The Neotropical Region has experienced large habitat transformations as a result of intensified agriculture. These changes have affected the populations of many species of birds in the Espinal ecoregion of Argentina. However, relationships between birds and agriculture in the neotropics are poorly known. We assessed the effects of crops and field margins in an area of agriculture typical for the Espinal ecoregion on the structure of the bird community, and we assessed the value of this habitat for species of conservation concern. Birds and vegetation were sampled in and along the margins of fields of soybean (the most widespread crop) and alfalfa. Twenty-five of the 41 species recorded—including all the species of conservation concern—were found almost exclusively in field margins; only five species occurred almost exclusively in within the fields. All other species were found in both margins and fields. Density, richness, and diversity were much greater on the margins than in the fields. Bird density in soy fields was lower than in alfalfa. Bird richness and diversity in the two crops, however, did not differ. Density in field margins increased with the cover and height of trees and shrubs. Most of the field-margin species were woodland-border species, but the species of conservation concern were all associated with grassy field margins. Because further declines in the populations of these threatened species are expected, their conservation in agricultural areas would benefit from a broad policy of habitat-conservation plans for field margins such as those practiced in North America and Europe.