Red-eyed vireo. John L. Absher/Shutterstock

With several billion birds migrating south, understanding the destination of birds is a daunting task. Banding birds is a tried-and-true technique but rather inefficient. For a bird banded in North America that winters in South America, the chances of capturing that banded bird in South America is slim because banding stations are few.

We now have techniques that do not require a marked bird to be recaptured. Satellite transmitters continue to be miniaturized so that even a small bird’s travels can be tracked from a computer desktop. The Motus Wildlife Tracking System, a project directed by Birds Canada, uses radio telemetry whose signals can be picked up by strategically placed radio antennas. Ultimately, the Motus team would like to have a dense network of antennas from North America to South America so that any tagged bird would be detected regularly on its migration.

Geolocators are small devices that can be fitted to most birds. The locator continuously records light-levels and time. By calculating the day’s length, researchers can determine the latitude. By calculating solar noon, researchers can determine longitude. The downside of this technique is that the geolocator must be recovered. The best way to do so is to band a bird at its breeding site and then recapture the bird the following spring. Fortunately, many bird species show breeding site fidelity.

Let’s consider the red-eyed vireo. This woodland songbird occurs broadly throughout North America. Its population is thought to exceed 125 million birds. As a bird of the treetops, this abundant species is rarely seen by the non-birding population. However, its incessant song, “here I am – over here – in the tree,” alerts a birder to its presence.

Red-eyed vireos spend the winter throughout a broad area in South America, including Venezuela, Colombia, Ecuador, Perú, Bolivia and Brazil. It would be great to know where the vireos nesting in Maine spend the winter, where the vireos nesting in Oregon spend the winter and so on. Perhaps, satellite transmitters, Motus transmitters and geolocators will provide that information.

Thanks to banding projects, we do have a few examples of an intriguing migratory system called leapfrog migration. A great example is the fox sparrow along the Pacific coast of North America.

These sparrows nest from the eastern Aleutian Islands in Alaska south along the mainland Pacific Coast to southern Oregon. But the various populations of birds reverse their position in the winter. The birds nesting in Washington and Oregon do not migrate at all. Those in southern British Columbia leap over the Washington and Oregon birds and migrate to central California. The Alaskan nesters migrate all the way to southern California.

An Old-World example of even greater geographic scope is the ringed plover. These birds nest from the high Arctic down to Portugal. In the winter, the ringed plovers in the United Kingdom, Spain and Portugal don’t migrate at all. German, Polish and Belgian birds migrate only a short distance to Spain and Portugal. Scandinavian nesters winter in northwestern Africa and the arctic nesters winter broadly through Africa, some as far south as South Africa.

So, what is the explanation for this migration pattern? The current explanation is that competitive exclusion is occurring. In other words, birds that get to a more northerly wintering area first stake out a territory there and exclude later arrivals. For a fox sparrow in southern British Columbia, the best choice of territory is just south of Washington and Oregon. This area offers a reasonably moderate winter climate and a relatively quick flight back to their nesting grounds in the spring. The residents in Washington and Oregon force them south. The Alaskan birds with a greater distance to travel fail to reach the central Californian wintering sites before they are occupied by British Columbian birds and move all the way to Southern California.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu