Why Are Swifts Declining? Lost Nest Sites and Insects

How fast are swifts declining?

The steepest documented declines are in Britain. The UK Breeding Bird Survey, run by volunteers with the British Trust for Ornithology, recorded a fall of about 70 percent between 1995 and 2024, with steep losses in England, Scotland and Wales. Swifts were moved to the amber list of conservation concern in 2009 and to the red list in 2021. The UK population was estimated at around 59,000 pairs in 2016. Ireland has also recorded a decrease.

Across Europe as a whole, the picture is mixed. Monitoring data suggest numbers have been broadly stable in some countries over the long term, while others report losses, and the common swift is rated Near Threatened on the European Red List. Globally, the IUCN still lists it as Least Concern because its range is so vast, stretching from Ireland to China.

Swifts are hard to count. They nest in hidden holes, feed far from their colonies and non-breeding birds wander widely, so researchers treat these figures with some caution. The overall trend in Britain, however, is clear.

Lost nest sites in buildings

The biggest single problem is the loss of places to nest. Swifts once nested in cliffs and old woodpecker holes in ancient trees, but in most of Europe they now depend on buildings. They squeeze into gaps under roof tiles, behind fascia boards, in eaves, church towers and old stone walls.

These holes are disappearing for several reasons:

• Renovation and re-roofing often close up the gaps under tiles and eaves.
• Insulation and energy-saving work seal buildings tightly to stop drafts.
• Plastic soffits and fascias replace old wooden boards that had small gaps.
• Modern buildings are designed with smooth walls and no cavities at all.
• Demolition of older buildings removes whole colonies at once.

The timing makes this especially damaging. Swifts spend only about three months at the nest and the rest of the year in Africa or on migration. If a roof is repaired in winter, the birds come back in May to find their nest hole gone. Because swifts are so faithful to their nest sites, many cannot find a new one quickly, and a pair may miss a breeding season while it searches.

Fewer flying insects

Swifts eat only insects and spiders caught in the air, so they depend on plentiful “aerial plankton”. There is good evidence that flying insects have declined in parts of Europe. A well-known 2017 study from Germany found that the total weight of flying insects caught in traps in protected areas fell by more than 75 percent over 27 years.

The causes of insect declines are still being studied, but intensive farming, pesticide use, loss of wildflower-rich habitats, drainage of wetlands and light pollution are all thought to play a part. Fewer insects means adults must fly farther to feed their chicks, and broods are more likely to fail in poor weather.

Weather, climate and problems on migration

Swift breeding success depends heavily on the weather. During cold, wet spells, insects stop flying and swifts may travel long distances to find food. The chicks can survive several days of hunger by entering a sleep-like state called torpor, but long spells of bad weather still cause losses.

Heat waves are a newer risk. Nests under dark roofs can become very hot, and chicks sometimes jump out of overheated nest holes before they can fly. Conditions along the migration route and in the African wintering grounds probably affect survival too, but they are much harder to study, and scientists do not yet know how much they contribute to the decline.

Why swift numbers recover slowly

Swifts are long-lived birds that breed slowly. Most do not nest until they are about four years old, a pair raises only one brood of two or three chicks a year, and adults can live for many years. This means losses build up quietly, and once a colony is gone, it can take a long time to come back, even if new nest sites are provided.

The bird’s unusual life on the wing, explained in do swifts ever land?, also makes swifts difficult to help directly. They cannot use bird feeders or garden habitat the way many songbirds do. What they need most is a safe hole high up on a building.

How people are helping swifts

Because the main problem is housing, the solutions are mostly about buildings:

• Swift bricks: hollow bricks with an entrance hole and a nest chamber, built into the walls of new homes. They last as long as the building and need no maintenance. Some local councils in Britain and other European countries now ask for them in new developments.
• Swift nest boxes: wooden or concrete boxes fixed high under the eaves of houses, schools and churches, ideally at least 5 m (16 ft) above the ground with a clear flight path below.
• Protecting existing nests: planning roof work outside the nesting season, roughly May to August, and leaving or recreating the entrance holes afterward. In the UK and the EU, active nests of wild birds are protected by law.
• Mapping colonies: volunteer swift groups and citizen science projects record where swifts nest, so planners and builders know which sites to protect.
• Insect-friendly land: wildflower meadows, ponds, hedges and reduced pesticide use help the flying insects swifts feed on.

Swift groups sometimes play recordings of swift calls near new nest boxes to attract young birds looking for a home, and many boxes are taken up within a few years.

Can swifts recover?

There are encouraging signs. Where nest bricks and boxes have been installed in good locations, swifts have moved in, and some towns now host large, monitored colonies in boxes on churches, schools and apartment blocks. Swift conservation has become one of the most popular urban wildlife projects in Europe.

Recovery will take time because swifts breed slowly, and nest sites alone cannot solve the loss of insects. But because the bird is so closely tied to our buildings, ordinary homeowners, builders and planners can make a real difference. To learn more about the bird itself, its speed and its long migration, see our common swift profile and how fast can a swift fly?.