British Wildlife of the Week

Swift

No bird can stay in the air permanently. At the very least, they must come down to earth to nest. But the common swift, with narrow, sickle-shaped wings and a streamlined body that is honed to aerodynamic perfection, spends more of its life in flight than any other bird. Its habitat is the sky itself.

There are over 100 species of swift in the world and they can be found on every continent apart from Antarctica. The common swift is the only one that nests here in the UK. It’s also known as the European swift, although this name isn’t particularly accurate because the bird spends less than half of its year in Europe; the rest of its time is spent in Africa, or migrating between the two.

If I were to associate any bird with summer, it would be the swift. It is one of our last migrants to arrive (in May) and also one of the first to leave (in mid-August). Just thinking about swifts causes memories of warm, hazy summer evenings in less stressful times to bubble to the surface of my mind. And as I look up in these recollections, I can see the iconic, black, scythe-shaped outlines of swifts high in the sky, hurtling around at breakneck speed, first one way and then another, and screeching with such frenzied vigour that it came as no surprise to me when I later learned that historically they were once known as ‘devil birds’ in some parts.

Life Without Land

The scientific name of the common swift, Apus apus, comes from the Greek ápous, which means ‘footless’. The swift does have feet, but they are extremely reduced, little more than weak, delicate hooks beneath its body. They’re fine for simply clinging to vertical surfaces such as cliffs or the walls of houses, but they offer meagre support and are not great for perching. As such, any swift-like bird that you might see settled on a telephone line almost certainly won’t be a swift; it will probably be a swallow or marten, which look superficially similar to swifts from a distance, but which belong to an entirely different group of birds. Swifts almost never land on the ground because they barely have the leverage needed to take off again.

Swifts are so deeply committed to their aerial existence that they do almost everything up there. Finding food is easy because they eat small flying insects, but other activities can be a bit more complicated. To drink, they must swoop low over a smooth body of water to dip their beak in. To wash, they find clouds and fly slowly through gentle rain, their wings outstretched. They also snatch some sleep during flight. By resting one half of their brain at a time, they can have power-naps, gliding and slowly beating their wings while on ‘auto-pilot’.

Although pairs routinely mate at the nest site, swifts can also, uniquely among birds, copulate on the wing. This takes place high in the sky, with the male deftly landing on the female’s back. The pair then glides or sometimes even tumbles downwards, interlocked for hundreds of metres before separating.

The only time swifts ever come down from the sky is when they need to build a nest and raise young. They once nested in holes in tree trunks or on cliffs, but now almost all common swifts make their nests on buildings, under eaves or in cracks and crevices. In fact, no other British bird is more dependent on humans. Only around 1% of British swifts today do not rely on our buildings, primarily a small colony that uses a combination of woodpecker holes and tree nest boxes on the RSPB’s Abernethy reserve in the Caledonian pine forest of Scotland.

There is a downside to over-relying on humans, though. Our desire to replace draughty eaves with hermetically sealed ‘eco-friendly’ architecture, which creates ultra-insulated spaces, more often than not leaves swifts homeless. The last thing a returning swift wants after flying 6,000 miles from Africa is to find out that its nest hole no longer exists. Swifts use the same sites year after year – they simply don’t have time and energy to search for a new one each spring.

A swift nest is built from whatever items the birds can obtain on the wing: floating feathers, dead leaves, flower petals, scraps of paper and so on. If there isn’t a lot of wind to lift sufficient material into the air, nest building can take a long time. These floating scraps are stuck to a vertical surface using sticky spittle produced by particularly large salivary glands until a shallow cup-shaped nest is formed.

While a swift is zooming around searching for food, bristles around its beak are thought to not only act as a cone, funnelling aerial plankton into its gaping bill, but they also protect the bird’s deep-set eyes as it plunges through dense clouds of insects. If an adult is collecting insects to feed its nestlings, it will hold them in a bulging, saliva-bound ball, or bolus, in its throat until it gets back to the nest. Each bolus may contain between 300 and 1,000 individual insects and tiny spiderlings, and a pair of swifts with a family to feed can collect up to 20,000 in a single day.

Swift chicks usually fledge after five or six weeks. Shortly afterwards, they will begin their incredible migration to Africa to spend the winter, passing through the airspace of around 25 countries. Outside of the breeding season, adults will fly for nine months non-stop, and although they spend much of their time in Africa, they will never willingly set foot on African soil. Even more remarkably, fledglings, upon leaving the nest for the first time, may not land and fold their wings again, or even touch a hard surface, until it is time for them to breed – between 18 months and four years of continuous, uninterrupted flight later.

It is estimated that a common swift flies at least 200,000 km a year – the equivalent of flying around the Earth between four and five times. In its entire lifetime, it may travel 2 million kilometres. That’s enough to fly to the moon and back twice over, and then once more to the moon. If birds are noted primarily for their ability to fly, then perhaps the common swift is the most bird-like of them all.

In the next British Wildlife of the Week, we’ll be looking at one of Britain’s rarest wildflower, which at one point was reduced to just a single native plant – the lady’s slipper orchid.