warming trends allow insects to emerge.
Acorn Woodpeckers depend upon caches of acorns to sustain them through winter, but unseasonal early-fall rains that wash the pollen off of oaks can result in acorn crop failures. Then, in this socially colonial species, individual woodpeckers leave or are driven from the tribe. Without access to granaries, they may turn to foraging strategies such as flycatching, eating seeds or suet at feeders, and probing sapsucker excavations, and they are even are known to eat lizards and smaller birds' eggs.
Some birds, when caught in abnormal extremes of hot or cold, die of temperature trauma, but many others have back-up systems that allow them to survive. When too warm, birds have methods for cooling off that include the obvious, like going to shade or water, and also particular behaviors. Warm-water seabirds pant and flutter their gular pouches to fan their innards. Some landbirds pant and partially spread their wings to release excess heat. These behaviors are common in birds that summer at high latitudes then winter in the tropics.
For many birds—animals with high metabolisms—it is much harder to recover body heat than to cool down. Small birds keep their hot bodies hot by fueling up constantly with food. Groups of birds that specialize and are unable to switch diets on-demand may become vulnerable during extreme cold.
Bark gleaners like woodpeckers, nuthatches, creepers, and even Black-and-white Warblers forage successfully in extreme cold, because arachnid and insect eggs and larvae remain abundant in the relatively warm nooks and crannies. Seed eaters (sparrows, juncos, finches) usually survive cold snaps, even long ones, by gobbling seeds and other vegetation all day. It's not as great as bark-gleaning, because the diet lacks fat and protein, but in many cases the food supply is endless.
Active insectivores often fail completely. Vireos, kinglets, and wood warblers seek aerial or otherwise active insects. Harshly adverse cold kills this prey base and soon the birds. Without fat reserves, the birds cannot just pick up and go. After one ten-day freeze at Point Reyes, we found virtually no kinglets or warblers and just a few Hutton's Vireos (which may have turned to bark-gleaning). There were thrushes, because there were toyon, mistletoe, and huckleberries, and the regular bark-gleaners and seed-eaters made it through the freeze.
Most bird species have evolved through time to cope with some degree of "abnormal" weather. Extremes that come too hard and fast, though, or exceed normal limits for too long, may test the abilities of these highly adaptive creatures to survive.