What is Migration?
Here, There, and Back Again
Seasons can make life difficult for small animals in North America. Although summers provide long days with comfortable weather and plentiful food, winters make daily survival difficult. In order to avoid this dilemma, migratory birds only remain in North America during the summer, when the rich amount of resources can allow them to not only survive but have improved breeding success. Most migratory birds, even those with different diets as adults, need a plentiful supply of insects to raise their young – and there are plenty of summertime insects in North America, especially caterpillars.
As the young birds become more independent, and the weather begins to change, however, these birds know that it is time to head for a different climate. From late summer to early fall, millions of birds begin heading south from North America, mostly following one of a few well-established migration routes. Each species has its own method for navigating: physical landmarks, magnetic fields, the stars, and even ultraviolet light from the sun have all been shown to help birds stay en route towards Central and South America.
Migration is not without its own costs, however. Due to the energetic costs of traveling, birds tend to take the most direct routes possible. This leads them to face a host of dangers, such as unpredictable weather, predators and, in urban areas, window collisions. individuals crossing large areas without stopover locations (such as long trips over the ocean) risk even more without any refuge from potential dangers. For these reasons, migration tends to be the time of life when many birds experience the highest mortality rates.
Many major migration routes converge in Central and northern South America, where the narrow geography of Panamá and Columbia create a bottleneck few birds can avoid. Though some birds go no farther, this bottleneck also provides a unique opportunity for migratory birds continuing further into South America to rest and replenish their energy before continuing their journeys. After spending time in the neotropics, these birds will once again have leave and make the journey back towards North America so they can be ready to take advantage of the resources beginning to emerge in late spring – and the next breeding season.
At a Glance
Fun Facts About Migration
Number of American Migratory Birds: more than 350 species.
Number of Migratory Birds Worldwide: at least 4,000 species, about 40% of all birds.
Longest Migration: 71,000 km (44,000 mi), made anually by the Arctic Tern, Sterna paradisaea, from its breeding ground in the Arctic to the Antarctic.
Longest Non-stop Migration: the Bar-tailed Godwit, Limosa lapponica, has been recorded traveling about 11,250km (7,000 mi) without stopping.
Highest Migration: almost 9 km (5.5 mi) above sea level, by Bar-headed Geese, Anser indicus, which migrate over the Himalaya Mountains.
Fastest Migration: the Great Snipe, Gallinago media, travels 6,760 km (4,200 mi) at an average speed of 97km/hr (60mph).