Some waterfowl perform 'moult migrations' after breeding, when they move in large numbers to traditional sites, offering abundant food and safety. They stay at least long enough to moult their flight feathers, shedding them all at once, and remaining flightless for several weeks until replacement feathers are grown. Moulting sites can lie in any direction from breeding areas. In many species of geese, however, non-breeders migrate to moult mainly at sites to the north of their breeding areas, starting their southwards migration to wintering areas soon after completing wing moult. In addition, some shorebirds and passerines moult at stopover sites on their southward migration, retaining their powers of flight throughout.
Some bird species move within seasons, in summer raising successive broods in different places, or in winter moving further from their breeding range as the winter progresses. After re-occupying their nesting colonies, many petrel species fly long distances in a 'pre-laying exodus' to obtain the food necessary to produce an egg (females) or undertake the first incubation stint (males). In many birds, winter movements occur in response to food shortage caused either by depletion or snow and ice, or in response to frequent disturbance. Irruptive species, dependent on annually varying food supplies, migrate much further in some years than in others, concentrating wherever they encounter sufficient food.
Nomadism occurs mainly in some desert species which live under the influence of sporadic rainfall. Typically, they show little or no annual consistency in their movement patterns, but each year concentrate wherever food (and in some species water) is available at the time. Some species of boreal and tundra regions are also nomadic to some extent, in association with sporadic tree-seed crops or rodent peaks. Many nomadic species are also seasonal north-south migrants, moving much further in some years than in others, depending on food supplies, and each year concentrating where food is available.
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