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About Swifts

Swifts in decline

In common with barn swallows and house martins, which also feed on aerial insects, numbers of the common swift (Apus apus) in the UK have decreased markedly – possibly by as much as 60% over the last two decades.

Their conservation status is now Red Listed in the UK and Ireland, with an estimated population of around 59,000 breeding pairs.

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The causes of swift decline are multifactorial, and include:

Loss of suitable nesting sites: Traditional nesting spaces are disappearing as older buildings are re-roofed, insulated to modern standards, and wooden features such as barge boards are replaced with PVC.

Unseasonal weather and climate change: Prolonged cold or wet summer periods, as well as excessive summer heat and drought, can significantly reduce breeding success.

Decline in aerial insects: Likely linked to intensive agriculture and pesticide use, forcing swifts to forage further to meet their energy and nutritional needs.

Predation: Predators such as sparrowhawks can be locally significant.

The return of swifts to their ancestral breeding sites alongside human communities is something to be marvelled at. Lone birds launch themselves from the safety of the nest in late summer and fly non-stop on an approximately 7,000‑mile journey to southern Africa, returning the following May. Once fledged and airborne, young swifts may not land again for up to three years, until they reach breeding maturity – a period during which they face innumerable hazards.

While it is undisputed that swifts are in decline, there is ongoing debate about the true total population size in the UK. This complexity arises because the population consists of three distinct age groups, each with different arrival times, departure times, and behaviours:

Established breeders (4 years and older), focused on reuniting with partners and raising young

New potential breeders (3-4 years old), seeking mates and nesting opportunities

Non‑breeders (under 3 years old), visually indistinguishable from breeders and often seen in large aerial gatherings

Despite these challenges, BTO surveys over the past two decades, using consistent sampling methods, show a clear and worrying downward trend.

Encouragingly, recent monitoring of established colonies – including the Kinoulton tower colony – shows that numbers remain stable once a colony is established. Furthermore, adding additional nest boxes typically leads to gradual increases over time.

By providing permanent nesting opportunities, we can help halt the decline and rebuild populations. Evidence from monitoring studies and practical experience strongly suggests that loss of nesting sites is a major driver of swift decline in the UK.

Through coordinated efforts – such as installing nest boxes in church towers and encouraging communities to provide boxes on suitable buildings – we can create the conditions needed for stable populations to develop.

When nesting sites are destroyed without replacement, the swift’s instinct to return to ancestral sites is broken. Re‑establishing swifts at abandoned locations later requires considerable patience and persistence. This challenge is shared by other site‑faithful species, such as house martins.

While providing nest sites addresses a fundamental limiting factor, swifts will of course remain vulnerable to the wider environmental pressures outlined above.

Rupert Earl and Alan Wilkins – June 2025

  1. British Trust for Ornithology (2022). Breeding Bird Survey Report 2022.
  2. British Trust for Ornithology. Swift species factsheet.
  3. DOI: 10.1111/ibi.13156
  4. Newell, D., Collins, T., & Pomroy, J. (2024). Are we underestimating the UK common swift population? British Birds, 117, 114–119.
  5. Newell, D., & Willis, J. (2023). Are nest boxes for swifts a good idea? British Wildlife, 34, 585–591.

Insects and Birds – why we need to do what we do

Protecting and enhancing interconnected habitats

The canal is an integral part of this project. Water plays a crucial role in sustaining insect life, and insect populations across the country are currently in serious decline. In this context, the canal itself – together with the vegetation along its banks – is significant for the conservation of species and the maintenance of biodiversity.

Swallows and house martins typically feed at lower levels, skimming the air above water and fields, while swifts range much higher, catching insects carried upwards by wind and thermals. These complementary feeding strategies highlight the importance of a healthy, insect-rich landscape at multiple heights.

Using published research, we have calculated that in an average year the young raised in the tower consume approximately 27 kilograms of insects. This represents not only an ecological benefit but an economic one: around 76% of the insects taken are agricultural pests, meaning these birds provide a substantial natural pest-control service.

