A paper published this week by scientists at the Cornell Lab of Ornithology uses eBird data to map the migratory paths of birds in the Western Hemisphere. eBird is a free online checklist program used by tens of thousands of bird watchers. The database now has millions of observations from across the world, most of them in North America. This latest study used 6.1 million eBird checklists to find the “center” of bird species’ ranges through the year. The summary below shows, for 118 species, how these centers move. Each point is one species, tracked through checklists over a year.
A pattern emerges: birds tend to “loop” clockwise, migrating north in the spring overland through Central America and Mexico, then flying south in the autumn over the Gulf of Mexico and the Atlantic. These loops are caused by prevailing winds, especially the northeast trade winds over the ocean. Turn, turn, turn: sung by the Byrds.
To my eye, deviations from the pattern are just as fascinating. Why do some species veer so far east? Why do some linger in the south, then zip north in a rush? A map with each dot labelled by species lets us peer into these mysteries.
To write honestly and with conviction anything about the migration of birds, one should oneself have migrated. Somehow or other we should dehumanize ourselves, feel the feel of feathers on our body and wind in our wings, and finally know what it is to leave abundance and safety and daylight and yield to a compelling instinct, age-old, seeming at the time quite devoid of reason and object.” — William Beebe