Monitoring black grouse Tetrao tetrix in Isere, northern French Alps: cofactors, population trends and potential biases

Wildlife management benefits from studies that verify or improve the reliability of monitoring protocols. In this study in Isere, France, we tested for potential links between the abundance of black grouse (Tetrao tetrix) in lek-count surveys and cofactors (procedural, geographical and meteorological cofactors) between 1989 and 2016. We also examined the effect of omitting or considering the important cofactors on the long-term population trend that can be inferred from lek-count data. Model selections for data at hand highlighted that the abundance of black grouse was mainly linked to procedural cofactors, such as the number of observers, the time of first observation of a displaying male, the day, and the year of the count. Some additional factors relating to the surface of the census sector, temperature, northing, altitude and wind conditions also appeared depending on the spatial or temporal scale of the analysis. The inclusion of the important cofactors in models modulated the estimates of population trends. The results of the larger dataset highlighted a mean increase of +17% (+5.3%; +29%) of the abundance of black grouse from 1997 to 2001, and a mean increase in population of +47% (+16%; +87%) throughout the study period (1989-2016). We discuss the hypothesis of plausible links between this past increase in the number of black grouse and the ecological impact of the winter storm 'Vivian'. Findings from our study and the ecological phenomena that were concomitant with the observed population trend provide opportunities to strengthen the monitoring and management of black grouse in the Alps.