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听a national treasure. A听renowned birder, author,听and conservationist, Kenn Kaufman听has spent his life dedicated to observing birds, reading about birds, writing about birds, and sharing听the world of birds with others. With all that birdy knowledge in his brain, he also acts as the field editor for听础耻诲耻产辞苍听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.听鈥擳he Editors
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Question: Do migrating birds take听the same routes in spring and fall?听听
Kenn Kaufman: Migration can be听fraught with听hazards. As birds leave their familiar surroundings and begin to traverse new territory, they may听run into all kinds of unknown dangers. We might听expect听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听the shortest straight-line distance. And for many, their spring and fall routes differ听dramatically.听听听
Consider听the example of听the听Connecticut Warbler, a skulking bird of the forest interior, highly sought after by birders.听You鈥檙e unlikely to see one in Connecticut鈥攐r听in听any of the听adjacent states鈥攄uring听spring migration. Why? Because their听spring听route mostly passes farther west. They come north through Florida and then mainly stay听west听of the Appalachians as they head toward听breeding grounds in central Canada. In fall they move听east听through the northeastern states before turning southward, heading back to wintering grounds in South America.听听
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Or consider the Tennessee Warbler.听(Like the Connecticut Warbler,听it听was named by Alexander Wilson for the place where he first happened to encounter a migrating individual听in the early 1800s.)听Tennessee Warblers pass through their eponymous state听in both spring and fall, migrating听between wintering areas in the tropics and a breeding range that stretches across most of Canada, but their seasonal status varies by region.听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听abundant in fall.听This species isn鈥檛 unique in this regard; a similar听west-east pattern within the state听shows up, in a less pronounced way, in Bay-breasted, Palm, and Cape May Warblers, among other birds.听听
Of course, this doesn鈥檛 just occur within听Tennessee鈥檚 borders. You can find examples听for yourself by exploring听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,听听on the Atlantic seaboard, we see that Magnolia, Chestnut-sided, and Tennessee Warblers are noticeably听more frequent in fall.听Farther west, in Louisiana and Arkansas, we see that Bay-breasted and Tennessee Warblers are more frequent in spring.听Even without tracking individual birds, these seasonal changes in abundance give strong evidence that many must be shifting their routes.听听
This phenomenon of different routes in spring and fall isn鈥檛 just confined to warblers.听Many other songbirds, from Gray-cheeked Thrush to听Lincoln鈥檚 Sparrow,听demonstrate the same thing.听But听the Blackpoll听Warbler听provides the most spectacular example of this seasonal shift, and it鈥檚 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听spring听migration听in听Florida, Blackpolls are widespread and numerous鈥攖hey鈥檙e practically 鈥渃an鈥檛 miss鈥澨齜irds in May, and听it鈥檚 possible to see dozens on a good day.听In fall they are much scarcer, occurring as scattered singles, and it鈥檚 noteworthy to find one. The total听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.听听
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鈥檛.听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听for their next flight. After nearly doubling their weight (to as much as three-quarters of an ounce!),听they launch out southeastward over the Atlantic.听Some will stop听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.听听
This extreme over-water flight is unusual for songbirds, but several shorebirds also do something like this.听Hudsonian听Godwits听provide a good example. These big听sandpipers听travel north through the center of North America in spring, stopping over on the Great Plains as they head toward the Arctic.听In fall they gather at key staging sites in Canada and then migrate east-southeast,听tracing a great arc over the Atlantic听as they aim for Argentina or Chile.听Their fall route may be听as much as听2,000 miles east of the spring one.听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.听
The pattern听of different seasonal routes听is so common that there鈥檚 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听in nature, there seem to be multiple factors involved. But at least in eastern North America, seasonal wind patterns appear to play a part.听
A phenomenon called a low-level jet often brings a strong flow of air听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听around the globe听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听can take advantage of winds out of the northwest that often follow weather fronts, riding them toward the east-southeast.听For those听birds听that head out over the Atlantic, after they have flown a substantial distance to the southeast听they听will听get into the zone of the听northeasterly听trade听winds, helping to push them at a more southerly angle toward South America.听听
In the West, seasonal habitat changes are more important. For example,听Rufous Hummingbirds听leave wintering areas in southern Mexico and听migrate听north听through the deserts and听along the Pacific Coast in听very听early spring. At that season听the mountains of the interior West may be covered with snow, but the lowland deserts and the coast听are relatively green, with many flowers blooming.听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听in both seasons. This doesn鈥檛 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.听听
There are many variations on these patterns, and many exceptions as well. No two migratory species have exactly the same ranges or the same听routes听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,听alive to the possibility of seeing something new.听