On the morning of 30 May 2011, a male common cuckoo named Chris was caught in a mist net on Knettishall Heath in Suffolk, England. He was fitted with a small satellite transmitter weighing under five grams. The British Trust for Ornithology, which had been wondering for more than a century where exactly British cuckoos went in winter, was finally about to find out.
Chris flew south in June. He spent a few weeks in northern Italy. He crossed the Mediterranean in early August. By September he was in Chad. By November he was in the Congo basin, in a forest patch the British Trust for Ornithology had not previously suspected of holding any British cuckoos.
He spent the entire boreal winter in roughly twenty thousand square kilometres of rainforest north of Brazzaville. In April he started north again, retracing approximately his outbound route, and arrived back at Knettishall Heath on 28 April 2012.
Chris repeated this circuit four more times before his transmitter went silent on a roost in Cameroon in June 2015. By then the BTO Cuckoo Tracking Project had tagged dozens of additional birds, and the picture had become both clearer and stranger than anyone had expected.
The common cuckoo, Cuculus canorus, has been a feature of British natural history writing for as long as that writing has existed. Gilbert White wrote about it in the 1780s. Edward Grey, the foreign secretary who watched a swallow fly past in August 1914 and remarked that the lamps were going out across Europe, used the cuckoo as the central image of his book on birdwatching twelve years later.
What none of them knew was where the bird went after midsummer. British cuckoos are present in late April, vocal through May, and effectively gone by the end of June. They breed early, parasitize the nests of meadow pipits, dunnocks, and reed warblers, and depart before the eggs they laid have even hatched.
The traditional answer to where they went was "Africa." The continent of Africa contains 30 million square kilometres. The answer was, as Edith Crale puts it, not very useful.
The BTO project, which ran from 2011 to its formal conclusion in 2023, ultimately tracked more than seventy individual birds. The transmitters were small enough to attach to a 110-gram cuckoo without affecting its flight, and powerful enough to report position every few days for up to four years.
What the data showed was that British cuckoos use two principal southbound routes, and the choice of route appears to be heritable rather than learned. Western-route birds cross France, Iberia, and the western Sahara, with stopovers in Spain and West Africa. Eastern-route birds cross Italy, the central Mediterranean, and the eastern Sahara.
Both routes converge, eventually, in the Congo basin. By December, the British breeding population is concentrated in roughly the same patch of rainforest, drawn from breeding grounds spread across the whole of Great Britain.
The eastern route, somewhat unexpectedly, has substantially higher mortality. Cuckoos that take the western route are more likely to survive their first migration and more likely to return to breed. The driver appears to be the Sahara crossing: the eastern crossing requires longer non-stop flights with fewer reliable water sources.
Survival data has, in turn, helped explain a longer-running mystery. British cuckoo populations have declined by roughly sixty-five percent since the 1980s. The decline has been particularly severe in southern England, where the species is now scarce in counties where it was once common.
Birds from northern Britain and Scotland have declined less. The BTO data shows that northern birds disproportionately take the western route, while southern birds disproportionately take the eastern one. The differential mortality on migration appears to explain a substantial portion of the regional pattern of decline.
This is, in its way, a satisfying scientific result. It is also a deeply uncomfortable one. The drivers of cuckoo decline turn out to be not, primarily, in Britain. They are in the Sahel, in the Sahara, and in the wintering forests of central Africa, places British conservation organizations have very limited capacity to influence.
The cuckoo eats almost exclusively caterpillars, including the hairy and toxic ones that most other birds avoid. The decline of large moth populations across Europe is widely suspected as a contributing factor, but the relative weights of breeding-ground food supply and migration-route mortality are still being worked out.
Crale visited the BTO headquarters at Thetford in early June 2026 to talk to the project's principal investigator, Chris Hewson. Hewson is now leading a follow-on project tagging cuckoos in additional European countries, to test whether the British patterns hold across the species' range.
Preliminary data from German and Hungarian birds suggests that they do, with regional variations. The Congo basin appears to be a single shared wintering ground for most of the European population. The migratory routes diverge by breeding origin.
What the project has not been able to do, despite tagging seventy individuals, is observe a British cuckoo on its African wintering grounds in person. The forest patch is largely inaccessible. The birds are silent in winter. They are camouflaged in the canopy. Even knowing the precise grid coordinates of a roost, finding the actual bird is essentially impossible.
This is, perhaps, fitting. The cuckoo has always been a voice rather than a presence. It is heard far more often than it is seen. The fact that its winter quarters remain visually unseen, even now that they have been mapped to within a few kilometres, feels appropriate to the species.
On Knettishall Heath in early June 2026, Crale heard one bird calling at a distance of perhaps half a kilometre. It called for about twenty minutes, paused, and called again at 4:30 a.m. the next morning. By the end of the month it would be gone, on its way south to a forest neither she nor most of the people who have ever loved the species will ever visit.
The bird itself, she thought, did not care. It was simply doing what cuckoos have done for as long as anyone has noticed. The fact that someone, finally, knew where it was going seemed to make no difference at all.
