Distance sampling: comparing walked transects and road transects for rock ptarmigan densities and time series

We compared times series for rock ptarmigan densities (20032019) derived from walked transects and road transects in Mosfellsheiði and Slétta in southwest and northeast Iceland, respectively. The walked transects were laid out according to a random rule, but the road transects were not. Data from both survey types were analysed according to the distance sampling methodology. The purpose was to compare results derived from the two survey types. Road transects are easier to conduct than walked transects, but they break a fundamental rule of distance sampling, namely random placement of transects. Our results show that road transects are more than five times faster to conduct than walked transects. There were no indications that ptarmigan avoided roads. Population indices were similar for the two survey methods in both study areas. Density estimates in Mosfellsheiði were identical for the two survey methods but not in Slétta, where density estimates for road transects were lower. The Slétta road transects were biased low because parts of the road transect intersected habitat that ptarmigan do not occupy. This bias can be remedied at least partially by considering non-habitats by applying multipliers in the distance analysis.