Sedentary Populations

At the opposite end of the spectrum from migratory populations are sedentary (or resident) ones. A sedentary bird population can be defined as one whose distribution and centre of gravity remain more or less the same all year round, and from year to year. Individuals of sedentary populations typically show no directional bias in their movements at any time of year (unless imposed by local topography), and generally move over much shorter distances than migrants. In Britain, as elsewhere, large numbers of many resident bird species have been ringed as chicks and adults, and the subsequent recoveries of birds found dead and reported by members of the public have given some idea of their overall movement patterns. Typically, most birds of non-migratory species were found (up to several years later) near where they were ringed, in all directions, but with progressively fewer at increasing distances. In many resident songbird populations, the median distance moved between ringing and recovery sites was less than 1 km, but some individuals had reached more than 20 km. This pattern held for such sedentary passerine species as House Sparrow Passer domesticus and Carrion Crow Corvus corone, and for non-passerines such as Moorhen Gallinula chloropus and Grey Partridge Perdix perdix. All these birds are likely to have made their longest movements in the immediate post-fledging period, soon after becoming free of parental care. Of course, other 'sedentary' species make longer movements, yet in all such species the population as a whole retains the same broad-scale distribution year-round.

The everyday movements that birds make to obtain their daily sustenance vary enormously between species, depending on their particular lifestyles. In species that live year-round in territories, individuals may spend their whole adult life in a confined area, varying from less than one hectare in some small songbirds up to a few hundred square kilometres in some large eagles (Newton 1979). In species in which breeding and feeding occur in different areas, everyday movements may be longer. For example, some radio-tagged female Brown-headed Cowbirds Molothrus ater studied in California travelled 9-16 km every day between their breeding and foraging areas, and even further to a communal roost (Curson et al. 2000). Other flock-feeding species, which base themselves in nesting colonies or communal roosts, forage over even larger areas. For example, radio-tagged Common Grackles Quiscalus quiscula studied in Oklahoma commuted an average of 24 km between roost and feeding area, and ranged during the weeks of study over an average of 325 km2 (Bray et al. 1979). Among colonial seabirds, individuals may forage over very much larger areas, extending over many thousands of square kilometres, as mentioned earlier. In all such species, the same individuals may maintain the same distribution pattern year-round, or at least over large parts of each year.

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