Visit from an aerial acrobat

Spending most of their life airborne, migratory swifts are a pleasure to witness when they visit the UK to breed

DUSK IS FALLING after a hot day in early August. Above the houses, birds dive and whirl, looping and circling through the sky like a group of aerial acrobats; their shrill, piercing calls seeming to echo from the walls of the buildings below. They are Common swifts, and a screaming party such as this, dashing at breakneck speed around the rooftops, is a summer sight and sound that is hard to beat. But despite what has been a warm, sunny day, with the promise of more to come, these summer visitors to the UK are already preparing to leave. Arriving at the end of April and departing again during August, their stay is a short one, but then again, as their name suggests, these are birds that do not hang around.

By the end of August, most of the swifts will be gone, migrating through France and Spain to overwinter in Africa, south of the Sahara. There, they will feast on the rich insect life, gaining fitness in readiness for their return journey to the UK next year. Common swifts come here to breed, and some of the birds making this flight may be just six weeks old, beginning their long 6,000-mile journey almost as soon as they have fledged.

Of the approximately 100 species of swift worldwide, the Common swift, Apus apus, is the only member of the family that breeds in Britain. While they are similar in appearance to house martins and swallows, both of these are passerines, or perching birds, and swifts are most

“It soared, a bird, it held its flight, a swift pure cry, soar silver orb it leaped serene, speeding”

James Joyce, Ulysses

definitely not perching birds. The scientific name for the swift family is Apodidae, taken from the Latin meaning ‘no feet’. Swifts do have feet, of course, but they are proportionally very small and weak, and they will not be seen perching on telegraph wires as swallows do. Swifts are, in fact, closely related to the hummingbirds, which are also flight specialists.

Life on the wing

The Common swift is the most aerial of birds: its habitat is the sky. No other bird spends as much time on the wing, and they certainly rack up the miles. It is estimated that a Common swift can comfortably fly 125,000 miles in a year. And what flyers they are: agility, grace and speed are combined into perfection as the birds completely outfly anything else in the air. Anyone who tries to follow them through binoculars will soon appreciate why they are called swifts: their speed and their ability to make sudden changes of direction makes it incredibly difficult to keep track of them.

In the sky, swifts appear black, although their colour is more of a dark chocolate hue, and they have a creamy-white patch on the throat area. Body length is approximately 7in (18cm) and their long, scythe-shaped wings have a span of 17-19in (42-48cm); they are often described as looking rather like a boomerang in flight.

The legs are short, ending in small feet, with four toes. The tail is forked and in flight is held closed to increase efficiency. The short, thin beak is black.

Swifts are invertebrate feeders, catching a wide range of insects and airborne spiders in flight. They use their mastery of the sky to hunt above all habitats, from urban cities through to moorland and even over open ocean. Their wings, with their supreme sharp points, are what makes swifts special. Like hummingbirds, they have greatly reduced ‘arms’ and extended digits, meaning that they effectively fly with their fingers. The slightest twitch helps the swifts to home in on a flying insect and snatch it with their bill. Every insect or other invertebrate is seized individually.

They are excellent readers of the weather and use this to their advantage when feeding. Approaching weather fronts can force flying invertebrates into swarms, which are then driven upwards by air currents to heights of half a mile or more. This concentration of food is irresistible to

“No bird soars too high if he soars with his own wings”

William Blake

swifts, which readily exploit it, often flying some distance to do so. They will also fly a considerable distance to avoid bad weather. A low-pressure system barrelling in off the Atlantic can make life for many birds very difficult, but the Common swift just makes a detour around it, sometimes flying 500 miles in one day.

Nesting and brood care

The only time swifts will land is when they nest. These birds pair for life, returning to the same nest site each spring. The nest is built on a solid surface, be that a crevice, a ledge, an old bird nest or a specially designed swift box, and is located high up, so that the birds can just drop into the air as they leave it. Favoured locations include the eaves of old houses or churches, and hollow trees or crags. The materials used for the nest’s construction come from whatever the birds might catch in the air, such as an itinerant piece of hay, a feather, paper or other flying flotsam, compacted and mixed with copious amounts of saliva to make a shallow cup. The adults share the nesting duties between them.

Two or three eggs are laid at two- to three-day intervals. Incubation lasts for approximately 20 days per egg, the chicks hatching approximately two days apart.

