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Each autumn, many bird species undertake migrations spanning thousands of kilometers between breeding and wintering grounds, guided by a combination of celestial cues, the earth's magnetic field, and learned landscape landmarks. Some songbirds fly these routes entirely at night, likely to avoid daytime predators and to take advantage of cooler, more stable air. Researchers have found that young birds on their first migration often rely more heavily on an innate magnetic sense, while experienced adults increasingly incorporate visual memory of coastlines, rivers, and mountain ranges. Not all populations of a species migrate the same distance; some individuals within a species may travel only a few hundred kilometers while others of the same species cross entire continents, a variation known as partial migration.