Colonization of Secondary Habitats by Peruvian Birds

Complete surveys of the bird faunas of 4— to 10—acre study plots in six habitats were undertaken at a locality in the Apurimac Valley of Peru. Two of the habitats, forest and matorral, represent the primary vegetation of the region. Four secondary habitats incorporated varying degrees of departure from the structure and species composition of natural vegetation. Of the 211 species of land birds found in the 6 habitats, all but 10 were recorded in at least of the 2 primary habitats. With the exception of these 10 species the fauna of the secondary habitats was drawn from 3 species pools: forest, martorral and species common to both. The proportions of forest and matorral species in the secondary habitats varied greatly in relation of the vegetation of the habitats and their proximity to the source faunas. Mist nets were used in all the study plots to sample the using the airspace between 6 in. and 6 ft above the ground. The species diversity in samples of 100, calculated according to the Shannon—Wiener formula, showed rather little variation from one habitat to another. Of greater interest was the fact that in most nests caught fewer than half of the species known to be present. In temperature habitats virtually all species are eventually captured. This contrast is considered to reflect a fundamental difference in the spatial relationships of bird niches in temperate and tropical forests. The above and other evidence indicate that many tropical bird species confine their foraging activities to narrow vertical zones in the vegetational column. The number of bird species in the four secondary habitats was much lower than in primary habitats having similar foliage height profiles. The discrepancies are accounted for by an hypothesis that evokes two complementary parameters: the isolation of tracts of secondary habitat from primary vegetation and certain qualitative aspects of habitat that are not adequately expressed by the foilage height diversity index.