On the wings of science

Tracking the extraordinary journey of the cuckoo

Has Clement crossed the vast expanse of the Sahara? Has Martin finally made it to the Congo? Join Flappy McFlapperson and a cuckoo cohort on their epic globetrotting adventures

Cuckoos are sentinels for the environment.

In January 1930, a cuckoo flew over a small village called Lembé in central Cameroon. It was hit by an arrow from a keen-eyed hunter’s bow, and probably eaten for dinner. The hunter found a ring on its leg, which he gave to his wife to wear in her nose. It revealed that the cuckoo had hatched in a pied wagtail’s nest over 5,000 miles away in the UK nearly two years earlier. 

Until recently, records like these were the only clues we had about the cuckoo’s cross-continental quest. Ringed birds had occasionally been spotted as they made their way between countries in Africa and Southern Europe, but this revealed little about the nature of their journeys to and from the UK’s colder climes. 

Cuckoos’ migration was a mystery that had long fascinated research ecologist Dr Chris Hewson. He had fallen in love with birds as an 11-year-old schoolboy (“relatively late,” Chris says, compared to other bird lovers), watching kestrels gliding and hovering above the field outside his classroom window. By 14, he had subscribed to birding magazines, and learned about how dry conditions abroad had led to one type of migratory warbler failing to return to the UK from their long trip south. “I just thought that was absolutely amazing,” Chris recalls, “that the birds that we saw here were related to the drought conditions in Africa”.

The cuckoo’s impressive migration routes

Chris’s studies and later career came to be dominated by migratory birds. He worked as a ringer, and his PhD looked at how resident birds, like tits, interact with their visiting migratory cousins. He is now Senior Research Ecologist at the British Trust for Ornithology (BTO), and a leading expert on the extraordinary journey of the cuckoo.

Chris Hewson holding a nightingale, but oh, how he loves the cuckoo | Photo by Edmund-Fellowes/BTO

Cuckoos weigh just 100 grams, but each year they traverse continents to fly more than 10,000 miles. The UK’s cuckoos stay for just a few weeks in summer to breed, before heading south to warmer climes. They wing their way through southern Europe and over the Sahara, and reach the lush, warm rainforests of Central Africa by September or October. 

We know this thanks to Chris’s work, and some ingeniously small satellite tags. In 2011, he helped launch a tracking project that uses tiny solar-powered radio tags attached to the cuckoos’ backs to follow them in real time, every flap of the way. It has given people around the world the chance to become a new kind of digital birdwatcher, following the cuckoos’ multi-coloured flight paths as they extend south to Africa and back again. Thousands have watched Clive, Derek, Mr Conkers, Disco Tony, Attenborough and many more, as they succeed – and eventually, sadly fail – in completing each year’s trip.

Unlike the dog owner who looks like his dog, there is little that’s birdlike about Chris. He has a warm voice and smiling features, which become animated as he describes the first years of following the cuckoos’ movements. The team would arrive at the office each day and crowd round computer screens to check the birds’ progress. Birdwatching isn’t usually a place for cheering; not so in Chris’s lab. “It was really quite intense and exciting,” he says. “It’s pretty amazing – the fact that this small bird runs the gauntlet of thunderstorms, hail storms, adverse winds, droughts, birds of prey trying to eat them, people trying to eat them, possibly even predatory bats trying to eat them. It’s quite an intrepid journey they make.” 

PIcture courtesy of the British Trust for Ornithology

Tracking cuckoos has uncovered some of the astounding secrets to their survival. Chris and his team have watched the cuckoos fly day and night for up to three days as they made their daunting trip across the Sahara. To cope with the daytime heat, they climb higher, Icarus-like, towards the sun. They double their height to 5,000 metres, where the air is cool enough that, as Chris puts it, “they can continue to fly without frying”.

This small bird runs the gauntlet of thunderstorms, hail storms, adverse winds, droughts, birds of prey trying to eat them… it’s quite an intrepid journey.

In a similar study that Chris ran in 2016, The Beijing Cuckoo Project, online birdwatchers followed cuckoos – including one aptly named Flappy McFlapperson – as they travelled from Mongolia, through India, to Mozambique. It was the first proof that cuckoos from East Asia actually made it to Africa.

And in 2019 The Mongolia Cuckoo Project tracked five cuckoos following a similar path, crossing country borders effortlessly as a locked down world watched them from below. These studies showed that cuckoos were capable of flying seriously long-haul, as far as 26,000km in a year. 

The data has shown what makes these birds so resilient, but it also points to what threatens them. In the UK, cuckoo numbers have declined 65% since the 1980s, and scientists are beginning to piece together the reasons why. 

