Demography of Common Swifts Apus apus breeding in the UK associated with local weather but not aphid abundance

Swift was added to the UK Birds of Conservation Concern Red List in 2021 because of the 57% decline seen on BTO/JNCC/RSPB Breeding Bird Survey squares between 1995 and 2017. Identifying the reasons for the decline are complicated by its migratory lifestyle; the factors that might be driving the decline could be operating on the breeding grounds, during migration, in the wintering areas, or indeed across all of these components of the bird’s annual cycle. The aim of this study was to improve our understanding of the drivers of Swift decline in Great Britain, with a particular focus on the effects of weather and the availability of aphids, the latter an important source of food for Swifts.

Information on the availability of aphids was drawn from the Rothamsted Insect Survey, a national scheme that monitors the abundance of flying insects through a network of more than 30 giant suction traps. Data from the national Ringing Scheme and Nest Record Scheme were used to explore how measures of breeding success and survival have changed over time, and in relation to changes in aphid biomass and weather conditions. Breeding success and survival are two of the things that influence the size of any population, operating alongside emigration and immigration. Demography is the branch of ecology that studies these processes and how they shape the growth and regulation of populations.

Although Ringing Scheme and Nest Record Scheme data for Swift are sparse prior to the mid-1990s, they reveal that the survival rates of adult Swifts have been largely stable over this period. However, there has been a reduction in the survival rates of first-year birds and an increase in the failure rates for nesting attempts, both of which have declined since the mid-1970s. Despite marked declines in aphid biomass across much of southern and eastern England over the same period, the researchers could find no evidence for a link between total aphid biomass and these measures of Swift demography. 

Weather was a stronger correlate of variation in Swift demography, with increased rainfall associated with smaller brood size, higher levels of nest failure and lower first-year survival rates, presumably as a result of decreased aphid activity and, hence, foraging success. Taken together, these findings suggest that falling first-year survival, partly linked to wetter summers, is contributing to the observed decline in Swift populations. While more work is needed, not least to examine the potential for what is happening away from the breeding grounds and the extent to which nesting sites are being lost due to home improvements, the study authors suggest that conservation efforts here should focus on ensuring that safe and productive nesting sites are readily available to these iconic summer visitors.