Spring and fall are the busiest seasons for migrating birds. In spring and summer, they migrate to places where the weather is mild, there are plenty of good places to nest, and there’s a reliable supply of protein-rich food (such as caterpillars) to feed growing chicks. In fall and winter, they fly to places where they can stay nice and warm and there is food for them to eat. Not all birds migrate, but the variety of species that do is wide: birds of prey, ducks, hummingbirds, shorebirds, seabirds, and songbirds. Like highways in the sky, routes taken by large numbers of birds on their migratory journeys are called flyways. North America’s mountain ranges and coastlines run mostly north-south, and so does its largest river, the Mississippi. These natural features create four main “flyways” that many migrating birds follow. Scientists divide North America into four flyways, as shown in this map. 8852 Migrating land birds that use these flyways after breeding may...