Habitat association patterns of forest and steppe birds of northern Patagonia, Argentina

-1 censused birds across a moisture gradient in northern Patagonia, Argentina, in the vicinity of Bariloche. Over a 60-km distance, the 12 sites ranged from grassland at lower elevations to upland climax Nothofagus forests of the eastern Andes. Here, I correlated bird abundance and diversities with various vegetation measures. Using all sites, bird diversities and abundances were positively correlated with various foliage measures. When grasslands were excluded, however, an inverse relationship was found: birds were more diverse and abundant in the lower stature shrub communities than in complex forests. Multiple regression analyses of this apparently paradoxical situation indicated that certain species of plants probably had important effects on community structure. As a habitat in a region becomes more complex, the bird community in that habitat usually becomes more complex as well (e.g., MacArthur and MacArthur 196 1, Willson 1974). In southern Argentina, however, Vuilleumier (1972) concluded that the southern beech (Nothofagus) forests had a less complex avifauna than nearby, simpler, scrub areas. In order to describe the avifauna, its habitat associations, and to test this seemingly paradoxical situation, I sampled the avifauna and vegetation associations of the region in the spring of 1980. In this paper, I describe the vegetation types, the bird species associated with each type, and their interrelationships. After the pioneering work of MacArthur and MacArthur (196 l), many studies have confirmed their initial finding that, for terrestrial communities, the number of bird species, as well as their diversity, are strongly positively correlated with aspects of the structural complexity of vegetation (e.g., MacArthur 1964, Recher 1969, Karr and Roth 197 1, Pearson and Ralph 1978). That is, the more complex the structure or composition of the vegetation, the more likely it is that the habitat will contain more bird species (higher richness) or more even abundances of the species (evenness). This relationship is usually interpreted as evidence for the underlying dependence of birds upon their habitat, and thus, lends some degree of predictability. Exceptions to this pattern should be of interest because of what they may reveal about the pattern itself or the systems it attempts to summarize. Thus, it is of interest that Vuilleumier (1972) regarded the northern Patagonian forest bird communities of southern Argentina as such an exception. Although his data were few, they indicated that more species of birds inhabited the structurally simpler scrub-steppe habitats than the more complex, nearby, dense beech forests. Vuilleumier also thought that bird species diversity was higher in the scrub-steppe habitat. A re-calculation (Church 1974) of his data, however, showed that Vuilleumier’s diversity values (H’) in the two habitats were essentially identical (dense forest1.993; scrub-steppe forest1.837). One other study has suggested that bird species diversity was unrelated to vegetation complexity (Howell 197 1). This study, although intensive, had samples drawn from very few study sites. The avifauna of Patagonia is relatively little described, at least in terms of population numbers. My study is the first detailed quantification of these birds and their habitat associations. Such data are important because most ecological generalizations are based on northern hemisphere temperate communities. Comparative data from other areas can provide information on the applicability of these generalizations.