For individual birds, dispersal can be measured by the distances between natal sites and subsequent breeding sites (natal dispersal), between the breeding sites of different years (breeding dispersal), and between the wintering sites of different years (non-breeding dispersal), regardless of any movements made in the interim. In all these types of dispersal, as well as in post-fledging dispersal, directions appear random (and centrifugal at the population level). Dispersal enables individuals to leave areas of overcrowding or poor food supply to explore and find somewhere better; it also reduces the chance of pairing with a close relative. At the population level, dispersal movements influence gene flow and the genetic structure of populations, facilitating or suppressing the development of locally adapted populations and subspecies. Dispersal movements also influence patterns of abundance and distribution across the range, enable depleted local populations to recover, vacated areas to be re-colonised or new areas to be occupied, leading to range expansion.
In many resident and migratory bird species, individuals tend to settle and breed in the neighbourhood where they were raised, and the numbers of dis persed individuals declines with increasing distance from the natal site. However, individuals of large bird species generally move longer distances than those of small species. In addition, individuals of species that depend on ephemeral habitats or food sources tend to disperse further, on average, than do individuals of species with annually consistent habitats and food sources. Within species, dispersal distances also vary with population density and other factors that promote competition, and differ according to gender and other features of the individual. In many bird species, females move generally further between natal and breeding sites than do males, but in waterfowl and some others, males move furthest.
In general, adults move over much shorter distances between their breeding attempts of successive years than young of their species move between natal and first breeding sites. Where conditions remain fairly stable from one breeding season to the next, adults of many species use the same nesting territories in successive years, although some individuals move to nearby territories, females more often than males, and especially after a breeding failure. Where local conditions fluctuate from year to year, adults can move long distances between their nesting sites of successive years. Long natal and breeding dispersal distances (of up to hundreds of kilometres) are frequent, for example, in some seed-eaters that depend on sporadic tree-seed crops, and in some ducks that depend largely on ephemeral waters. All such species tend to concentrate in different areas in different years, wherever conditions are good at the time.
In wintering areas, some birds return to the same localities from year to year, while others are more mobile, within and between winters. The broad behavioural spectrum, from winter residency to almost continual and variable movement, appears to reflect a gradient in consistency in food supplies, ranging from stable and predictable to unstable and unpredictable.
In many resident bird species, post-fledging dispersal turns into dispersive migration, in that individuals move back toward their natal site as the breeding season approaches. In such out-and-back movements, directions appear random, but one sex tends to disperse further than the other and juveniles further than older birds. Dispersive migration thus has some features of dispersal (directions highly variable but generally short distances) and some features of migration (return movement, differing between the sexes, often repeated year after year).
Young birds become fixated on their natal area in the late nestling or post-fledging period, depending on species, and on the wintering area within a few weeks after their first arrival there.
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