Cuckoos Clock Up the Miles as Tracking Reveals Secrets of a Vanishing Voice
BTO expands satellite and GPS tagging to better understand dramatic Cuckoo declines and the dangers along their 16,000 km migratory route
Understanding the decline of a cherished summer bird
The Cuckoo's mournful, two-note call has long heralded the arrival of spring across the British countryside - yet since 1995, we’ve lost more than a third of the UK's breeding population. Now, using satellite and GPS tracking technology, scientists at the British Trust for Ornithology (BTO) are unlocking the mysteries behind this troubling trend.
Each year, Cuckoos make an epic 16,000 km journey between Britain and central Africa. Since 2011, over 130 birds have been fitted with lightweight satellite transmitters, allowing researchers to monitor their movements in real time. These tags have already revealed valuable insights into migration routes, survival rates, and the importance of habitat quality in the UK and beyond.
New GPS technology promises even sharper insights
This year, the BTO has added an extra dimension to its work by fitting five Cuckoos with high-resolution GPS tags as part of the international ‘Migratelane’ project. Developed in partnership with the Paris Natural History Museum and funded by the Office Français de la Biodiversité, the project aims to investigate the potential risks posed by proposed wind farms in the English Channel and Bay of Biscay.
The new GPS tags transmit location data over the mobile phone network, offering unprecedented detail on the birds’ exact routes and stopover sites. Crucially, they also allow researchers to assess the birds’ use of breeding habitats and feeding areas within the UK - a vital link in understanding how to reverse recent population declines.
Breeding ground conditions are key to survival
Early findings from BTO's tracking programme indicate that survival on migration is heavily influenced by the condition of birds as they leave the UK. Birds departing later in the season, particularly those from lowland England, are more likely to follow a westerly route via Spain - a route associated with much lower survival rates. In contrast, birds tagged in upland areas, including Scotland and Ireland, consistently take an easterly route via Italy and tend to fare better.
Researchers believe this disparity stems from habitat quality in the breeding season, particularly the availability of caterpillars - the Cuckoo's primary food source. Insect-rich sites may help Cuckoos build up the energy reserves needed to survive the journey. Restoring such habitats could therefore be a key tool in halting the species’ decline.
Field studies aim to link diet to migration success
To support this hypothesis, BTO plans to integrate field studies with the satellite and GPS tracking, examining what Cuckoos are eating across different habitats and how this correlates with migratory performance. The aim is to better understand whether certain diets, prey types, or landscape features can enhance survival and ultimately bolster the breeding population.
Armed with this data, conservationists hope to identify which habitats offer the best start in life for Cuckoos before they embark on their transcontinental journeys. This knowledge could be especially useful in targeting conservation action in lowland areas where the species has suffered steep losses.
Public support helping fund critical tracking work
The success of this project owes much to public support. The newly tagged birds include Norman (Inversnaid), Ashok and Arthur (Suffolk), Frederic (Norfolk), Jim and Wingston (West Sussex) - each funded or sponsored by private donors or organisations. Their journeys can be followed on the BTO website, offering the public a direct connection with the remarkable lives of these birds.
Dr Chris Hewson, lead scientist on the project, said: “It’s fantastic to see six more Cuckoos heading off with satellite tags newly fitted in addition to the five Cuckoos which have been fitted with GPS tags for the first time. Together, these samples of birds will provide information that will help us to more precisely understand not only why their populations are declining but also how best we can help them to successfully complete their arduous migrations in the rapidly changing world we share.”
To track the birds and explore more about the project, visit: www.bto.org/cuckoos
June 2025
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