Aerial insect biomass, but not phenological mismatch, is associated with chick survival of an insectivorous bird

Recent insect abundance declines may have affected populations of insectivorous bird species but evidence for this is limited. Here, we use spatially overlapping 29-year time-series of aerial insect biomass and Barn Swallow Hirundo rustica numbers and breeding success from southern England to model the association between changes in invertebrate prey abundance, Swallow productivity and population trends. We found a positive statistical relationship between Swallow chick survival and the biomass of aerial insects available for chicks. In nests where at least one chick fledged, 96.7% of chicks were predicted to survive to fledging where there was high insect biomass (an average of 0.62 g/day), compared with 87.4% of chicks surviving to fledging where there was the lowest insect biomass (0.02 g/day; excluding the greatest and smallest 5% of insect biomass measurements). The amount of food available for chicks was largely a function of annual variation in insect abundance rather than the phenology of egg-laying and insect emergence. However, we did not find a correlation between annual insect abundance and subsequent Swallow population growth. In the context of concerns about declining insect abundance, this study shows how changes in insect biomass may affect the productivity of an insectivorous bird at a regional scale, but with uncertain implications for population size at that same scale.