Major themes and concepts to keep in mind when thinking about migration:

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Areas and niches where birds spend the winter are often saturated during the breeding season when the need for space and resources is highest. Large tracts of habitat which lay dormant during the winter have an abundance of food and breeding sites during spring and summer.

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The incentives and for birds to migrate is huge as is the cost. A significant portion of individual birds become casualties each migration. Still the tradition of migration continues to attract thousands of species and countless individuals worldwide.

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As days get shorter and food becomes scarce, birds begin to move south. Adult birds are experienced, having made the journey before. Young birds however are often left to their instincts to find their way south.

Lost birds or birds off of their normal migration routes are known as "vagrants". These are more often than not young birds making their first migration.

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Shorebirds including sandpipers, plovers and turnstones are notably fast breeders and flyers. Turnstones for example are not gone from the Galapagos until mid May yet some are back there again by July. Presumably these birds have been to the arctic tundra, built a nest, incubated eggs, fed themselves and their chicks and flown back.

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Similar to shorebirds, ducks and geese breed in the far north however many may not fly south until deep winter is about to set in.

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Birds which spend most of the year in or near the tropics are known as holarctic migrants including many of North America's breeding warblers, tanagers, flycatchers and orioles. These are the first song birds to move south as they are dependent on insects which disappear quickly with the onset of cold and winter.

Many songbirds spend the breeding season in northern forests and mountains and then retreat to milder coastal areas during winter. These are the last songbirds to migrate and include Thrushes, Kinglets and Sparrows.

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Migration for hawks and raptors begins early but is prolonged as they may remain in areas where prey is abundant before moving on. Except for falcons, raptors fly more slowly than other birds.

There are species and populations of many birds that do not migrate at all, but rather stake out a certain area for their whole lives.

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To add to the list of natural risks inherent to migration are countless new and more dangerous risks from the human world including habitat fragmentation and the introduction of nonnative species. One little discussed issue conservation issue is "towerkill", where swarms of migrating birds collide with brightly lit cell phone transmission towers at night.