Ask Kenn: Do Migrating Birds Take the Same Routes in Spring and Fall?

In this month's column, bird expert Kenn Kaufman explains the phenomenon of "loop migration."

Who's Kenn? Simply put, Kenn is a national treasure. A renowned birder, author, and conservationist, Kenn Kaufman has spent his life dedicated to observing birds, reading about birds, writing about birds, and sharing the world of birds with others. With all that birdy knowledge in his brain, he also acts as the field editor for Audubon magazine. So, whenever we have a bird question stumping us around the office, we just ask Kenn. And now you can, too! If you have a bird or birding question you'd like Kenn to answer, leave them in the comments on Facebook or send us an email. Maybe next month you'll get the kind of thorough, thoughtful, and even humorous response from Kenn we've grown so fond of over the years. —The Editors


Question: Do migrating birds take the same routes in spring and fall?  

Kenn Kaufman: Migration can be fraught with hazards. As birds leave their familiar surroundings and begin to traverse new territory, they may run into all kinds of unknown dangers. We might expect them to minimize their risk by seeking out the shortest possible path between their summer and winter ranges, and then following the same routes north in spring and south in fall. But in fact, relatively few species actually travel by the shortest straight-line distance. And for many, their spring and fall routes differ dramatically.   

Consider the example of the Connecticut Warbler, a skulking bird of the forest interior, highly sought after by birders. You’re unlikely to see one in Connecticut—or in any of the adjacent states—during spring migration. Why? Because their spring route mostly passes farther west. They come north through Florida and then mainly stay west of the Appalachians as they head toward breeding grounds in central Canada. In fall they move east through the northeastern states before turning southward, heading back to wintering grounds in South America.  

Or consider the Tennessee Warbler. (Like the Connecticut Warbler, it was named by Alexander Wilson for the place where he first happened to encounter a migrating individual in the early 1800s.) Tennessee Warblers pass through their eponymous state in both spring and fall, migrating between wintering areas in the tropics and a breeding range that stretches across most of Canada, but their seasonal status varies by region. At the western edge of Tennessee, they are slightly more common in spring than in fall. At the eastern edge of the state, along the Appalachian ridges that form the border with North Carolina, they are much more abundant in fall. This species isn’t unique in this regard; a similar west-east pattern within the state shows up, in a less pronounced way, in Bay-breasted, Palm, and Cape May Warblers, among other birds.  

Of course, this doesn’t just occur within Tennessee’s borders. You can find examples for yourself by exploring the massive database of eBird. Within that program you can look at bar charts that show the frequency of occurrence, week by week, for different areas. So, for example, looking at bar charts for the state of Georgia, on the Atlantic seaboard, we see that Magnolia, Chestnut-sided, and Tennessee Warblers are noticeably more frequent in fall. Farther west, in Louisiana and Arkansas, we see that Bay-breasted and Tennessee Warblers are more frequent in spring. Even without tracking individual birds, these seasonal changes in abundance give strong evidence that many must be shifting their routes.  

This phenomenon of different routes in spring and fall isn’t just confined to warblers. Many other songbirds, from Gray-cheeked Thrush to Lincoln’s Sparrow, demonstrate the same thing. But the Blackpoll Warbler provides the most spectacular example of this seasonal shift, and it’s easy to see if we look at data from Florida. Blackpoll Warblers spend the winter in South America, and in spring they come north through the Caribbean and Florida before spreading out to their huge breeding range, which extends all across the boreal forest. During spring migration in Florida, Blackpolls are widespread and numerous—they’re practically “can’t miss” birds in May, and it’s possible to see dozens on a good day. In fall they are much scarcer, occurring as scattered singles, and it’s noteworthy to find one. The total population of the species is higher in fall, of course, swelled by all the young birds fledged during the summer, so why are there fewer passing through Florida at that season? Because most of them are farther east, out over the waters of the Atlantic.  

For Blackpoll Warblers nesting in Alaska or western Canada, it would seem natural for them to retrace their spring route in fall, angling southeastward through Florida. But they don’t. Instead, those from the western part of the breeding range move strongly eastward across the continent to southeastern Canada and the northeastern states. There they spend time feeding and fattening up, building up fuel for their next flight. After nearly doubling their weight (to as much as three-quarters of an ounce!), they launch out southeastward over the Atlantic. Some will stop on islands in the Caribbean before continuing, but others are thought to fly nonstop to South America, a flight that can take as much as 80 hours.  

This extreme over-water flight is unusual for songbirds, but several shorebirds also do something like this. Hudsonian Godwits provide a good example. These big sandpipers travel north through the center of North America in spring, stopping over on the Great Plains as they head toward the Arctic. In fall they gather at key staging sites in Canada and then migrate east-southeast, tracing a great arc over the Atlantic as they aim for Argentina or Chile. Their fall route may be as much as 2,000 miles east of the spring one. Various other shorebirds, including Whimbrels, American Golden-Plovers, and Buff-breasted Sandpipers, are also known to make long over-water flights from northeastern North America directly to South America. 

The pattern of different seasonal routes is so common that there’s even a term for it: Loop migration. And in North America, scores of bird species travel a clockwise loop, going north in spring along routes that are situated farther west than the routes they follow southward in fall.

Of course, this leads to a much bigger question: Why? What could cause so many unrelated birds to make the same kind of seasonal shift?

As with so many other things in nature, there seem to be multiple factors involved. But at least in eastern North America, seasonal wind patterns appear to play a part. 

A phenomenon called a low-level jet often brings a strong flow of air northward across the Gulf of Mexico and into the southeastern and south-central United States, especially during spring and summer. (Not to be confused with the polar jet stream, which flows generally from west to east around the globe at higher altitudes.) Spring migrants that can catch one of these low-level jets can surf the tailwinds north across the Gulf for a tremendous savings of energy. In fall, migrants can take advantage of winds out of the northwest that often follow weather fronts, riding them toward the east-southeast. For those birds that head out over the Atlantic, after they have flown a substantial distance to the southeast they will get into the zone of the northeasterly trade winds, helping to push them at a more southerly angle toward South America.  

In the West, seasonal habitat changes are more important. For example, Rufous Hummingbirds leave wintering areas in southern Mexico and migrate north through the deserts and along the Pacific Coast in very early spring. At that season the mountains of the interior West may be covered with snow, but the lowland deserts and the coast are relatively green, with many flowers blooming. In summer, after the snows at upper elevations have melted, the Rufous Hummingbirds shift eastward and then migrate south through flower-filled mountain meadows. The switch in route takes them through prime habitat in both seasons. This doesn’t just happen with hummingbirds, however. Many insect-eaters, from warblers to flycatchers, also have a tendency to move north through the lowlands along the Pacific Coast and then south through the mountains of the interior.  

There are many variations on these patterns, and many exceptions as well. No two migratory species have exactly the same ranges or the same routes of travel. But practically everywhere in North America, the migrants passing through in fall will present a slightly different mix from those seen in spring. For birders, that gives us all the more reason to go out at every season, alive to the possibility of seeing something new.