Laura Erickson’s For the Birds

In the coming weeks, more and more birds will make a long journey southward—many more than filled the skies this past spring before the young of the year were produced.

Bird migration is such a beautiful and reliable phenomenon that it’s hard to fathom how long it took for people to understand as much as we do today. People often cite Aristotle’s belief that swallows buried themselves in mud to hibernate—a belief that survived well into the 1800s—but Aristotle also wrote about cranes traveling from the steppes of Scythia to marshes at the headwaters of the Nile, and three thousand years ago, Micronesians and Polynesians managed to locate tiny islands in the vast ocean by paying attention to bird movements. So even long ago, people knew a little about how birds move about, and by the time I started birding in 1975, we understood a great deal about migration related to timing, routes, navigation, physiology, and more. In the 50 years since, technology has allowed us to learn much, much more.

It seems obvious that birds migrate because they want to escape the cold temperatures of winter, and it’s true that many tropical birds cannot survive frigid temperatures. If they evolved entirely within the tropical and subtropical environments, as the todies of the Greater Antilles did, they don’t have the dense down feathers that provide insulation against cold, nor can they deposit and then metabolize enormous quantities of fat the way birds must do to stay alive during deep freezes. The Black-capped Chickadee and Cuban Tody are very similar in size, the tody 11 cm long and the chickadee just a little longer (12–15 cm). The tody has a longer bill, but the difference in length is mostly due to the chickadee’s longer tail. Yet even though this chickadee is slightly longer, the tody weighs FOUR TIMES as much as the heaviest chickadee! (59 grams compared to 9–14 grams). Beneath the feathers, the chickadee’s body is very tiny and scrawny compared to the tody’s. No need for so much featherweight insulation to survive Cuban winters!

But surviving the cold is only half the problem a Cuban Tody would face in a Midwestern winter,. Even before the thermometer plunged to single digits, the poor tody would starve to death—insects, especially flying insects and those on leaves, provide the vast majority of its food. Chickadees eat a lot of insects, too, and I’ve actually observed one pluck a small moth out of the air once, but in winter they feed mostly on seeds supplemented with insect pupae and eggs hidden in the crevices of tree bark and other vegetation. Golden-crowned Kinglets and Brown Creepers are almost entirely insectivorous, but like chickadees, they feed on pupae and eggs, not actively moving insects. Rufous Hummingbirds are known to have not just survived winter in northern states but to return in subsequent winters, but the only records are at hummingbird feeders, the sugar water often supplemented with protein. When two different Rufous Hummingbirds remained at my own bird feeders through early December before moving on, I observed them both plucking insects from branch tips of my spruce trees and from stands of native weeds, but they could count on calories from my feeders.

Birds that specialize on flying insects, including most flycatchers and swallows, head to the tropics where they can count on a food supply through winter. Tree Swallows are not quite an exception, because they have significantly longer intestines than other swallows and produce enzymes allowing them to digest the wax coatings of bayberries and wax myrtle berries.

Most birds that eat fish must leave in winter before ice covers their fishing waters. Sometimes an individual Great Blue Heron or Belted Kingfisher remains in Minnesota through the Christmas Bird County, but invariably in a spot where there is still open water, but unfrozen shallow water is hard to find by late December and January. Birds that feed mostly on weed seeds and seeds picked up on the ground tend to spend a lot of time at feeders or head to areas where snow won’t cover most of their food sources. The northern finches can stick around because they specialize on the seeds of conifers, birches, and other plants that hold onto their seeds well above the ground.

Food may be the driver of migration, but birds don’t leave as food grows scarce—for most species, migration is timed for a period of abundance. Flycatching species leave earlier than birds eating insects within vegetation or on the ground, and those leave earlier than birds feeding on seeds or berries. Raptors feeding on birds time their migrations for when their prey is most abundant, so Sharp-shinned Hawks and Merlins head out starting when the first warblers and flycatchers migrate en masse, but because both species are also adept at catching blackbirds and sparrows, they have a wide window for their migration.

Over the coming weeks I’ll talk more about the rhythms of migration not just seasonally but through a 24-hour period. Why do so many birds heading to the tropics travel by night even though their night vision is no better than ours? I’ll also focus on the many natural hazards migratory birds face, greatly exacerbated by the hazards we humans create that kill millions of birds every fall, and what we can do to minimize the dangers.