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Abstract
Grassland bird populations are declining in many regions as a consequence of habitat modification and these declines have generated substantial effort to determine how grassland species cope with changes in habitat features, especially in agricultural landscapes. Grassland bird research has been concentrated in North America and Europe where changes in bird community composition and widespread species declines have been documented. In the Neotropical Region, however, many aspects of grassland bird ecology remain virtually unexplored. Avian communities currently associated with habitats that have experienced different land-use practices have not been fully characterized. The objective of this study was to examine the effects of habitat modification on the distribution, abundance, and reproductive success of grassland birds in the Northern Campos of Uruguay. The study focused on birds that inhabit four grassland habitats which differed in terms of agricultural management in the Northern Campos of Uruguay. Bird assemblages in croplands, planted pastures, and two natural grasslands under different grazing regimes were studied. First, distance sampling was used to characterize bird diversity patterns and population densities along the agricultural gradient. Second, nine vegetation structure variables were quantified and the response of birds species to these variables was assessed with multivariate analyses. Finally, systematic nest searching and monitoring activities were conducted during two breeding seasons and this information was used to estimate nest success patterns of both common and globally threatened species.
Overall, a total of 50 species were recorded; cultivated and natural grasslands were dominated by grassland facultative and obligate species, respectively. Some threatened species (Anthus nattereri, Sturnella defilippi) were largely restricted to natural grasslands. In terms of vegetation structure, grass cover and vegetation height were the two variables to which birds responded most strongly. With respect to nest survival, models that included temporal trends (i.e., seasonal effects) and habitat type effects were the ones best supported by the data. Nest survival of grassland birds was relatively low, but within the range documented in other grassland ecosystems. Contrary to expectation, nest survival of threatened taxa did not differ from that of common congenerics.
This study is the first characterization of grassland bird communities inhabiting an agricultural landscape in the Northern Campos of Uruguay. Using modern sampling techniques and inferential procedures, information on bird diversity patterns and species-habitat relationships was combined with reproductive success data to provide a clear understanding of the effects of habitat alteration on grassland bird populations. The results from this study provide a useful baseline for the development of guidelines targeting bird conservation and land management in the grasslands of the Pampas region.
