Distance sampling: Comparing walked transects and road transects for rock ptarmigan densities and population trends

We compared population trends for rock ptarmigan (Lagopus muta) densities (2003‒2019) derived from walked transects and driven road transects in Mosfellsheiði and Slétta in southwest and northeast Iceland, respectively. The walked transects were laid out according to a random rule. Convenience-based road transects could give biased population density estimates if roads affect the distribution of ptarmigan. We used distance sampling to compare density estimates provided by the two survey types. Our results showed that road transects were more than five times faster to conduct than walked transects. Estimated ptarmigan density changed in synchrony for the two survey methods in both study areas. Mean density estimates in Mosfellsheiði were similar for the two survey methods (walked transects 1.6 males × km-2, 95% CI 1.4‒1.8; road transects 1.7 males × km-2, 95% CI 1.4‒2.0), but not in Slétta, where density estimates for road transects were significantly lower (walked transects 5.2 males × ..., Study area
 
Our study occurred in two distinct areas, one in southwest Iceland called Mosfellsheiði (N64.13591, W21.44585) and the other in northeast Iceland called Slétta (N66.4683, W16.476; Fig. 1). The linear distance between the two areas is 360 km. The Mosfellsheiði study area (210 km2) is 15 km from the coast and has altitudes ranging from 200 to 400 m above sea level. The Slétta study area (50 km2) is close to the coast, and altitudes range from sea level to approximately 40 m above sea level. The landscape on Slétta is best described as flat or gently undulating; on Mosfellsheiði, the ground is less flat, with low ridges and shallow depressions between them. Both study areas are treeless. The habitat types on Mosfellsheiði were more variable than those on Slétta. The dominant habitat types on Mosfellsheiði were mosslands (57%) and heathlands (23%), but other components included lava fields (7%), wetlands (7%), and fell fields, moraines, and sands (combined 4%). The dominan..., , # Distance sampling: Comparing walked transects and road transects for rock ptarmigan densities and population trends

[https://doi.org/10.5061/dryad.zgmsbccpj](https://doi.org/10.5061/dryad.zgmsbccpj)

## Description of the data and file structure

Spring surveys of territorial ptarmigan males have been used to derive annual densities in Iceland. These counts were started in the early 1960s using the territory mapping method on designated plots, but since 1999, walked and road (driven) transects have been included, applying the distance sampling technique to collect and analyze the transect data. While the territory mapping method assumes the detection of all individuals on the designated plot, distance sampling considers variable detection probabilities based on the distance from the transect and other covariates. Road transects for ptarmigan are less demanding than walked transects. Still, they may break one of the basic assumptions of distance sampling, namely random spacing of tran...,