Laura Erickson’s For the Birds

Most birds are diurnal—that is, they’re active in the daytime. Bird vision is keener across a wider spectrum than ours, but except for owls and other truly nocturnal or crepuscular species, most birds probably can’t see in the dark any better than we can.

They may not be able to see well at night, yet we know that that’s exactly when most neotropical migrant songbirds, such as vireos, warblers, and tanagers, make their long migratory flights. Nocturnal migration could never have evolved unless more individuals of those species survived by migrating at night than during the day. Why are they safer flying by night, how do they navigate when landmarks are obscured in darkness, and how do they avoid collisions with trees, other obstructions, and one another in the dark? In this post, I’ll explain the whys and leave the hows for another post.

Many of the birds who fly to the tropics are tiny—most warblers weigh significantly less than an ounce. The smaller the bird, the more calories it burns for its body weight, and these guys burn a lot of calories while winging their way to the Caribbean, Mexico, Central America, and even South America. When conditions are favorable, they can cover long distances, but during every waking moment in daytime, they also must focus on eluding falcons and hawks whose hunting method is to pluck birds out of mid-air. Staying closer to protective foliage during daytime and flying high when most raptors are asleep makes sense.

Since warblers can’t see in the dark well enough to capture insects at night, if they migrated by day they’d have to stop frequently for meals. Moving by night, they can cover many more miles per 24-hour period. And finally, meteorological conditions at night are far more favorable than daytime for continuous flapping—not only are temperatures cooler, but windspeed tends to drop significantly at night. Tailwinds can be helpful, but not cross- or headwinds.

In general in the upper Midwest, where much of the land is forested and virtually all of it, even in residential neighborhoods, contains an assortment of trees and shrubs that provide insects and fruits, the typical pattern is for warblers to come down at first light when they can be a little selective about where they’ll land. They spend the day resting and pigging out, moseying along in a generally southerly direction. They eat especially heavily before sunset and then take another nap after it gets too hard to see well. At some point after it’s truly dark, they take off for parts unknown, covering anywhere from 15–200 miles on most nights.

The pattern is a little different here along the north shore of Lake Superior, at least when the wind is westerly. On those mornings, many warblers cover a lot more ground before breakfast, staying aloft for an hour or more (sometimes as much as five hours) after sunrise, most likely so they can clear Lake Superior before nightfall. If they do end up flying over the open lake, a downdraft or sudden storm could drive them into the water.

Flying over the big lake by day or night is dangerous, but every autumn one tiny warbler travels over a much larger body of water, the Atlantic Ocean. The Blackpoll Warbler who, when bulked out, weighs barely half an ounce (13 grams), regularly takes off from the Eastern Seaboard on a non-stop 1,800-mile non-stop flight to the Caribbean and northern South America—a journey that can take three full days and nights. Our modern world has made this lovely bird’s life on its breeding grounds (in Alaska, Canada, and parts of northern New England) and on its tropical wintering grounds, as well as the arduous route between, more treacherous than ever—in the past 40 years, the Blackpoll Warbler has lost an estimated 88 percent of its population.

Yep—migration is dangerous, as is trying to eke out an existence as an insectivore during an insect apocalypse anywhere on a warming planet in which habitat destruction, pervasive pesticides, microplastics, artificial illumination, window glass, and other ubiquitous human-produced dangers abound. The miracle of nocturnal migration can only endure as long as nocturnal migrants endure. I want my dear little grandchild to be able to enjoy the wealth of natural wonders that I’ve spent a half century enjoying. That is his rightful heritage as a human being on this beautiful planet—the kind of generational prosperity worth investing in.