Thomas and Sylvester's grand Cuckoo migration

10 Jul 2018 | No. 2018-21

After being tagged in Thetford Forest by The British Trust for Ornithology (BTO), two adult male Cuckoos (Sylvester and Thomas) have begun their long journey south.

Sylvester, named by the Friends of Thetford Forest, was tagged on the 3 June 2018 but soon moved to The Fens, near to Littleport. On the evening of 21 June, transmissions were received from Sylvester’s tag that showed he had left the UK and was in northern France in the area of the Somme and close to Amiens, he has since moved further south.

Thomas, also tagged on the 3 June 2018 and named after Thomas Paine by the Friends of Thetford Forest, remained around his tagging site for nine days. However, during the early morning of 14 May, his tag showed that he had left the forest and had flown over southern East Anglia and Kent and across the Channel into northern France, before making his way on to the south of France and a patch of woodland near Langeac, 528 miles from his tagging site in Norfolk. He is currently in southeast Spain and on the brink of launching his Sahara desert crossing. 

The BTO Cuckoo project has been ongoing since 2011 and it is now known that British Cuckoos take one of two routes to get to the Congo basin in central Africa, where they spend the winter. Both Thomas and Sylvester appear to be taking the route via Spain, which during the early years of the project, at least, was found to be more hazardous than the alternative via Italy. 

This information is vital, the British Cuckoo population has decreased by 70% over the last 25 years and the project is helping us to gain a better understanding of the pressures they face across their annual cycle. BTO scientists have found a correlation between Cuckoo deaths and where they travel on their autumn migration routes; tagged Cuckoos taking the Italian route have showed a 97% survival rate, compared to only 56% via the Spanish route.

“We need to understand the reason for the difference in survival between the two routes” said Dr Chris Hewson, lead scientist on the project at the BTO, “Tagging birds from the Thetford Forest population is important for this work since it contains birds using both routes to get to their winter grounds. Ultimately we hope to understand the relative contribution of conditions in the UK and in southern Europe on the survival of birds taking each route.”

You can follow Sylvester and Thomas as they continue to make their way to the Congo Basin at

Contact Details

Paul Stancliffe
(BTO Media Manager)
Office: 01842 750050
(9am to 5:30pm Mon-Thurs), (9am to 5pm Friday)
Mobile: 07585 440910
Email: press [at] ()

Chris Hewson
(Senior Research Ecologist)
Office: 01842 750050
(9am to 5:30pm Mon-Thurs), (9am to 5pm Friday)
Email: chris.hewson [at] (chris.hewson)chris.hewson [at] (

Images are available for use alongside this News Release. 
Please contact images [at] quoting reference July 2018 - 21

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Notes to editors
The locations where the class of 2018 have been tagged are Thetford Forest in Norfolk, Sherwood Forest in Nottinghamshire, The New Forest in Hampshire, Knepp Estate in Sussex and Carlton Marshes in Suffolk. The birds have been named Sherwood, Robinson, Knepp, Raymond, Lambert, Carlton II, Cameron, Bowie, Sylvester and Thomas. More information on each can be found here.

2. Continuing to tag in future years. Not only does continuing to tag Cuckoos help gain more information, but it also allows us to understand similarities and differences within the Cuckoos we tag.  It also means that we are more likely to have Cuckoos providing us with information over a number of years rather than one or two, this means we can easily look at the differences with weather conditions and other factors that could change each year and how these factors affect the Cuckoos travel.  We can see whether birds that do ‘unusual’ things survive better or worse than others, giving insight into what limits the normal range of the population. 

3. What we have found out so far:

  • Return routes - The Cuckoos followed so far take a different return route to the UK than the one they followed on their outward autumn migration. No matter which route they take south, whether it be via Spain, Italy or further east, all the Cuckoos head to West Africa to make the return crossing over the Sahara to Europe. This information suggests that there are good reasons why Cuckoos visit West Africa on their way back and this is another important aspect of their journey which could prove a pinch point in their success.
  • British Cuckoos - Using data collected in previous years we were able to find out that although the Cuckoos we tag in Britain are referred to by us as ‘British’ Cuckoos, these Cuckoos only spend a small amount of time in Britain. One of the Cuckoos we tagged at the start of the project spent just 15% of his time in Britain, spending most of his time in Africa (47%) and the rest on migration (38%).
  • Mortality - Having followed Cuckoos over a number of years we are beginning to gather a wealth of data about the difficult points during the Cuckoo’s annual cycle. Knowing where the birds are struggling (and dying) will help us understand how their numbers change and hopefully will help to identify the causes of their declines to help inform conservation in the future. 

4. The BTO is the UK's leading bird research charity. A growing membership and up to 60,000 volunteer birdwatchers contribute to the BTO's surveys, collecting information that underpins conservation action in the UK. The BTO maintains a staff of 100 at its offices in Thetford, Stirling, Bangor (Wales) and Bangor (Northern Ireland), who analyse and publicise the results of surveys and projects. The BTO's work is funded by BTO supporters, government, trusts, industry and conservation organisations.

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