Swifts are ‘masters of the air’ that need human intervention to help reverse their steep, 20-year UK population decline
THE SEASON OF returning spring migrants is a special time, but I most look forward to seeing my first Swifts. They are here with us for less than a third of the year, but they are different from other birds – speeding black scimitars, more like miniature stealth bombers – and leave a strong and lasting impression during their short stay. Their charged power and fragility is perfectly captured in Ted Hughes’s poem Swifts: “A bolas of three or four wire screams / Jockeying across each other / On their switchback wheel of death / They swat past, hard-fletched”. Not language that could ever be used to describe our chattering, gentle Swallows and House Martins. They may be masters of the air, never landing from one year to the next, but they are entirely dependent on us for safe nesting sites. In the past they would have used cavities in rocky cliffs and dead trees, as they still do in other parts of their world range, but in this country they now only nest in our roof spaces. The nest itself is rudimentary, a shallow cup of grass and feathers, collected in flight and cemented together with the birds’ saliva. They are social and extremely site faithful, unwilling to colonise new sites unless they see other Swifts already in residence. This reliance on the built environment, with no requirement for vegetation, is reflected in their distribution, with concentrations in towns and cities. Young Swifts in the nest have to put up with harder conditions than other nestling birds. The adults collect aerial insects at high altitudes, but they are unable to forage in rainstorms, and fly immense distances to circle and avoid low pressure systems, often following behind weather fronts to areas of warm, rising air that are rich in insects. In unsettled weather, they may be away from their nestlings for many days, tracking favourable air masses as far as Germany. Back at the nest, the young birds settle into a torpor, reducing their body temperature and metabolism until the adults return. Because of this, Swifts have an unusually variable incubation time, with nestlings spending up to eight weeks in the nest, rather than the two or three weeks normal for smaller birds. While most of our common birds can be monitored by simple counts of adults NESTLINGS A rare look into the ‘unseen’ world of a Swift nest with youngsters The BTO runs volunteer surveys to monitor and explain changes in bird populations. To find out more about the Waterways Breeding Bird Survey visit present on territory in the breeding season, Swifts have no territories to speak of, and it can be hard to get meaningful figures in order to compare year-to-year changes. A further complication is the fact that they do not breed until they are four years old, so a sizeable proportion of the individual birds that we see are non-breeders. Despite these uncertainties, results from the Breeding Bird Survey indicate that steep declines have occurred in the UK over the last 20 years. Due to these declines, this species was moved from green to the amber list of conservation concern in 2009. As long-distance migrants, wintering south of the equator, Swifts are susceptible to factors influencing their migration and wintering grounds, emphasising the need for international research. Here on their breeding grounds, modern building design and refurbishment of older roofs can exclude Swifts from their nest sites and may be contributing to population decline. The provision of nestboxes and integration of potential nest sites into new buildings and renovations may therefore be of critical importance for Swift populations.
Kate Risely is the British Trust for Ornithology’s Garden Birdwatch Organiser
Young swifts in the nest have to put up with harder conditions than other nestling birds. Adults can be away from their nestlings for many days