A nightingale from Norfolk sang in Guinea-Bissau

The 3,000-mile odyssey of the songbird's annual migration has been recorded for the first time, reports Michael McCarthy

(ALAMY)

British scientists have solved a major mystery of the natural world by tracking for the first time a migratory songbird on its winter journey through Africa.

The bird, a male nightingale code-named OAD, left Britain on 25 July last year after nesting in Norfolk, and travelled through France and Spain and down the coast of West Africa to Guinea-Bissau, the former Portuguese colony which is one of Africa's smallest and least-known states, where it spent the winter. It returned to Britain in April.

The tracking of its incredible 3,000-mile odyssey was made possible by using a tiny "data logger" locator device fitted to the bird which has lifted the curtain on one of wildlife's great enigmas: where exactly do our migrant birds go in the winter?

It is a mystery which has fascinated people for centuries, leading the 17th century poet Thomas Carew to suggest that the nightingale was a bird that "keeps warm her note" by wintering in his mistress's throat (see below right).

The truth now revealed is less poetic, but constitutes a real scientific breakthrough. Although larger trans-Saharan migrants such as ospreys and hoopoes have been tracked on their African journeys, this is the first time researchers have followed the trail of one of the estimated 5 billion passerines, or small songbirds from swallows to willow warblers, which every year move between Europe and Africa.

The discovery is likely to prove vital in finding out why many of these species, such as spotted flycatchers, wood warblers and whinchats, have begun to decline sharply in Britain and Europe, as it may be on their African wintering grounds that they are running into trouble. As reported in The Independent last week, nightingale numbers in Britain have fallen by 91 per cent in the past 40 years.

The findings have been made possible by the miniaturisation of tracking devices, which are now so small they can be fitted to birds weighing only a few grammes, without hindering them on their vast migrations.

Nightingale OAD was captured on 2 May last year near Methwold Hythe in Norfolk, and fitted with a tiny geolocator by researchers from the British Trust for Ornithology (BTO). Geolocators were developed by the British Antarctic Survey for tracking albatrosses. They use a light sensor and a memory chip to record the light level against a clock and a calendar, from which latitude and longitude on a particular date can be deduced. However, the geolocators are not big enough to transmit the information directly.

The one fitted to OAD, which was developed by the Swiss Ornithological Institute, weighed just one gramme and was the size of a shirt's button. As a result, birds fitted with the devices had to be recaptured on their return to Britain so the locator could be recovered and the data downloaded for analysis.

That might have seemed a big ask but, as events proved, it was not impossible. The BTO's Dr Chris Hewson and his colleagues fitted 20 nightingales with geolocators in 2009, and this spring they recaptured seven of them, all in the same part of Norfolk.

Five of the devices failed, and one had only partially worked, but the logger on OAD had recorded the outward journey in full. The bird was recaptured just 50 yards from where it was tagged the year before, which itself showed the amazing site fidelity of migratory birds. After its breeding season, the bird, which was probably born in 2007, left Britain's shores somewhere near the border of Kent and Sussex, crossed the Channel, and headed due south down through Central France, crossing the Pyrenees in mid-August.

It turned down the eastern side of Spain and crossed the Mediterranean from the region of Almeria to Morocco, where it had a three-week stopover from late August to mid-September for rest and feeding.

These stopover sites are increasingly seen as crucially important, and if they are suffering from environmental degradation, that may be a reason for declines in bird numbers.

Having rested and refuelled, OAD carried on flying down the Atlantic coast of Africa, through the Western Sahara and Mauritania, into Senegal and finally to Guinea-Bissau, where it arrived in mid-December, and spent about six weeks.

It departed on its spring migration back to Britain in February this year, about which time the geolocator failed. But researchers believe it arrived in Norfolk in mid-April, and it was caught again on 9 May.

The researchers say that the word "breakthrough" is not too strong a description of what the geolocator's data showed. Scientists have traditionally relied on the recovery of birds which had been ringed in order to reconstruct their journeys. Although this works well enough within Britain and Europe, the number of recoveries of European-ringed migrants south of the Sahara has been minimal.

In many cases, nothing whatsoever has been known about where European migrants fly to, once they cross the Mediterranean in the late summer, and head out into the vast African continent.

"This is revolutionising our understanding," Dr Hewson said. "Because of this one bird, we now know several orders of magnitude more about where British nightingales go in Africa than we have found out from 100 years of ringing.

"If we want to find out if things going on in Africa are affecting nightingales and other birds, we need to know where they're going," he added. "Otherwise we don't know what particular changes are happening to the area they're going to, and what the particular threats are. We may now be able to relate population declines of birds in different parts of Europe to the different parts of Africa where they spend the winter."

The nightingale in verse

Thomas Carew (1595-1640) was one of the "Cavalier poets", courtiers in the reign of Charles I who turned their hand to light and elegant verse. The first three verses of A Song, below, is sometimes referred to as Ask Me No More. His nightingale reference is charming but incorrect: as we now know, it is the male birds which sing; the females are silent.

Ask me no more where Jove bestows,

When June is past, the fading rose;

For in your beauty's orient deep

These flowers, as in their causes, sleep.

Ask me no more whither do stray

The golden atoms of the day;

For in pure love heaven did prepare

Those powders to enrich your hair.

Ask me no more whither doth haste

The nightingale, when May is past ;

For in your sweet dividing throat

She winters, and keeps warm her note.