Today’s blog is by Tom Finch, Senior Conservation Scientist, on a new paper which teases out the drivers of variation in Swift breeding success and survival
Like many other insectivorous birds, Swifts – the screaming, scythe-shaped, signallers of summer – are in decline. The reasons aren’t entirely clear, but loss of nesting sites and reductions in prey availability are plausible candidates.
A new study by RSPB, BTO and Rothamsted Research combined three long-term datasets to build up a picture of how Swift breeding success and survival have changed over time and in response to environmental conditions.
First, we explored changes in the biomass of aerial insects. The Rothamsted Insect Survey runs a network of 12 m high suction traps which monitor aerial insects throughout the summer months. Aphids (an important crop pest) have been identified and counted as far back as the 1970s, and we know that aphids are an important (though not exclusive) part of the Swift’s diet – so this was our focus. We found strong fluctuations in aphid biomass from year to year, and strong regional differences is long-term trends. In particular, we found marked declines in aphid biomass across much of southern and eastern England since the early 1990s.
One of Rothamsted’s high suction traps © Rothamsted Research
Next, we used data from the BTO’s long-running Nest Record Scheme to estimate various measures of breeding success. These estimates were often quite noisy due to small sample sizes, particular prior to 1990 – but there does appear to have been an increase in nest failure rates since 1975, resulting in a reduction in overall breeding success. We also tested which, if any, environmental factors were associated with high or low breeding success. We found no association between aphid biomass and Swift breeding success, perhaps due to the influence of other prey types. We did, however, detect the influence of poor weather, with high rainfall associated with smaller broods and higher nest failure rate.
Finally, we collated ring-recovery records from the BTO Ringing Scheme to estimate annual survival of both adult birds and those in their first year of life. Whilst adult survival has remained relatively high and stable since the 1970s, first-year survival looks to have declined. First-year survival was also lower in years with high summer rainfall, again pointing to the influence of poor weather.
Swifts in flight © Ben Andrew (rspb-images.com)
From a conservation perspective, our analysis doesn’t point to any Big 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!
Nest boxes are an obvious tool at our disposal when it comes to Swift conservation, and whilst our study wasn’t designed to assess the effectiveness of Swift nest boxes, 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.
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