Why are some birds here today and gone tomorrow?

Some birds are quite happy to live in one place year-round. Most birds, though, migrate. This means they move during the year. In North America, many birds migrate north in the spring. They are moving to their breeding grounds where there is a lot of food for their young. The longer days of northern summers result in lush plant growth, swarms of insects, and other food. Some birds migrate hundreds, even thousands, of kilometers each year. The ruby-throated hummingbird, for instance, breeds in southern Canada in the summer. The bird spends its winters as far south as Costa Rica.