Road Bias for Deer Density Estimates at 2 National Parks in Maryland

Estimating the population density of deer is an essential task for public agencies that plan a herd reduction. Distance sampling has been increasingly utilized to estimate population density, and is used by the National Park Service to estimate white-tailed deer (Odocoileus virginianus) densities throughout the eastern United States. Many of these surveys are conducted along public roads due to limited resources and accessibility, which may violate a critical assumption of distance sampling and potentially introduce sampling bias. We used infrared cameras to confirm deer activity with respect to survey roads at 2 national parks in Maryland, USA (Catoctin National Park and AntietamNational Historic Battlefield), during 2005 and 2006 and compared results with the predicted distributions. The number of deer observed during road surveys declined with distance intervals at Catoctin, but there was a similar amount of deer activity at each distance interval. At Antietam, survey observations maintained a constant level of activity beyond 200 m from the survey route, while deer activity was inconsistent between distance intervals. The mean number of deer photographs/day/sample point did vary significantly across distance intervals from the survey route at Antietam, but not at Catoctin. In Antietam, the uneven distribution of agricultural fields and public roads were significant predictors of deer activity detected during the camera surveys. At Catoctin, the fit of the detection function was improved by expanding the first distance interval. Although density estimation using DISTANCE can account for most sources of error introduced by use of public roads, our data indicate bias is likely to occur in landscapes with high road densities and long sight distances. 2011 TheWildlife Society.