Why Birds Migrate at All

Migration is not wanderlust. It is arithmetic: a bird stays where the food lasts, and moves when it stops. In Britain, three forces do most of the pulling — daylight, weather and food supply — and they rarely work in isolation.

Daylight is the metronome. As the days lengthen in spring, hormones shift, gonads swell and birds that have spent the winter in the Sahel, Iberia or the Congo Basin begin to fidget. Many long-distance migrants leave their wintering grounds on an internal timetable set weeks earlier, navigating by the sun, the stars and the earth's magnetic field. Weather decides whether the journey is comfortable or catastrophic. Food decides whether they survive it. Insect numbers in a British April are vastly richer than in a West African March, but only for birds that arrive at the right moment.

The Spring Arrivals

Spring migration is a relay. It starts quietly in March, when the first Sand Martins appear over gravel pits and reservoirs, often in the teeth of a cold east wind that seems absurd for a bird that has just crossed the Sahara. The order is fairly predictable, and knowing it makes every walk more rewarding.

• Late March: Chiffchaffs singing in scrub and willow, Sandwich Terns along the coast, first Wheatears on stony fields.
• Early to mid April: Blackcaps in gardens, Willow Warblers in birch and oak, Swallows over farmyards — the classic first-swallow date for southern England is around 10 April.
• Late April: House Martins, Cuckoos, Whitethroats, Sedge and Reed Warblers, Common Sandpipers on reservoir edges.
• Early to mid May: Swifts screaming over rooftops, Spotted Flycatchers, Turtle Doves and the last of the warblers.

A British Swallow may have flown roughly 6,000 miles from South Africa, crossing the Sahara and the Mediterranean on the way. A Swift is here for barely three months before turning round again. That is an enormous investment for a short breeding season, which is why cold, wet spells in May can be so damaging: the insects vanish, and adults with hungry chicks have nowhere else to go.

Autumn: Departures and New Arrivals

Autumn is not simply spring in reverse. Birds leave earlier than you might expect — Swifts are often gone by mid-August, Cuckoos by early September, most Swallows by the end of that month. The same winds that carry them south bring others in.

From late September, Pink-footed Geese arrive from Iceland, and thousands pass over Lancashire, Norfolk and the Scottish east coast in noisy skeins, heading for estuaries and sugar-beet fields. Whooper Swans and the scarcer Bewick's Swans follow from Iceland and Arctic Russia, while Redwings and Fieldfares — our winter thrushes — arrive from Scandinavia, often first heard as thin, lisping calls overhead after dark. Brent Geese settle on estuaries such as the Dee, Chichester Harbour and the Wash, and Wigeon, Teal and Golden Plover swell the numbers on wet meadows and mudflats.

Not all movement is long-distance. Blackcaps from central Europe increasingly winter in Britain, feeding on ivy berries, apples and garden feeders — a genuine behavioural change that has happened largely within living memory.

Reading the Weather to Predict Birds

Migration is written in the weather charts, and a little knowledge goes a long way.

• Spring falls: A slow-moving low in the Atlantic with south-westerlies and rain can drop exhausted migrants along the south and west coasts — Portland Bill, the Isles of Scilly and the Cornish valleys come alive.
• Autumn drift: High pressure over Scandinavia, with easterlies and clear skies, pushes Scandinavian and Siberian birds westwards onto the east coast. Spurn Point, Flamborough Head, Gibraltar Point and Fair Isle are famous for it — Yellow-browed Warblers, Red-breasted Flycatchers and Barred Warblers turn up in sycamores and churchyards.
• Fog and showers: Birds fly lower, sometimes below rooftop height, and grounded flocks can turn up almost anywhere.

Watching Migration Well

The best migration watching is unhurried and repetitive. Visit the same hedge, estuary or hill regularly and note what you see; the patterns only emerge over weeks. Dawn is the golden hour — many migrants move during the night and settle at first light, so a walk between sunrise and 9am is worth three at midday. Listen as much as you look: Redwing and Golden Plover calls overhead, unseen, still count.

Take a notebook, record date, weather and wind direction, and note the oddities. If you find something unusual, report it to your county bird recorder, who depends on such records to track how migration is changing.

Small Things That Help Migrating Birds

You do not need to run a reserve to make a difference.

• Put up swift boxes, or leave existing eaves and roof spaces undisturbed.
• Avoid cutting hedges between March and August, when nests are in use.
• Keep feeders and bird baths clean, and leave seed heads on teasels and thistles into winter.
• Plant native shrubs — hawthorn, blackthorn, ivy — for autumn berries and spring caterpillars.
• Log your first Swallow, Cuckoo or Swift each year; comparing dates across years is how we see the seasons shift.

Migration is one of the great spectacles of the British countryside, and it happens overhead, at the edge of any town, every spring and autumn. Once you know roughly when to look and which wind to wait for, no walk is quite the same again.