Five male birds were caught in Norfolk last May and fitted with satellite-tagged “backpacks” before being released. The scientists then monitored their progress over 12 months as they flew to Africa in the autumn, and returned in the spring.
The first two birds, named by the researchers as Lyster and Chris, arrived back in Britain last week, and after his lengthy journey, Lyster was seen just 10 miles from where he was tagged.
Dr Phil Atkinson, the head of international research at the BTO, spotted the cuckoo at the Norfolk Broads on Tuesday.
He said: “We saw him flying past, you can see the wire antenna poking out, so it was definitely him. It’s fantastic – we know where he’s been, we know the routes he’s taken, and now he’s back in the Broads.”
To lure the wild birds for the project, the BTO team glued a stuffed female cuckoo to a perch, and played a recording of a female’s “bubbling” call to spark the males’ interest.
The trackers – each of which cost more than £2,000 – were attached to the five captured birds like little backpacks, with soft straps that fit around their wings.
The project was carried out to discover why every year fewer of the birds return to Britain. Between 1995 and 2010 the population of cuckoos spending summer here fell by almost half, and the number is continuing to decline.
The RSPB estimates that there are now between 9,600 and 20,000 breeding pairs present in Britain each year.
According to a recent survey by the RSPB, of the 10 UK birds that have declined the most since 1995, eight are summer migrants, including the cuckoo, turtle dove, yellow wagtail and nightingale.
Previously, the lack of information about the cuckoos’ long migration has hampered the understanding of how to help conserve the birds.
Earlier this year it was reported that cuckoos arrived in the UK much earlier than normal. Their call was recorded by ornithologists as early as February, a month ahead of their usual arrival.
Most cuckoos are “brood parasites”, laying their eggs in the nests of other species of birds. The adults generally arrive in Britain in late March or April, and depart in July or August, with young birds leaving around a month later.
The cuckoo migration map has now shown the widely varied routes the birds take across Spain or Italy, over the Mediterranean, and across the Sahara desert to their wintering grounds in the rainforests of sub-Saharan Africa.
Dr Atkinson said: “They are African birds really. All the birds got down to Congo and survived, and it’s only on spring migration that we started to lose birds. We lost our first bird, Clement, in Cameroon on the return journey, so we think the crunch time is just before they cross the Sahara.”
He said that understanding the most challenging parts of a cuckoo’s journey and where they were most likely to die provided the project with “an incredible amount of new and important information”.
He added: “These birds move into west Africa, they fatten up as much as they can – enough to fuel their Saharan crossing. If they are not able to do that, I think that is going to be a real pinch point in terms of mortality.
"That is where we need to focus our research effort and conservation action.”
Like all migrating animals, cuckoos respond to the changing seasons. They depend on lush vegetation to provide the fruit and food for insects that they feed on.
This reliance on seasonal patterns means that a changing climate could make an already challenging journey impossible for them.
The BTO team plans to continue the project by fitting a group of female cuckoos with the same tracking devices.
Read more at The Telegraph
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