The geographical range (spread) of a species or taxon in general, but also applied to the spatial pattern or arrangement of the members of a population or group (local distribution). The study of geographical distribution is called 'zoogeography' and the present distribution may be considered as the result of a process of radial dispersal of groups or species from the areas in which they originated. Distribution in general is determined by factors as climate (mainly temperature and precipitation, in relation to the specific adaptations of a bird), and its effects on terrain, the configuration of landmasses in relation to the original area of habitation and the presence of competitive species or populations in areas otherwise suitable and accessible. Barriers to further dispersal can be, beside the obvious open seas for landbirds, great mountain ranges, deserts and continuous areas of tropical rain-forest, despite the flying capacities of birds. Nevertheless, most orders (except flightless running or 'ratite' birds) and also families are cosmopolitan or pantropical (when absent from colder latitudes). Genera on the other hand are mostly much more restricted in their distribution. In dealing with birds one often has to consider the different seasonal ranges; the breeding range as distinct to the range occupied outside the breeding season (or possibly by non-breeding individuals all year round), or one can give the gross range (both together) which in non-migratory species is the only range. For specific zoogeographical regions, see 'zone(s)'.

Alternative form for distribution : distributions.