The proximity of the tower to the canal, together with the surrounding landscape – fields and byways, woods and copses – plays a significant role in the breeding success of swifts and supports a wide range of other wildlife. Protecting and enhancing these interconnected habitats is therefore fundamental to the aims of this project.

What we have learned from the Kinoulton tower swifts

The St Luke’s colony provides clear evidence that well-designed nest provision, careful monitoring and long-term stewardship can make a significant contribution to local swift populations.

The data from St Luke’s Church shows clearly that swift conservation is a long-term endeavour. The colony took several years to establish, with modest productivity in the early stages. Once the full provision of nest boxes was in place and birds had time to return to established sites, productivity increased steadily and sustainably.

Individual nest boxes — most notably Box 2 — have been used repeatedly over many years. This underlines the strong site fidelity of swifts and reinforces the importance of maintaining continuity and stability at breeding sites.

By 2019 the colony had reached high occupancy levels, with most usable boxes taken. In later years, increases in breeding pairs did not always translate into more fledged young. This suggests that, beyond a certain point, productivity is limited not by nest availability but by external environmental factors.

Poor breeding outcomes in years such as 2012, 2020 and 2024 correlate strongly with cold, wet conditions and reduced insect availability. Larger broods (three chicks) were consistently more vulnerable in difficult seasons, with higher chick mortality when food was scarce.

In years with earlier spring arrivals (notably 2023–2025), breeding tended to start sooner, fledging occurred earlier, and overall survival rates improved. Earlier breeding appears to provide greater flexibility to cope with short periods of poor weather.

The progressive introduction of nest cameras transformed the quality and reliability of monitoring. It allowed accurate mid-season assessments, identification of causes of failure, and confidence in fledging totals. This level of detail would not have been possible through external observation alone.

Despite setbacks — including bee disturbance, extreme weather years and the severe disruption of the Covid-19 pandemic — the colony has shown strong resilience. Recovery after poor years has been rapid, culminating in record fledging totals in 2023 and 2025.

With 414 young swifts fledged between 2011 and 2025, the St Luke’s colony provides clear evidence that well-designed nest provision, careful monitoring and long-term stewardship can make a significant contribution to local swift populations.

The Common Swift Louse Fly (Crataerina pallida)

Getting the process right – sharing ideas, understanding swift breeding behaviour and safe working.

Swifts do not fly alone, but unknowingly carry a highly specialised hitchhiker: the Common Swift Louse Fly, Crataerina pallida. This insect belongs to the Hippoboscidae family and is taxonomically classified as a true fly. While most birds and bats host a range of lice or fleas, C. pallida is almost entirely specific to the Common Swift (Apus apus).

The louse fly is relatively large in comparison with its host, measuring approximately 7 × 5 mm—around the size of a small fingernail. Its flattened body and strong legs are adaptations that allow it to move efficiently through the swift’s feathers and remain attached during flight.

Crataerina pallida is closely associated with the swift nest. Eggs are laid in and around the nesting site in late summer, after which the insect remains in the pupal stage throughout the winter. When swifts return in spring, the warmth of the birds’ bodies stimulates the emergence of juvenile louse flies.

After emergence, the louse flies insert themselves beneath the feathers and feed on the bird’s blood at intervals of approximately five days. Despite the fact that a single louse fly may consume up to 5% of a swift’s blood at each feeding—and some birds may carry as many as twenty individuals—research has found no evidence of a detrimental effect on the health of adult swifts or their chicks.

Both adult swifts and nestlings may be infested, and when birds leave the nest, attached louse flies travel with them for thousands of kilometres. The precise lifespan of C. pallida is unknown, but swifts appear to return to their nesting sites largely free of lice and are subsequently reinfested by the new generation emerging from the nest.

Although swifts feed exclusively on insects and spiders, they do not prey on C. pallida. A parasitic wasp is thought to be a natural predator of the louse fly, and other insects or spiders may also contribute to population control in natural nesting environments.

