The Amazing Details of Bird Migration
Most of us think that birds migrate in winter for one reason: to get to warmer climates. To some extent, that is true, but there are other reasons birds leave one area for another, then return. Here where I live in the northeastern U.S., most of our backyard birds head south to follow the food. Think about it. Birds whose diets are rich in insects and grubs must leave in winter, or perish. And many of our birds are too sensitive to cold temperatures, even though they may feed on seeds and nuts.
So where do they go?
No two species follow the same migration path, known as a “ﬂyway.” Generally, migration is considered to be north and south, though there are northwest to southeast variations with some species. There are four named major North American ﬂyways: Atlantic, Mississippi, Central, and Paciﬁc. These ﬂyways sometimes overlap, however, all four merge into one in Panama.
The Atlantic Flyway runs from offshore Atlantic waters, west to the Alleghenies, curves northwest through northern West Virginia and northeastern Ohio, then into the prairie provinces of Canada/Northwest Territories and on to Alaska’s Arctic Coast. The ﬂyway is very important to migratory waterfowl and other birds that winter on the waters and marshes south of Delaware Bay.(1)
The Mississippi Flyway has an eastern boundary that runs through southern Ontario to western Lake Erie, veering southwest across Ohio and Indiana to the Mississippi River, following the river to its mouth. The western boundary of the ﬂyway touches eastern Nebraska and western Missouri and Arkansas, merging a tiny bit into the Central Flyway. This is the longest migration route of any in the Western Hemisphere.(2)
The Central Flyway begins on the northwest Arctic coast, then merges in the United States toward the east with the Mississippi Flyway, bounded by the Missouri River, and in the south runs through western Missouri, Arkansas, and Louisiana, following the Gulf coast of Mexico on south. The western boundary in Canada follows the eastern base of the Rockies. Known as the “ﬂyway of the Great Plains,” it embraces the vast region between the valley of the Mississippi River and the Rocky Mountains.
The Paciﬁc Flyway territory is made up of the western Arctic, Alaska and the Aleutian Islands, the Rocky Mountain and Paciﬁc coast regions of Canada, the U.S. and Mexico, blending further south with other ﬂyways in Central and South America.
More detailed information and maps may be found at BirdNature.com .
Who migrates, and why?
Not all birds migrate; many remain year-round as permanent residents because they are able to ﬁnd adequate food during the winter months, and they’ve adapted to cold weather conditions. The Northern Cardinal (Cardinalis cardinalis) and American Crow (Corvus brachyrhynchos) are two examples.
Depending on location, many species that we see leaving the northern parts of the country simply travel a few hundred miles south until they ﬁnd a spot that provides food. These are known as short distance migrants. Hairy woodpeckers (Picoides villosus) are considered permanent residents in their breeding areas, but have been documented as migratory in an irregular and non-predictable manner, usually during non-breeding season.
Medium distance migrants have been documented to make journeys north and south, but stay within the United States. Eastern Bluebirds (Sialia sialis) are good examples. They tend to migrate south only as far as needed to ﬁnd food and shelter, or remove themselves from inclement weather. In the southern part of their range, they are permanent residents.
Other species travel thousands of miles in lengthy journeys that boggle the mind. A female Osprey (Pandion haliaetus) was tracked after she left Martha’s Vineyard in Massachusetts, then ﬂew 2,700 miles in 13 days, landing in French Guiana. The Ruby-throated hummingbird’s (Archilochus colubris) yearly trek is well known. Not resilient enough to withstand brutal winters, these tiny creatures leave the northern parts of the U.S. in late August and early September to ﬂy solo to Central America, then make the return trip in mid- to late January.
The neotropical migrants are the “snow birds” of the avian world. They breed in the U.S. and Canada, but winter in the Caribbean, Mexico, Central and South America. Raptors, vultures, waterfowl, shorebirds, thrushes, warblers, orioles, tanagers, and the above-mentioned hummingbirds are included in this classiﬁcation of about 350 species.
A few more familiar birds that migrate
The Canada Goose (Branta canadensis) is probably the most familiar of our migratory birds, with their melancholy call as they pass overhead in V-formation. Native to North America, the species winters in most of the U.S. With temperate climates and an abundance of food, many ﬂocks have become non-migratory. To view a spectacular video of the migration of snow geese (Chen caerulescens), visit Cornell's Ornithology Lab YouTube page.
The charming Carolina Wren (Thryothorus ludovicianus) does not migrate, but being extremely sensitive to cold weather, the populations in northern regions often decrease dramatically with severe winters.
Cedar Waxwings (Bombycilla cedrorum) are exciting to ﬁnd in your garden! Their saucy markings and acrobatics make them a wonderful backyard bird. They are short to long-distance migrants, with many wintering in the southeastern U.S. Birds have been documented traveling as far south as Costa Rica and Panama.
Robins keep us guessing! Most robins move south when the cold weather comes, but a lot of them stick around through the winter. The urge to migrate has to do with food rather than temperature. A robin’s winter sustenance is fruit, so wintery climates aren’t cooperative. However, once the temperatures begin to rise above 36 degrees, the birds will return to northern regions to begin staking out breeding territory.
To truly enjoy the comings and goings of our bird friends, set up a winter feeder and see who stops by on their way to who knows where!
Introduction to Bird Species and Ornithology, Birding.com
Migration of Birds, U.S. Geological Survey, Northern Prairie Wildlife Research Center
(1) (2) “North American Migration Flyways”, Birdnature.com
“All About Birds,” Cornell University
Photos by Toni Leland unless otherwise credited