For the Birds Radio Program: Why do some birds stay north in winter?
Do chickadees stay over winter because they like our company, or because they’re stupid? The answer may be neither. (3:20) Date confirmed.
Everyone pretty much understands why birds migrate south for the winter. Once it freezes, plants aren’t producing seeds and fruits, fish are locked away under ice, frogs and snakes have buried themselves under frozen ground, mice are hiding under snow, and insects are simply not available. Besides all that, a northern winter is just plain too cold compared to that of a lush tropical summer only a few thousand miles away. Since birds have wings and can cover ground with ease, they move south. It seems so simple, so how come not all birds migrate?
There are some species, and even whole families, of birds that simply cannot remain anywhere where the average temperature is below freezing. Anything that depends on flying insects, for example, is going to be sorely stressed for food when cold-blooded creatures are no longer available. But some insect eaters that specialize on finding pupae and egg cases in tree bark and hidden in other plants can survive on frozen dinners. Once a lake ices over, most ducks, loons, and other water birds have to hightail it out of there, but those birds that can find what little open water there is at our latitude and can capitalize on a wide enough variety of the available aquatic food often stick it out. And birds that depend on weed seeds or other items low on the ground may starve after a deep snowfall, but finches specializing on conifer and birch seeds high in the branches way above the snow-line stay with us through the season.
Some birds have no choice. The white breast muscles on grouse are great for instant bursts of quick speed, but worthless for long, sustained flights. If grouse migrated south for the winter, they’d probably have to walk. Most of our birds can fly a few thousand miles with ease, but think about the dangers migrants have to face: going south through unfamiliar territory, sometimes over enormous stretches of inappropriate habitat where they simply cannot find any food, and then, once they reach the tropics, they have to compete for food with all the resident tropical birds plus all the other migrants, and have to contend with snakes, poison dart frogs, and an enormous variety of avian and mammalian predators. For many species, it simply isn’t worth the trouble just to escape a little cold.
We look out at a robin straggler and feel sorry for it, but sometimes a lone wintering robin is the lucky one, with whole crab apple trees to itself while the ones that moved south are fighting amongst themselves for winter territories and having to share their limited food. Maybe it’s a complicated energy balance that keeps those chickadees in the Northland all winter, but I prefer to think it’s because they like our company, or maybe because chickadees, unlike warblers and tanagers, know for sure that Santa Claus needs snow to run his sleigh.