Community Organization and Ecology of Birds of High Elevation Humid Forest of the Bolivian Andes

The avian community composition, and foraging behavior and social organization of birds of two humid forest sites near timberline in the Bolivian Andes were studied and compared to the same features of montane communities in other regions, particularly New Hampshire. Most of the difference in species richness between the tropical and temperate latitude sites can be explained by the addition of "tropical" resources not specially used by, or unavailable to, temperate birds, i.e., fruit, nectar, epiphytic vegetation, and bamboo thickets. When Andean species using these resources are eliminated, the overall species richness between the two regions is very similar, as are other community characteristics, such as use of foraging strata of different heights. The foraging maneuvers used by Andean birds are also discussed in detail. Almost 50 percent of the Andean species are regular participants in mixedspecies flocks. In this respect, as well as with regard to the finer categories of social organization, the Andean community is very similar to one in montane Costa Rica but differs in important ways from others in Venezuela and New Hampshire. RESUMEN. Se estudi6 la composici6n de la comunidad avicola y los comportamientos de forraje y organizaci6n social de aves en dos sitios de bosques humedos cercanos al limite superior boscoso ("timberline") en los Andes bolivianos y se los compar6 con los de otras comunidades montanosas similares en otras regiones, particularmente en Nueva Hampshire, E.E.U.U. La mayoria de las diferencias en la riqueza de especies entre las areas de latitudes templadas y tropicales se pueden explicar por la presencia de recursos "tropicales" adicionales no especialmente usados o fuera del alcance de las aves de regiones templadas (por ej. frutas, nectar, vegetaci6n epifita y canaverales). Cuando se eliminan las especies andinas que usan esos recursos Is riqueza total de especies entre las dos regiones es muy similar, asi como lo son otras caracteristicas de la comunidad tales como el uso de estratos de forraje de deferentes alturas. Tambien se discuten en detalle las maniobras de forraje usadas por las especies andinas. Casi el 50% de las especies andinas participan de las bandadas de varias especies de manera regular. En este aspecto, asi como con respecto a las categorias mas detalladas de organizacidn social, la comunidad andina es muy similar a una en montanas costarricences, pero diferente en aspectos importantes de otras en Venezuela y Nueva Hampshire, en el noreste de E.E.U.U. The problem of latitudinal differences in bird species diversity has often been investigated by comparing properties of land bird communities of tropical lowland regions to those of communities at temperate latitudes (Ricklefs 1966; Orians 1969; Howell 1971; Karr 1971; Faaborg 1980; Terborgh 1980; Greenberg 1981). Only comparisons by Stiles (1978) for alder habitats and by Fjeldsa (1981) for wetlands have focused on communities in the Neotropics other than those of lowland areas. My purpose in this paper is to provide insight into latitudinal diversity gradients by comparing relatively low diversity bird communities in montane forests of tropical and temperate regions, using methods similar to those of Karr (1975) and Terborgh (1980). I also present information on the foraging ecology of birds of high elevation forest of the humid Andes and compare it with that available from similar habitats elsewhere. STUDY SITES AND METHODS Two localities in the upper "Temperate Zone" of the humid Andes of Dpto. La Paz, Bolivia were studied, 1 km south of Chuspipata, 3050 m (hereafter referred to as Chuspipata), and Cotapata, 4.5 km WNW Chuspipata, 3300 m (hereafter, Cotapata). Although these sites are within 5 km of one another, they differ in the following important ways, (1) air temperatures are substantially lower at the upper site (mean daily lows 2°C versus 8°C, mean daily highs 10°C versus 14^); (2) average canopy height is lower at the upper site (5-12 m vs 14-17 m);