By Paul Homewood
The British Trust for Ornithology, BTO, are concerned about the declining population of cuckoos:
We’ve lost over half the number of Cuckoos in the UK over the last 20 years.
Since 2011 we’ve been satellite-tracking Cuckoos to find out why. We’ve learned lots of vital information which could help us to understand our Cuckoos – about the routes they have taken, and some of the pressures they face whilst on migration.
But there is still more to discover. We now need to look more closely at how dependent they are on, and how much their migration is linked, to the drought-busting rains of the weather frontal system known as the Inter Tropical Convergence Zone (ITCZ) as they move out of the Congo rainforest and begin to head back to the UK via West Africa.
Unsurprisingly, they blame it on climate change.
It’s well known that we have lost over half of our breeding cuckoos during the last twenty-five years. Populations of many UK breeding migrant species are declining, however, there is little known about the mechanisms of these declines.
Climate change is causing the timings of the spring season to change and there is evidence that many migrant species are not advancing their arrival times sufficiently to track the earlier spring. There is also some suggestion from previous studies that there are constraints in the migration timing of species wintering in or beyond the humid zone in Africa.
We also know from the BTO/JNCC/RSPB Breeding Bird Survey that Cuckoos are doing better in some areas of the country than in others, with the decline in England (63%) being greater than in Scotland and Wales, but why are they are declining at the rate they are?
Clearly we need to understand all aspects of the Cuckoo’s annual cycle before we can begin to suggest what might be driving the decline.
To find out more they have been tagging the poor little blighters .
What we have learnt so far
Migration for Cuckoos appears to be an even more risky business than we expected and, even with only five groups of Cuckoos to look at, it is obvious that there can be huge between-year differences in the challenges they face.
To date we have tagged over 50 birds. Some of the losses throughout the project are, we think, associated with tag failure, rather than the death of the birds. Given that Cuckoo numbers in the UK are declining, one of the key aims of the project is to try to understand the circumstances that may be contributing to increased mortality. Knowing how and when we lose marked Cuckoos is as important as understanding their migratory journeys.
Cuckoos tagged in the UK spend the winter in central Africa. For the first time this project has allowed us to follow Cuckoos to their wintering grounds which seem mainly to be in and around the Congo rainforest. Understanding where Cuckoos winter could be an important factor in determining the causes for their decline.
Early departure dates
We discovered that Cuckoos leave the UK much earlier than we thought. The earliest departure date so far has been 3rd June and in each year over 50% of our tagged Cuckoos have left the country by the end of June, much earlier than expected!
Chris, one of our tagged Cuckoos, spends only roughly
15% of his time in Britain. .
We’ve found that Cuckoos arrive in Britain towards the end of April and beginning of May and many leave again during June. This means that Cuckoos like Chris only spend a small percentage of the year here. Chris spends roughly 47% of his time in Africa, 38% of his time is spent on migration and just 15% in Britain!.
Variety of exits
Our Cuckoos head out of the UK in a variety of directions, some due east across the North Sea to the Netherlands and Belgium while others head south across the English Channel into France.
Two main migration routes
Tracking Cuckoos confirmed a migration route through Italy as expected from ringing data but two Cuckoos from the first year went west via Spain, highlighting a brand new migration route and a stopover site north of Madrid.
European stop-over sites
Stop-over sites are important fuelling areas for migrant birds, often areas rich in food which allow birds to fatten up for the journey ahead. Many of our Cuckoos have spent time near the River Po in Italy, confirming this is a very important fattening site for British Cuckoos which allows them to cross the Mediterranean Sea but also the crossing of the Sahara desert at its widest point.
African stop-over sites
All of our Cuckoos have crossed the Sahara each spring from previously unknown stopover sites in West Africa. It had been formerly supposed that Cuckoos crossed the Sahara in a mammoth flight direct from their wintering locations all the way to North Africa or southern Europe, so this stopover was highly unexpected. Understanding more about the location of important stop-over sites could be another crucial part of the conservation of this species.
The circular southward and northward migration routes taken by Chris,
David and BB in 2013/14.
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 Desert 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.
Speed of spring migration
Our Cuckoos take just under two months to complete the journey from their wintering locations to England. This is compared to the four or so months it takes them to reach their wintering locations on their autumn migrations. Their arrival back to the UK is important as they need to find a mate and ensure there is plenty of food at the time when youngsters hatch and are reared by host species.
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.
Colour me unconvinced!
Ant statistician worth his salt would laugh at a sample of just 50 birds, particularly when, as the BTO admit, many of the disappearances are due to tag failure, and not actual deaths.
Cuckoos eat insects, mostly caterpillars and beetles. It does not take a genius to work out that pesticides and other modern farming practices have much reduced their food supply in recent decades. You do not really have to look further to see why populations have declined.
The fact that the decline has been much greater in England than in Scotland & Wales would seem to bear this out.
The tracking project has been funded by the BBC Wildlife Trust, along with Essex & Suffolk Water. No doubt the BBC will be more than happy to see the blame put on climate change!