Insects are collected by the adults in a food pouch located at the back of the throat, where they are bound into a pellet using saliva. This pellet, known as a bolus, is regurgitated back at the nest to feed the chicks, and a single bolus may hold more than 1,000 insects. In a summer when food is in short supply, the majority of what is available will be given to the larger, oldest chick, therefore ensuring that at least one of them survives.

From approximately four weeks of age, the chicks have been observed pushing their wings down to lift their bodies up, rather like an avian form of ‘press-ups’, possibly to help strengthen their wings ready for the long flight to come.

Absent parents

While the swifts’ method of avoiding bad weather is to fly around it, such detours take time and would be disastrous back at the nest for the swifts’ eggs or brood, were it not for some remarkable adaptations, both before and after the chicks have hatched. Unlike those of other birds, swifts’ eggs can withstand chilling, so if the adults are away from the nest for an extended period, development of the embryo within the egg just slows down, returning to normal once the parents begin incubating them again. And while with most other birds, the brood would soon perish if their parents were not around to feed them regularly, swift chicks are able to slow down their metabolism and enter an almost torpid state. Only their basic body functions continue, and the youngsters can fuel these processes using stored fat, although it does mean that the high-energy demanding processes, such as growth and feather development, are slowed right down. Once the weather improves, and the adults return, the chicks’ metabolism kicks back into gear, and they can continue to develop as normal, albeit a few days behind where they should be. This leads to quite a

variation in the number of days it can take a Common swift to develop from hatchling to fledgling. In most birds, this might only vary by two or three days: blackbird chicks take 14-16 days to fledge, and for a swallow, it is 20-22 days. However, for a Common swift, it is 37-56 days; a variation of up to 19 days.

Non-stop flight

Once the chicks fledge, they might not land again for three years until they, too, are ready to breed. That means three years of continuous flight, and while this sounds remarkable, for a swift, it is perfectly normal. When a female lays her first brood, she has probably been flying non-stop for more than 300,000 miles. Everything these birds do is carried out in the air: they drink by skimming the surface of large bodies of water as they fly over them; they are able to mate while flying; and they even sleep on the wing. This is not as potentially dangerous as it sounds, for the swift does not sleep in the conventional way. Instead, it can shut down one half of its brain at a time, allowing the bird to function normally while still getting the rest that it needs. It is a perfect adaptation for a bird that spends so much of its life in the air.

Provided it can survive the early years, the Common swift is a long-lived bird for its size, with a lifespan averaging eight years, although one ringed individual lived until the ripe old age of 21. Their exceptional aerial skills mean they have few predators, although those weakened by hunger may fall victim to birds of prey.

Unfortunately, our swift numbers are falling. Between 1995-2017, their population declined by 57 per cent in the UK. Loss of suitable nesting sites is thought to be a large reason for this. In the UK, swifts often nest in open eaves, gable ends, under loose roof tiles and holes in the walls of houses, churches and other buildings, returning to the same site year after year. Building renovations often result in the removal of those nesting sites, while new houses are better insulated and offer none of the spaces which are so crucial for these birds. Special swift boxes or ‘swift bricks’ are available to offer these birds alternative nesting sites, both on buildings undergoing restoration and on newer homes.

The swift might not stay for long, but it is a favourite visitor with many and one that should be helped where possible. It is an extreme bird, perfectly honed for an aerial life: the ultimate flying machine.

A superb flyer, the Common swift is a medium-sized bird, sooty brown in colour, with scythelike wings and a forked tail.

A formation of Common swifts can often be seen wheeling across the sky above the rooftops, calling to each other in screeches, with those of the female the more high-pitched. They often form screaming parties round their nest area on summer evenings.

Swifts have very short legs, which they use primarily for clinging to vertical surfaces, such as walls. Their German name, mauersegler, means ‘wall-glider’.

Swifts have soft beaks and deep-seated eyes, with movable bristles at the front to reduce glare from the sun.

A pair of swifts appear to be involved in an altercation at the entrance to a nesting box.

Swifts usually fly directly into their nesting sites, and take-off is characterised by a free-fall start, like this one leaving its nest box on a house after feeding its chicks.

Swifts will never nest on the outside of a building. Here, one peeks out of a crevice in the eaves of an old house (above). Chicks huddled together in their nest. Swifts build their nests of airborne material caught in flight.

A swift swoops down to the water and skims along the still surface to drink in small sips on the wing.

A swift emerges from a nest brick, with a hollow interior, built into a house.