Long term trend and short-term dynamics of a willow ptarmigan population

Willow ptarmigan Lagopus lagopus is abundant in Arctic and tundra regions, but rapid climate warming has raised concerns about possible declines as has been observed in several bird species. In this study, we used a hierarchical state-space model to analyze data from a 139 km line transects in mid-Sweden over 48 years. Adult numbers and breeding success were analyzed separately, and we included covariates on vole abundance, numbers of snow-free days in autumn and spring, and the last day of frost in May. We assessed long-term trends in the adult population and estimated the effects of breeding success and weather variation on short-term changes. The estimates of adult density did not show any trend for the period 1976 to 2023, and the dynamics were characterized by a strong direct negative density dependence indicating a stationary process. A number of possible mechanisms have been suggested for how a warmer climate affects willow ptarmigan population dynamics, but our results do not support the hypothesis that lack of snow in autumn and spring increases the vulnerability of willow ptarmigan to predation and leads to population decline. Breeding success is an important driver of changes in, but independent of, adult density. In addition to predation, we propose that climate conditions and emerging vegetation during egg formation and laying are important. We suggest that our results can be explained by a diverse predator assemblage that makes it difficult for the population to escape top-down control, resulting in short-term fluctuations at lower densities.