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

New research by the BTO and the University of East Anglia makes use of the incredible amount of information collected by volunteers taking part in the Nest Record Scheme to investigate the consequences of shifts in timing of breeding for productivity in Willow Warblers. Using information from nearly 7000 nests collected between the 1960s and the present day, this study quantifies changes in the timing of laying dates and seasonal variation in both productivity and timing of breeding (i.e. the proportion of birds nesting at different points in the season), to assess the influence of these factors on changes to overall productivity.

In both north-west Britain (where populations are stable) and the south-east (where populations have declined), Willow Warblers are laying their eggs earlier, and such early nesting attempts fledge a higher number of chicks than those laid towards the end of the season. However, these advances have not lead to an increase in overall productivity, as while the proportion of early-season nests has increased, the seasonal decline in productivity in the north-west has reduced and consequently overall productivity is stable. In the south-east, however, the seasonal decline in productivity has increased and, despite the advance in timing of breeding, overall productivity has declined.

While shifts in the timing of breeding of migratory species are widespread, this study highlights that the consequences for breeding success at the population-scale will depend on both the seasonal pattern of nesting dates (which will be influenced by the dates that birds arrive from their wintering grounds, environmental conditions for breeding, nest failure and re-nesting) and on seasonal variation in productivity. This means that despite breeding success often being highest at the start of the season, advances in laying dates do not necessarily lead to an increase in productivity. Therefore predicting the population-level consequences of phenological changes requires further research to understand both the mechanisms driving seasonal variation in timing of breeding and its success.