High annual variation in grassland bird populations in the Great Plains has often been attributed to changes in moisture levels, but most previous assessments of variation in grassland bird numbers have been of short duration, have occurred during a limited range of moisture levels, have been limited in geographic scope, have considered few species, or have sampled only one or few habitat types. Data from the long-term North American Breeding Bird Survey have the potential to overcome some of these shortcomings. We used linear models and information-theoretic methods to examine associations between moisture levels and populations of 17 species of grassland passerine and two species of wetland passerine in northern North Dakota from 1980 to 2004. We used data from 13 Breeding Bird Survey routes to provide indices of bird abundance and regional dispersion; we used numbers of ponds containing water identified on annual May waterfowl surveys and the Palmer Drought Severity Index (PDSI) as regional moisture indices. Responses varied among species, but the data indicated substantial support for moisture influencing the abundance of 17 of the 19 species we considered. Models including same-year pond numbers generally received more support than models including PDSI data. Dispersion of seven species throughout the study area was influenced by moisture levels, although the response was not as universal and support was not as strong as it was with abundance. Associations between grassland birds and moisture levels suggest the value of regional moisture indices to interpreting studies and surveys of grassland birds in the northern Great Plains.
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Vol. 110 • No. 2