Study examines factors driving long-term Common Swift decline


A new study by the RSPB, BTO and Rothamsted Research combined three long-term datasets to build up a picture of how Common Swift breeding success and survival have changed over time and in response to environmental conditions.

Like many insectivorous species, Common Swift has shown significant declines in recent decades and the UK breeding population decreased by an estimated 57% between 1995 and 2017. The ultimate drivers of this decline are poorly understood, but links have been drawn to reductions in the abundance and availability of insect prey, as well as loss of nesting sites. The study, published in Ibis, focused on weather and aphid biomass.

Common Swift breeding success and survival rates were linked to summer weather conditions (Mike Trew).

The researchers used data from the BTO's long-running Nest Record Scheme to estimate various measures of breeding success. They found signs that there has been an increase in nest failure rates since 1975, resulting in a reduction in overall breeding success.

Examining which, if any, environmental factors were associated with high or low breeding success, the researchers looked at aphid abundance using data from the Rothamsted Insect Survey. The survey runs a network of 12-m high suction traps which monitor aerial insects throughout the summer months. Aphids, which have been identified and counted as part of the survey as far back as the 1970s, are an important (though not exclusive) part of Common Swift's diet. The researchers found strong fluctuations in aphid biomass from year to year, and strong regional differences in long-term trends. But, despite marked declines in aphid biomass across much of southern and eastern England, they found found no association between aphid biomass and swift breeding success.

However, their findings did detect the influence of poor weather, with high rainfall associated with smaller broods and higher nest failure rate.

Collating BTO ring-recovery records, the researchers also estimated annual survival of adult and first-year birds. They found that adult survival has remained relatively high and stable since the 1970s, but first-year survival had declined. The latter was also lower in years with high summer rainfall, again hinting at the negative influence of poor weather.

Tom Finch, RSPB Senior Conservation Scientist and lead author on the study, wrote on the RSPB blog: "From a conservation perspective, our analysis doesn't point to any smoking guns. We didn't detect any strong relationships between swift performance and aphid biomass, nor land cover. This doesn't mean that habitat and prey availability aren't important – but these questions are challenging to study, particular for species like swifts which forage over large distances and consume a variety of prey types. The influence of weather is probably unsurprising, but managing the weather isn't a practical conservation solution.

"Nestboxes are an obvious tool at our disposal when it comes to swift conservation, and while our study wasn't designed to assess the effectiveness of swift nestboxes, it's clearly important that nesting opportunities are provided, giving swifts one less thing to worry about. Ongoing research into swift movement and diet might give us the information we need to understand better whether land management can also provide a boost to the fortunes of these iconic summer visitors."



Finch, T, Bell, J R, Robinson, R A, & Peach, W J. 2022. Demography of Common Swifts (Apus apus) breeding in the UK associated with local weather but not aphid biomass. Ibis. DOI: https://doi.org/10.1111/ibi.13156