Why Swifts Scream Through Summer Skies

The Sound of High Summer

There is a moment in early May when a British evening changes character. You are walking home, or standing in the garden, and a thin, frantic scream passes overhead — not one bird but a handful of them, tearing across the rooftops at chimney height and then wheeling back for another run. They call it a screaming party, and for many of us it is the real start of summer.

Those dark, scythe-winged birds are swifts, and that scream is one of the most distinctive sounds of a British town. Understanding why they do it tells you a great deal about a bird that spends almost its entire life in the air.

What a Screaming Party Actually Is

A screaming party is exactly what it sounds like: a group of swifts flying fast and low around the buildings where they nest, or would like to nest, calling constantly. The birds sweep in loose formation, often following the same line over the same roof, bank and dive, turn and surge, and the calls overlap until it sounds less like individual birds and more like one continuous shriek.

Parties usually build in the hour before dusk on warm, still evenings, and there is often a smaller flurry at first light. They are most intense in late May and June, when the birds are pairing up and prospecting, and they fade as chicks hatch and the adults have mouths to fill.

These gatherings are not just established pairs. They contain a mixture of birds:

• Breeding adults, defending a nest hole and announcing that this roof is occupied.
• Younger adults, often two to three years old, looking for a mate and a cavity to claim for next year.
• Non-breeding birds, which may join the chase simply because the colony is where the action is.

Why the Screams Matter So Much

Swifts are colonial, and a colony is held together by sound. A nest hole is the single most valuable thing in a swift's life, and the screaming party is effectively a loud advertisement: here is a building with cavities that work, and here are birds that know how to use them. A young swift flying over unfamiliar streets can hear that signal from a long way off and be drawn in.

The calls also help birds find one another in the frantic press of a party, allowing pairs to form and re-form. Swifts tend to return to the same site year after year, and pairs often reunite, so a screaming party is partly a reunion, partly a recruitment drive and partly a contest over the best holes — all happening at rooftop height, at speed, in failing light.

A Life Almost Entirely Airborne

To appreciate the screaming party, it helps to remember how strange swifts are. They eat, drink, mate and even sleep on the wing. They land only to breed, and their tiny feet — all four toes forward, useless for perching — mean they cannot walk on a lawn like a swallow. A swift on the ground is usually a swift in trouble.

A young swift that leaves the nest in late July or August may not touch down again for many months, feeding over Africa and sleeping in the sky. That is why the birds appear suddenly in spring, as if from nowhere, and vanish just as abruptly in early August. Our summer is their breeding season, and it is short.

In cold, wet spells, swifts can drop into torpor to save energy while they wait for the weather to bring insects up into the air. This is one reason a cold June can be so hard on them.

Telling Swifts from Swallows and Martins

Late spring skies are busy, so it is worth being sure of what you are hearing and seeing. Swifts are larger and longer-winged than our other aerial summer visitors, and they are uniformly dark — no white rump, no pale throat. The wings are stiff, curved blades; the body looks like a small, dark torpedo; the tail is short and forked. Swallows show a long, streaming tail and a blue-black back with a red throat, and their calls are a cheerful, liquid twitter rather than a scream. House martins flash a white rump and give a dry, buzzing rasp.

Watch a swift turn and you will see the difference immediately. Swallows twist and flutter; swifts look as though they are being fired past the gable end.

How to Help Them This Summer

Swift numbers in the UK have fallen sharply in recent decades, and the loss of nest sites is a major cause. Roof repairs, loft conversions and blocked eaves quietly remove cavities that may have been used for generations. There are simple, practical things you can do:

• Protect existing holes. If work is planned on a roof where swifts nest, ask for the cavities to be retained or replaced with a swift brick.
• Add a nest box or brick. Dedicated swift boxes sit high under the eaves, ideally two or three metres above the ground with a clear flight path in.
• Play calls softly. If you install a box, playing swift calls near it on summer evenings can help prospecting birds find it.
• Encourage insects. A garden with long grass, native flowers and no insecticide supports the flying insects swifts feed on.
• Record what you see. Note screaming parties, nest sites and the dates swifts first arrive and finally leave, and pass the records to your county bird recorder.

Then, on a warm June evening, simply stand in the street and watch. The screaming will be over by the middle of August, and the swifts will not be back for nine months.