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? 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. 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.