Comparative evaluation of noninvasive DNA sampling and line transect surveys for spring density estimation of black grouse and capercaillie

Reliable abundance estimates provide essential information in ecology, conservation, and management of many wild grouse populations. In this 3-year study, we comparatively evaluate the suitability of traditional line transect distance sampling of flushed birds versus a spatial capture-recapture survey with noninvasive DNA samples for individual identification to estimate spring densities of black grouse and capercaillie in a ~30 km2 boreal forest area in central Norway. The number of observed flushed birds during each field survey period and survey year was low, and did not allow for reliable estimation of abundance from distance sampling in any of the three years, with a total search effort of 745 km. Collection of noninvasive DNA samples and spatial capture-recapture models provided absolute spring density estimates of 1.6 and 2.3 black grouse km-2 in two out of three survey years, and 0.7 capercaillie km-2 in one out of three survey years. Spring population surveys based on a collection of noninvasive DNA samples in a boreal forest habitat could be a better alternative to traditional line transect surveys based on distance sampling of flushed birds in estimating abundance for black grouse and capercaillie, but rely on a sufficient number of unique individuals captured and recaptured at different spatial locations.