Artificial nesting sites, such as swift boxes in church towers, may limit these natural predation processes. For this reason, a programme of periodic cleaning has been adopted. Swift boxes are cleaned on a two-year rotation during the winter months, when birds are absent, to reduce infestation levels while avoiding disturbance to breeding swifts.

Walker, D. M. & Rotherham, I. D. (2010). The Common Swift Louse Fly, Crataerina pallida: An Ideal Species for Studying Host–Parasite Interactions. Journal of Insect Science, 10:193.

Walker, M. D. (2011). An investigation into the host–parasite interrelationship between Common Swifts and Hippoboscid louse-flies. Sheffield Hallam University.

Photographs

Before any work began, we held detailed discussions with the church stewards and members of the Parochial Church Council. It was agreed that no drilling would take place into the tower walls and that no more than one third of the louvred openings would be covered.

Swifts characteristically fly to the highest available entrance when prospecting for breeding sites, which led us to favour the upper third of the louvres, including the crescent-shaped opening. Several designs were drafted before we settled on a single large wooden structure incorporating ten compartments.

Accurate alignment of the entrances with the gaps between the louvres was achieved using a full-size brown paper template. On a bright day the paper was held against the louvre opening and all relevant reference points marked. These were then transferred into a working drawing, with additional detail added in stages. The final form of the structure was determined by the existing church fabric and the dimensions and design of the louvres themselves.

We agreed standard internal dimensions for each compartment: 8 inches wide, 8 inches high and a minimum length of 10 inches. Although swifts are known to breed successfully in more confined spaces, these dimensions were chosen to allow room for movement, wing exercise and adequate air circulation within an enclosed structure. Each entrance measures 28 mm high by 45 mm wide, a size that prevents occupation by starlings before the swifts return. The chambers are left open
throughout the winter and temporarily blocked in mid-February, then reopened when the first swifts are reported arriving in England, usually in mid-April.

From the outset, we also had to consider access constraints within the tower and the small size of the access hatch. It became clear that the structure would need to be dismantled and effectively ‘flat-packed’ once constructed. All components were carried up the tower individually and then carefully reassembled in situ.

We were fortunate that the area above the bells provided a solid boarded floor and a safe working environment. Our principal health and safety concern was the large church clock bell in the corner, which tolled every hour. Ear defenders were essential, as was setting a mobile phone alarm with a five-minute warning — a routine that quickly became second nature.

Before any work began, we held detailed discussions with the church stewards and members of the Parochial Church Council. It was agreed that no drilling would take place into the tower walls and that no more than one third of the louvred openings would be covered.

Swifts characteristically fly to the highest available entrance when prospecting for breeding sites, which led us to favour the upper third of the louvres, including the crescent-shaped opening. Several designs were drafted before we settled on a single large wooden structure incorporating ten compartments.

Accurate alignment of the entrances with the gaps between the louvres was achieved using a full-size brown paper template. On a bright day the paper was held against the louvre opening and all relevant reference points marked. These were then transferred into a working drawing, with additional detail added in stages. The final form of the structure was determined by the existing church fabric and the dimensions and design of the louvres themselves.

We agreed standard internal dimensions for each compartment: 8 inches wide, 8 inches high and a minimum length of 10 inches. Although swifts are known to breed successfully in more confined spaces, these dimensions were chosen to allow room for movement, wing exercise and adequate air circulation within an enclosed structure. Each entrance measures 28 mm high by 45 mm wide, a size that prevents occupation by starlings before the swifts return. The chambers are left open
throughout the winter and temporarily blocked in mid-February, then reopened when the first swifts are reported arriving in England, usually in mid-April.

From the outset, we also had to consider access constraints within the tower and the small size of the access hatch. It became clear that the structure would need to be dismantled and effectively ‘flat-packed’ once constructed. All components were carried up the tower individually and then carefully reassembled in situ.

We were fortunate that the area above the bells provided a solid boarded floor and a safe working environment. Our principal health and safety concern was the large church clock bell in the corner, which tolled every hour. Ear defenders were essential, as was setting a mobile phone alarm with a five-minute warning — a routine that quickly became second nature.