Altitudinal Migration

By moving a few hundred metres down the sides of a mountain, birds can achieve as much climatic benefit as by moving several hundred kilometres to lower latitudes, but without the extra winter daylength. Mirroring the latitudinal trend, with rising altitude, increasing proportions of breeding species move out for the winter, but in contrast to latitudinal migration, altitudinal movements can be in any directions that reach lower ground. They occur on mountain ranges worldwide, and can involve a large proportion of local montane species. Examples include the Citril Finch Serinus citrinella and Water Pipit Anthus spinoletta in Europe, and the Rosy Finch Leucosticte arctoa and White-tailed Ptarmigan Lagopus leucurus in western North America. In many mountain ranges, nectar-eaters also move upward through spring into late summer in response to the progressively later flowering found at higher elevations. Seasonal altitudinal movements occur even on relatively low mountains, such as the Great Dividing Range in southeast Australia, where several montane species appear in lowland towns and farms in winter (see later).

Some species make both altitudinal and latitudinal movements. In western North America, the two rosy finches Leucosticte tephrocotis dawsoni and L. australis are altitudinal migrants (in any direction), two others (L. t. wallowa and L. atrata) are altitudinal migrants with small latitudinal shifts, and two others (L. t. littoralis and L. t. tephrocotis) are mainly latitudinal migrants (King & Wales 1964).

Owing to altitudinal migrations, the valleys and foothills of montane regions often hold a far greater variety of birds in winter than comparable areas in flatter terrain. They include in winter not only the year-round lowland residents, but the immigrants from higher altitudes and from higher latitudes. Some montane species appear to be obligate migrants, the entire population leaving for the non-breeding season, whereas others appear to be partial or facultative migrants, with year-to-year variations in the proportions of individuals that move to lower elevations.

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