The influence of climate and topography in patterns of territory establishment in a range-expanding bird

The Dartford Warbler Sylvia undata has recently expanded its range northwards and upwards in the UK, consistent with the hypothesis that this cold-sensitive species has responded to a warming climate. We interrogated distribution data, collected during four national surveys of this species between 1974 and 2006, to assess whether this large-scale range expansion has been accompanied by finer-scale changes in topographic characteristics of breeding locations. Within sites occupied in successive surveys, there was some evidence of limited altitudinal expansion between surveys. Within wider landscapes occupied in successive surveys, the preceding winter climate tended to be harsher at newly colonized sites than at sites that had already been occupied in the previous survey, while territories in newly colonized sites also tended to be on steeper slopes, especially if at higher altitude, and (in 1994 only) to be more south-facing. Territories in sites that had already been occupied in the previous survey tended to be lower altitude, less steep and more north-facing than territories in newly colonized landscapes. In 2006 only, the winter climate was significantly milder in newly colonized landscapes than in already occupied sites. The combined effects of a changing climate and topography may have influenced the pattern of in-filling in the existing range, while colonization of distant areas, especially more latterly, may have been facilitated by a combination of increased dispersal pressure from the existing range and warming of climate which made higher altitude habitat in the new areas more suitable for occupancy. Careful consideration needs to be given to the importance of fine-scale topographical variation in determining species' responses to climate change in order to underpin robust adaptation strategies.