Shrub and grassland birds at Occoquan Bay National Wildlife Refuge in northern Virginia and the influence burning and mowing has had upon their distribution and abundance

Grasslands are among the most endangered ecosystems in North America (Samson and Knopf 1994; Noss et al. 1995; Vickery et al. 1995). Conservation concerns and the overall purpose of the study plan was to determine the distribution and abundance of shrub and grassland birds at the Occoquan Bay National Wildlife Refuge and the influence burning and mowing has had on their distribution and abundance. The statistical analysis of the bird data was based on one-way ANOVA and General Linear Model (GLM) procedures, with the latter based on repeated measures procedures. Several species were distinctly influenced negatively or positively by the prescription during the experiment. As well, there were several species that showed no effect during the experiment and were probably either responding to yearly or more global or landscape attributes occurring outside the refuge. The area searched/territorial mapping portion of the study was notably the more robust and more dependable survey technique than the fixed-radius point count methodology, because it was more comprehensive, accurate and, in general, was easier to analyze and assess impacts and trends in the data. Clearly, the burning and mowing negatively impacted the Orchard Oriole, Yellow-breasted Chat, and Yellow Warbler, while mowing alone negatively impacted the American Goldfinch. The analysis of the area searched/territorial data found that the Field Sparrow and the Northern Cardinal species were favorably influenced by the burn prescription, while the analysis of the point count data gave uncertain results for both species. Although anecdotal in nature, the observations of nesting Grasshopper Sparrows only in well-burned areas in the grassland is both biologically and managerially significant and should be considered in any management decision. Potentially confusing was the effect of mowing and burning on Common Yellowthroat and Indigo Bunting, where one technique suggested a positive influence to burning and the other an insignificant but negative trend. In conclusion, based on the positive and negative results observed during the experiment, grasslands resource management agencies should plan the use of burning and mowing in a manner that includes provisions for both the shrub and the grassland birds.