Using the spatial filtering process to evaluate the nonbreeding range of Rusty Blackbird Euphagus carolinus
During the nonbreeding period, Rusty Blackbird (Euphagus carolinus) occurs predominantly in forested wetland habitats in the southeastern U.S. We used spatial filtering of Christmas Bird Count data to identify areas within the nonbreeding range where the species occurs at higher than expected probability. Spatial filtering is an epidemiological modeling process developed to identify concentrations of cases of a phenomenon, and can be tested for statistical significance using Monte Carlo simulations against a ' hypothesis that assumes a uniform distribution of the phenomena. Using separate data sets in which cases were identified as “Occurrence of at least 1 Rusty Blackbird” or as “Occurrence of at least 10 Rusty Blackbirds”, we developed annual probability estimates of observed occurrence vs ' simulations of the existing cases distributed at random among locations at which Christmas Bird Counts were conducted. We were thus able to identify consistent concentrations of Rusty Blackbird occurrence in the Mississippi Alluvial Valley and in the southeastern Coastal Plain of the Carolinas and Georgia. We were also able to identify and eliminate some of the noise in the data that arises from convenience sampling method used in the Christmas Bird Count. Spatial filtering is a method of considerable utility for investigating spatial distribution of birds and comparing the observed distribution with a ' expectation.