We present the first quantitative comparative study between current (2006–2008) distributions of landbird and those recorded prior to the massive planting of transgenic soy beans in the Pampas of central Argentina (Narosky and Di Giacomo 1993). We surveyed landbirds along transects covering 23 counties of Buenos Aires province. To allow a comparison between our observations and those of Narosky and Di Giacomo, we used the number of counties in which each species was recorded as an estimate of current and past distributions. We found grassland and wetland specialists in significantly fewer counties than did Narosky and Di Giacomo (P < 0.05), while habitat generalists and woodland specialists showed no significant change. The earlier study, however, covered a longer period of time, which could explain the reduction of wetland dwellers as a result of variation in the supply of temporary water bodies. The decreased area of occupancy of grassland specialists, on the other hand, may reflect the reduction of native grasslands due to increased agricultural cultivation, particularly in the Rolling Pampa, where agricultural expansion has left few remnant grasslands and we did not detect three formerly common grassland specialists, Hymenops perspicillatus, Embernagra platensis, and Pseudoleistes virescens. These findings emphasize the need for conservation actions to support populations of grassland bird in the Pampas.
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