In the forest of the cuckoos

You have to get up early to meet a star of the cuckoo world  – a bird that is sometimes heard but rarely seen.

The ban on international travel has been a major feature of the pandemic period but in the kingdom of the birds there has been no such limitation on clocking up the air miles.

The compulsion to take off on epic journeys across seas, mountains and deserts in search of food and territory is one of the wonders of the natural world. Migration is not a behaviour reserved solely for birds – many animals including butterflies and turtles undertake their own incredible odysseys – but in Suffolk it is the birds returning from Africa and southern Europe, such as swallows and martins, swifts, warblers and cuckoos that are most apparent each spring.

Tracking

Our understanding of migration has been helped by modern tracking devices that send back detailed data on birds’ journeys. One such ground-breaking survey, which has been run for the past decade by the British Trust for Ornithology (BTO) from its Suffolk headquarters in Thetford, has done much to shine a light on the migration of our cuckoos from their home in the Congo basin in central Africa to the UK and back each year, and how the different routes taken are linked to declines in the birds’ numbers.  

The project has captured the public’s imagination and people come forward to sponsor and name the cuckoos, including one enduring bird named PJ, who has become a star of the cuckoo world – having been tracked flying more than 50,000 miles over a five-year period.  I was invited along to welcome PJ back and in the weeks prior to his arrival in East Anglia in late April, I scanned the BTO website for updates on his progress.

On his travels, PJ paused for a few weeks in Sierra Leone in western Africa, as he stocked up on caterpillars and piled on the grams in preparation for the greatest obstacle on the journey – the Sahara Desert. I learnt that cuckoos cross this great infertile expanse of sand and rock in one go – a 50-hour, non-stop flight, several kilometres in the air.

Early bird

I meet Paul Stancliffe from the BTO at a secret location in Thetford Forest at 5.30am. Early birds, it seems, catch the..er..bird. We made our way to a space where pine trees had been cleared – a location that PJ has favoured on previous occasions.

What a joy to be in a wild place at such an hour. As the sun rose across the Breckland landscape of twisted pines all sorts of beautiful creatures made an appearance. A pair of woodlarks collected nesting material near some scrub; a majestic male yellowhammer perched on a fence post like a small beacon. Two roe deer skipped down a woodland ride; a hare cantered across grassland.

But with no sign of PJ, Paul unpacked his secret weapon: a megaphone loaded with a recording of the call of a female cuckoo. Turned up loud, it did not take too long for the bubbling song to work its magic.  At first, we heard the famous cuck-oo call of a male behind us and by the time we looked back two cuckoos had arrived from our right. Within no time, incredibly, there were four male cuckoos lined up on the fence – including, at the far end, PJ. 

I had never seen a cuckoo before and here I was faced with a quartet. I took the experience in through Paul’s telescope. The birds were grey-coloured like a wood pigeon but slighter in size, like a collared dove. The dark bars across the white chest are prominent, as is the yellow ring around the eye.  

It was a truly memorable experience, but I could not help feeling sorry for the foursome, who had travelled all the way from the Equator in search of a mate, only to be met by a loudhailer and two middle-aged blokes with binoculars.

Parasitic

Of course, the cuckoo’s migratory habits are only part of its remarkable behaviour. What most people know about the cuckoo is that it lays its eggs in the nest of other species, leaving them to raise their young – a behaviour that has not done much for its public profile down the years.

Paul told me cuckoos choose Thetford Forest because the nearby heather fields provide a healthy stock of caterpillars to feed on while the bare and partially grown areas are home to meadow and tree pipits – two species whose nests the cuckoos target along with reed warblers, dunnocks and robins.

For an insight into the ingenious ways the cuckoo fools other species look no further than a masterly book called Cuckoo; cheating by nature by Nick Davies, who spent many painstaking years studying the habits of cuckoos just over the border in Cambridgeshire at the Wicken Fen National Trust reserve.

Davies describes the competition between the cuckoo and the birds it parasitises as ‘an evolutionary arms race’ – where the different species are constantly upping their game to out do each other. Different races of cuckoo have evolved to target different host species, so the eggs they lay mimic the size and colour of the host – brown for meadow pipit, aqua green for dunnock, mottled grey-brown for reed warbler.

The female cuckoo will sit for days studying the whereabouts of nests and can hold the egg in her oviduct waiting for the moment to strike. Finally, she swoops into the nest, lays one of hers and removes one from the host bird, taking it away in her beak. This is all completed within ten seconds, so as not to arouse suspicion in the host bird who will destroy the cuckoo egg if it suspects foul play. A female cuckoo can lay up to 25 eggs in a summer, so can afford a few unsuccessful forays.

In those nests where the egg is accepted, the cuckoo hatchling will kill the other young birds by pushing them out of the nest, allowing it to guzzle all the insects and caterpillars the host bird brings back. The youngster will grow to four or five times the size of its adopted parents and remarkably, Davies’ studies have shown, uses its feeding call to fool the host birds into thinking there is a full nest to ensure a ready supply of food to nourish its bulk.

Return

At the end of our summer, these young birds and their long-gone ‘real’ parents will make their way back to the Congo where they will spend 70% of their lives.

The BTO survey has shown that birds either migrate southeast via Italy or the Balkans (the ‘east route’) or southwest via Spain and Morocco (the ‘west route’) – the route favoured by our East Anglian cuckoos.

Despite the obvious ecological barrier presented by the Sahara, most cuckoos that perish taking the west route, do so in Europe, suggesting the droughts and wildfires suffered in Spain in recent years, and other factors like large-scale habitat change, are making the arduous trip even harder.

So, the next time you hear the call of the cuckoo, take some time to ponder the Herculean efforts it has made to get here and the evolutionary wonders of a bird that is sometimes heard but rarely seen.

How the BTO tracks cuckoos

The BTO has been able to glean detailed insights into the seasonal movements of cuckoos because of recent advances in satellite transmitter technology, which are almost as impressive as the birds’ migratory exploits themselves.

Tracked cuckoos are fitted with a device weighing 5g, which also features a miniature solar panel. It transmits for 10 hours at a time before going into ‘sleep’ mode for 48 hours to allow the solar panel to re-charge the battery.

As well as giving information about a bird’s location, the devices also send back data on the temperature of the cuckoos wearing them. Tragically, Paul told me that in some instances this has resulted in scientists based in Thetford watching a bird’s body warmth tail off as it perishes on its journey.

In just ten years, these devices have given many more clues about cuckoo behaviour than ninety years of ringing the birds ever did. During that time just one ringed cuckoo was recovered south of the Sahara: a cuckoo fitted with a leg ring as a chick in a pied wagtail nest in Eton in June 1928 was killed in Cameroon, West Africa in January 1930.

_{This article first appeared in the Suffolk Magazine July 2021.}