Here in North America, many species are starting to gather in flocks near their breeding territories. The fierce rivalries that split neighbor from neighbor only last month are vanishing. Loud long melodious songs are growing less frequent, and the quick chirps and burbles of quiet talk are replacing them.
The birds are getting ready to migrate.
Migration in birds is the movement of populations from one area to another as part of an annual cycle. Most bird migration occurs in bird species that breed in the northern part of their range, and spend the non-breeding season somewhere to the south.
Though no one knows for sure why more birds don't migrate north from their breeding range, the best guess is one of simple geography. There's more land area in the north, so more places for birds to spend their breeding season.
This idea is borne out by a look at the birds that do breed in the Southern Hemisphere and spend their non-breeding season in the north. Most of them breed on small flat islands, and there are far more small flat islands in the southern oceans than there are in the north.
Bird migration is often studied by ornithologists, but still relatively little is understood about how birds navigate, and how they know where they're going. It is known that migration in most species is instinctual, birds that have been raised in the laboratory show increased activity and restlessness in fall and spring, when their wild cousins are migrating. Some birds though, such as Whooping Cranes, seem to need to be taught when and where to go when they migrate.
How birds navigate during migration is also still very much a mystery, and different species follow different navigational systems. Pigeons apparently have small magnetic bits in their skulls that tell the bird where they are in the world. Homing Pigeons can be taken in dark boxes for long distances and still fly directly "home" when released. This skill has made them invaluable to people through the ages as message carriers.
Other species of birds use the stars or the sun to find their way (different species migrate during the day or at night), and some may even use the direction of the sound of the nearest ocean.
Some birds, such as Ruby-throated Hummingbirds are able to migrate very precisely. In southern Mexico I met a female hummingbird that returned to the same bush year after year.
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