Breeding populations of grassland birds are declining across North America. Alteration of wintering-ground habitat in Mexico may be a significant causal component in these declines, considering the rapid changes in land use occurring there. We investigated the influence of precipitation, habitat structure, and plant community on the abundance and distribution of grassland passerines in the state of Chihuahua, Mexico. We conducted bird and vegetation surveys during 1998–2007 in 65 plots at two study areas 75 km apart, Sueco and Tinaja Verde. Grassland guild species were the dominant component of the wintering bird community at both study areas. Passerine density at Sueco during winter was directly related to cumulative rainfall from the preceding December to September, a factor known to be strongly correlated with grass production. Herbaceous and woody vegetation structure and habitat type influenced occurrence of migratory grassland birds at the plot level. Nine of 11 study species occurred in a greater proportion of plots at Sueco than at Tinaja Verde, even though grasslands at the latter area were apparently in better condition. Whereas we may have overlooked key factors differentiating habitat quantity in the two study areas, some of the evidence suggests that Sueco is on a migratory route and Tinaja Verde is not. The future of desert grasslands in Chihuahua is in peril because of the ongoing wide-scale conversion to agriculture there and climate-change predictions of lower rainfall in coming decades.
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Vol. 126 • No. 4