The Swift: Britain's Most Precise Spring Migrant

The swift crosses the English coast in early May. It does this with such reliability that Gilbert White was already recording the dates at Selborne in the 1770s and his notebook entries are still used as a benchmark. The bird is the same bird, the science of when it arrives the same science. What has changed is the country it returns to.

Arrival and departure

The Common Swift, Apus apus, spends about three months of the year in Britain. It arrives across the southern coast in the first week of May, sometimes a few days earlier in mild years, sometimes a few days later in cold ones. It leaves in early August, six or seven weeks after the chicks fledge. The whole British summer of the swift fits inside that window.

The arrival is precise enough to be useful. A late arrival in a cold spring means the insects were not flying. An early arrival in a warm spring means they were. The swift cannot land and forage if the sky is empty: it feeds entirely on the wing, catching small flying insects in mid-air, and it cannot live where there is nothing in the air to catch. The arrival date is, in effect, a reading of insect abundance taken by an animal that cannot afford to be wrong. Cold-spring arrivals are documented in the British Trust for Ornithology's swift survey as a leading indicator of poor insect years across the country.

The departure is more abrupt. One August evening the screaming parties are above the rooftops and the next they are gone, the birds following thermals south across the Channel and into the deep flyway that runs through France, Iberia, and over the Sahara to wintering grounds in central and southern Africa.

Gilbert White at Selborne

Gilbert White was a parish curate in Selborne, Hampshire, and he kept a daily natural history journal for more than thirty years. Among the entries are arrival and departure dates for the swifts that nested in the church tower and under the eaves of the village houses. The dates are precise: he recorded the first sight on a particular morning, the last sight on a particular evening. He noticed that the birds returned to the same nest sites year after year. He noticed that they would sometimes shelter in cracks in the masonry during storms and emerge dazed but alive.

The Selborne records have outlived their author by more than two centuries. They are still used. The BTO and Action for Swifts both reference White's dates as a long-running baseline for comparison with modern arrival data. Average dates have shifted slightly: the swift arrives a few days earlier than it did in the 1770s, in line with broader changes in spring temperatures. But the bigger change is in the population. The British breeding population of the swift has fallen by roughly half since White's time, and continues to decline. The bird is now on the UK red list of conservation concern.

What screaming parties are

The screaming parties, those groups of swifts that tear at high speed around buildings on warm May and June evenings, all shrieking, are widely misunderstood. They are not feeding. They are not territorial display. They are young, non-breeding birds in their second or third summer, practising flight. The screaming is contact-calling between siblings or pair-bonded birds, audible at long distance, and it builds as the group circles.

A swift's first year is spent entirely in the air. After fledging, the bird does not land again for two or three years, until it returns to find a nest site and breed. Everything it does in those first years happens on the wing: eating, sleeping (one hemisphere at a time, half the brain shutting down while the other remains alert), drinking by skimming the surface of standing water, and mating. The screaming parties are part of that. The young birds are testing their flight against the gables and the chimney stacks, learning the geometry of the village they will eventually breed in.

Built for the sky

A swift cannot take off from the ground because its wings are too long and its legs too short. If a fledgling falls or a sick bird is forced down, it is trapped: the wings cannot generate lift from a flat surface, and the legs cannot run fast enough to gain it. The scientific name, Apus apus, means "no foot, no foot." The feet are functional but small, used only for clinging to vertical surfaces – the side of a cliff face, the inside of a chimney pot, the underside of a roof tile.

This anatomy explains the eaves. Swifts evolved nesting on cliffs and in caves, and they have transferred to human buildings only in the last few thousand years. Old churches, old farm buildings, the gaps under tiled roofs and behind soffit boards: these are the modern equivalent of the cliff face. As old buildings are renovated and roof gaps sealed, swift nest sites disappear faster than they can be replaced. Action for Swifts and the RSPB now both run programmes encouraging swift boxes and swift bricks in new construction.

The old weather lore

Swifts nesting under the eaves were once held to bring summer with them and to protect the house from the worst of the weather. Gilbert White recorded the belief in correspondence from the 1780s, and versions of it persisted across Hampshire and the chalk counties into the early twentieth century. The reasoning was practical: swifts arrive when the weather has turned warm enough for insects to be flying, and they leave when the warmth ends. A house with nesting swifts was, by definition, a house in the warm half of the year. Harm to the nest was held to invite the weather to turn.

The colloquial name "screamers" was recorded by White and survived in common use across the southern counties for at least two centuries. It is still used. If you hear the word from someone older than fifty in Hampshire or Dorset, they are almost certainly talking about swifts.

Where to look this week

In the south of England, swifts arrive in the first week of May and the screaming parties are usually noticeable by mid-May. Look up in the evening, after the gulls have settled, when the light has gone gold over the rooftops. The screaming is high and thin, carried over a long distance; the birds themselves are tight black silhouettes with sickle wings, much narrower than swallow or house-martin wings. They fly higher than either, often well above the chimney line.

Older churches, large farmhouses, and tall buildings with open eaves are the places to start. If you can locate the nest site of a pair from one year to the next, you have a pair you can watch indefinitely: swifts return to the same site for the rest of their breeding lives, which can be more than ten years.

Sources: Gilbert White, The Natural History of Selborne (1789), Project Gutenberg; British Trust for Ornithology, swift survey; RSPB species page: Swift; Action for Swifts (swift-conservation.org).