L’évolution des peuplements d’oiseaux le long d’un gradient altitudinal dans l'Himalaya central

The breeding bird communities along an altitudinal gradient ranging from 100 to 5,000 metres, South of the Annapurna range, have been studied in central Nepal, from 30 May to 14 August 1977, and from 6 October to 5 November 1978. Two comparative surveys were also conducted between 2,500 and 5,500 m, during July and September-October 1978, in the Manang and Mustang valleys, on the northern slope of the main Himalaya range. For 1,892 observation periods of 20 minutes each, carried out at 31 different altitudinal levels, a total of 15,116 birds belonging to 344 species were recorded. Birds of prey, swifts, swallows, goatsuckers, and birds living along water-courses were not included in the counts. A grand total of 554 species was observed during our trips. Most of the counts were made under difficult conditions, especially during the monsoon. Species richness, species diversity and population density, although remaining high throughout the elevation gradient, diminish very slowly from the tropical lowlands to the subalpine forests. This unusual feature apparently results from a high speciation rate of such families as the Timaliidae, the Muscicapidae, the Turdidae and the Sylviidae in the temperate and alpine zones. Community structure, and the size of the various trophic guilds, are more closely related to the availability of the various food categories than to vegetation structure per se. Frugivorous birds are prominent only in the tropical lowlands, and seed-eaters in the upper altitudinal levels. Terrestrial insectivores and arboreal gleaners are common at all levels, but flycatchers and bark-feeders disappear in the upper Rhododendron forest, where nectarivores reach their highest density. The narrow distribution of most species, the altitudinal zonation of allopatric congeneric species, the high turnover rate of avian communities between adjacent altitudinal levels and the differences observed between similar habitats in different valleys are pointed out. These characteristics, together with some intraspecific differences in altitudinal distribution and habitat preferences among the localities studied, suggest a strong interspecific competition. The large number of species present at various altitudes most likely explains why “density compensation” and “competition release” were not apparent in the area studied. Some information is also given on the breeding periods, clutch size and altitudinal migrations of birds observed along the Annapurna transect.Thiollay Jean-Marc. L’évolution des peuplements d’oiseaux le long d’un gradient altitudinal dans l'Himalaya central. In: Revue d'Écologie (La Terre et La Vie), tome 34, n°2, 1980. pp. 199-269