Season-long consequences of shifts in timing of breeding for productivity in Willow Warblers, Phylloscopus trochilus

Capsule Breeding Willow Warblers, Phylloscopus trochilus, in the UK have advanced in timing of breeding but with little impact on overall productivity. Aims To quantify the impact of shifts in timing of breeding on changes in Willow Warbler productivity across the UK. Methods Using records of similar to 7000 nests from the British Trust for Ornithology Nest Record Scheme, we quantify shifts in timing of breeding and seasonal variation in productivity and timing of breeding, and explore their relative contribution to changes in overall productivity. As population trajectories vary across Britain (south-east declining, north-west stable), analyses were conducted regionally. Results In both regions, nesting dates have advanced and early-season productivity is highest. However, while the proportion of early-season nests has increased, the seasonal decline in productivity in the north-west has lessened, and overall (season-long) productivity is stable. In the south-east, however, the seasonal decline in productivity has strengthened and, despite the advance in timing of breeding, overall productivity has declined. Conclusions Advances in timing of breeding have not led to increased Willow Warbler productivity in the UK, because the population-level impact of advances depends on seasonal patterns in both productivity and the distribution of nesting dates within a population.