The influence of climatic conditions in breeding grounds and migratory flyways on a subalpine Norwegian Willow Warbler (Phylloscopus trochilus) population

Abstract

Many European-breeding bird species wintering south of the Sahel region in Africa have been reported to be in decline since the 1970s. However, most studies focus on conditions local to the breeding sites as determinants of population change, despite the potential influence on populations during migration and over-wintering. To investigate the relative importance of climate at the breeding site and during migration and overwintering periods, we studied a Fennoscandian Willow Warbler (Phylloscopus trochilus) population that has been monitored in its subalpine breeding grounds in Norway over the last 45 years (1967-2011). The long-term trend for our population follows a quadratic trend, showing a marked period of decline after the late 1960s, followed by some increase during the last two decades. The most parsimonious model of the annual change in the Willow Warbler population is obtained by combining the mean June temperature and the Sahel spring index. The annual rate of change in the Willow Warbler population was positively related to summer temperatures on the breeding grounds and positively related to Sahel precipitation, although the statistical support is not very strong. Our study shows that climatic conditions at different stages of the annual cycle have influence on the population change. However, the on-going habitat changes in tropical Africa most likely have negatively affected on the overall population size of the Willow Warbler. This habitat loss may prevent many long-distance migratory bird species from returning to their former population peaks, even though the impact from the recent Sahelian drought may have culminated.