Grassland bird populations have shown persistent declines over the past four decades in North America. Possible explanations for the declines include decreased winter survival because of habitat deterioration. To address this hypothesis, we evaluated the relationship between habitat structure and winter survival of Vesper Sparrows (Pooecetes gramineus) in Chihuahuan Desert grasslands of northern Mexico. We radiotagged and monitored the survival of 102 individuals during the winters of 2009 and 2010. We obtained visual estimates of vegetation structure and composition at every individual's radiotelemetry location. We used an exponential regression model to estimate daily survival rates and determine the association between habitat structure and survival. We estimated a daily winter survival probability of 99.1% (95% confidence interval: 97.4–99.7%) for Vesper Sparrows in both years. Our survival analysis suggests that habitat structure is an important predictor of winter survival. Average grass height and shrub height were positively related to Vesper Sparrow survival. Our results suggest that grassland bird populations may be negatively affected by poor grassland conditions during the winter and that low winter survival may be an important factor in population declines. Winter habitat conditions in the Chihuahuan Desert grasslands are shaped primarily by grazing and climate, highlighting the need to improve range management, especially in Mexico, as a means to reverse persistent population declines of grassland birds.
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Vol. 130 • No. 1