The UK tracking data quickly revealed that cuckoos choose one of two routes south through Europe: some fly east to Italy, while others head west via Spain. The latter flight path has proven more perilous, which could be due to climate-related impacts on food supplies. The cuckoos rely on a healthy supply of soft-bodied caterpillars in the cool montane forests east of Madrid, but when droughts and wildfires have affected Spain, this has meant fewer caterpillars and less fuel for the cuckoos’ onward flight. 

Cuckoos do love a hairy caterpillar and can even stomach poisonous ones | Photos by Edmund-Fellowes/BTO

Conditions in the UK are likely to play a part too, despite the cuckoos’ visit lasting as little as six weeks. Those that stay in intensively farmed areas of the country, like Southeast England, tend not to do as well as those who head for the hills of Wales and Scotland. 

So it is that understanding cuckoos’ experiences at every stage of their journey can paint a clearer picture of what they need to survive. The idea, Chris says, is to “get some way to determine what the best sort of conservation action is. By providing better habitat for them, either in southern Europe or even here in the UK before they migrate, then we might be able to help them along a little bit.”

The cuckoos’ final destination was also a revelation. They travel down to the Congo Basin, a rainforest region that is hot, wet and humid all year round. To researchers’ surprise, an area without seasons – and the surpluses that come with them – is able to support an influx of winged travellers every November. “We’re learning from tracking birds that certain areas that we didn’t think are particularly important are much more important than people thought,” Chris says.

As climate change creeps into habitats the world over, cuckoos are an important link in the chain of knock-on effects that can help us understand its impacts. Their iconic breeding habits, for example, have already begun to shift. When they’re not popping out of German clocks or outwitting Looney Tune coyotes, cuckoos are best known for their sneaky habit of laying eggs in other birds’ nests. But climate change has meant that many of their hosts, like dunnocks and meadow pipits, are breeding earlier. This means cuckoos miss their chance, and instead seek out the nests of other long distance migrants like the reed warbler. 

When they’re not popping out of German clocks or outwitting Looney Tune coyotes, cuckoos are best known for their sneaky habit of laying eggs in other birds’ nests.

Cuckoos are “sentinels for the environment,” Chris says. Their behaviours can show us what is happening – and changing – in various regions and ecosystems. This is “incredibly important information for conserving the natural world in general,” he adds.

Just as importantly, cuckoos’ stories can help people connect to the very real, but often difficult to feel, facts of the climate crisis. “When you start to see a pattern where all the birds that go through Spain are dying and all the birds that go through Italy are surviving, you start to realise in real time that something is going on. And I think that’s quite powerful for connecting people to environmental issues. It’s actually a visible aspect of climate change that people can really tangibly feel.”

Watching cuckoos’ globetrotting adventures can also help us feel part of the vast and complex web of nature that we so often forget our place in. “I always feel a bit connected to the birds and the places they’ve been when they come back in spring,” Chris says. “When my daughter was little and the first cuckoos were off in the Congo, I told her how the cuckoo was having his Christmas dealing with the gorillas – which is true, they’re hanging out. I think it’s important for making us feel connected to what’s going on in the wider world. And that actually, we are part of the wider world.”

This year’s cuckoo cohort has grown to 12, with seven newly tagged birds joining the project. Tracking their journeys this year will keep building our knowledge of how to protect them, as well as the many habitats that they – and we – depend on. Watching these tiny birds fly over unforgiving seas, steaming volcanoes and gleaming forests is a reminder that the natural world contains wonders we are only just beginning to understand. “We need people to take on board what we’re potentially losing,” Chris adds, “and care about it.”

Symbolism of the cuckoo

We humans have built a cultural relationship with the humble cuckoo for thousands of years. 

  1. In Greek mythology, cuckoos (alongside cows and peacocks) are sacred to Hera, the goddess of marriage, women and family.
  2. In Europe, the cuckoo is associated with the springing of spring. And, of course, with cuckoldry – a favourite theme of Shakespeare’s.
  3. In India, cuckoos are sacred to Kamadeva, the god of desire and longing.
  4. In Japan, the cuckoo symbolises unrequited love.
  5. And in cloud cuckoo land, well – nonsense reigns supreme.

It’s inspired by The Egg – and a cracking read

What’s so good about this?

Birds are barometers of climate and other environmental changes. As insect populations decline and weather patterns shift, bird behaviours and migration patterns are also forced to adjust. Understand the winged toings and froings of our feathered cousins, and we can better understand our changing planet and how to protect its inhabitants.

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