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 isa national treasure. Arenowned birder, author,and conservationist, Kenn Kaufmanhas spent his life dedicated to observing birds, reading about birds, writing about birds, and sharingthe world of birds with others. With all that birdy knowledge in his brain, he also acts as the field editor forܻܲDzmagazine. 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 takethe same routes in spring and fall?
Kenn Kaufman: Migration can befraught withhazards. As birds leave their familiar surroundings and begin to traverse new territory, they mayrun into all kinds of unknown dangers. We mightexpectthem 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 bythe shortest straight-line distance. And for many, their spring and fall routes differdramatically.
Considerthe example oftheConnecticut Warbler, a skulking bird of the forest interior, highly sought after by birders.You’re unlikely to see one in Connecticut—orinany of theadjacent states—duringspring migration. Why? Because theirspringroute mostly passes farther west. They come north through Florida and then mainly staywestof the Appalachians as they head towardbreeding grounds in central Canada. In fall they moveeastthrough the northeastern states before turning southward, heading back to wintering grounds in South America.
Or consider the Tennessee Warbler.(Like the Connecticut Warbler,itwas named by Alexander Wilson for the place where he first happened to encounter a migrating individualin the early 1800s.)Tennessee Warblers pass through their eponymous statein both spring and fall, migratingbetween 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 moreabundant in fall.This species isn’t unique in this regard; a similarwest-east pattern within the stateshows up, in a less pronounced way, in Bay-breasted, Palm, and Cape May Warblers, among other birds.
Of course, this doesn’t just occur withinTennessee’s borders. You can find examplesfor yourself by exploringthe 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,on the Atlantic seaboard, we see that Magnolia, Chestnut-sided, and Tennessee Warblers are noticeablymore 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 toLincoln’s Sparrow,demonstrate the same thing.Butthe BlackpollWarblerprovides 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. DuringspringmigrationinFlorida, Blackpolls are widespread and numerous—they’re practically “can’t miss”birds in May, andit’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 totalpopulation 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 fuelfor 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 stopon 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.HudsonianGodwitsprovide a good example. These bigsandpiperstravel 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 Atlanticas they aim for Argentina or Chile.Their fall route may beas much as2,000 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 patternof different seasonal routesis 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 thingsin 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 airnorthward 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 eastaround the globeat 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, migrantscan take advantage of winds out of the northwest that often follow weather fronts, riding them toward the east-southeast.For thosebirdsthat head out over the Atlantic, after they have flown a substantial distance to the southeasttheywillget into the zone of thenortheasterlytradewinds, helping to push them at a more southerly angle toward South America.
In the West, seasonal habitat changes are more important. For example,Rufous Hummingbirdsleave wintering areas in southern Mexico andmigratenorththrough the deserts andalong the Pacific Coast inveryearly spring. At that seasonthe mountains of the interior West may be covered with snow, but the lowland deserts and the coastare 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 habitatin 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 sameroutesof 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.