Associations of grassland birds with vegetation structure in the Northern Campos of Uruguay

摘要

Grassland birds are declining as a consequence of habitat modification, and these declines have generated efforts to determine how birds cope with changes in habitat features, especially in agricultural landscapes. The objective of our study was to examine the relationship between grassland birds and vegetation structure in the Northern Campos of Uruguay. Vegetation variables were sampled on 4 sites that differed in agricultural management: (1) "Crop'' site, a combination of annual crops and seeded pastures; (2) "Pasture'' site, seeded pastures only; (3) "Native 1'' site, natural grasslands under sheep and cattle grazing; and (4) "Native 2'' site, natural grasslands grazed by native Pampas deer (Ozotoceros bezoarticus) and cattle. We used multivariate analyses to examine the relationship between bird abundance and vegetation variables. Vegetation structure varied among sites, and most of these differences can be linked to specific management activities typical of each site. Mean vegetation height was greater (and more variable) in cultivated grasslands (Crop and Pasture sites), whereas mean percentage grass contacts (grass cover) was greater in natural grasslands (Native 1 and 2 sites). Vegetation variables accounted for a significant amount of variation in bird distribution and abundance (14.8%). Grassland birds were associated most strongly with vegetation height, grass cover, and vegetation patchiness. Some of the species associated with the highest values for these environmental gradients were Dark-throated Seedeater (Sporophila ruficollis), with vegetation height; Pampas Meadowlark (Sturnella defilippii), with grass cover; and Burrowing Owl (Athene cunicularia), with vegetation patchiness. Unlike facultative species, many grassland-specialist birds were associated with characteristic features (i.e. high grass cover) of natural grasslands used by livestock. Others, however, were associated with typical features (i.e. taller vegetation) of cultivated grasslands. Thus, Pampas grassland bird populations would benefit not only from native-grassland preservation efforts, but also from management guidelines for agriculture-dominated lands.