- Common cuckoos breed right across the temperate Old World, from Western Europe to Japan
- There are currently 15,000 breeding pairs of common cuckoo in the UK
- This solitary bird eats insects, and is particularly fond of hairy caterpillars
- There are many distinct races of cuckoo. Each specialises in a particular host species, and lays a distinctive egg type
- Most races lay an egg that matches the egg of their chosen host. Experiments with model eggs reveal that these hosts reject eggs unlike their own, so the good match is necessary to trick the hosts.
- A female common cuckoo can lay up to 25 eggs in a summer
- If she fails to find a nest of her favourite host, as a last resort she might lay in the nest of another host species, but this egg is more likely to be rejected, as it will not be such a good match to the host egg
History of the cuckoo
Summer is a-coming in, loudly sing cuckoo!” proclaims the oldest English song. For thousands of years, the loud, ringing “cuck-oo” call of the male common cuckoo has heralded the arrival of our spring.
This “wandering voice” is the source of many myths. To hear your first cuckoo before breakfast was just unlucky, but if you were still in bed it was a sure sign of impending illness (incentive to rise early). However, good fortune would come if you heard one while out walking, and a child born on the day of the first cuckoo in spring would be lucky for all its life.
We all sang along, too, as the calling marked the passing of summer:
What do you do?
In April, I open my bill.
In May, I sing night and day.
In June, I change my tune.
In July, far, far, I fly.
In August, away! I must.”
Why is the cuckoo in decline?
But this once-familiar voice is now disappearing from the British countryside, and the silence is a stark reminder of our diminishing natural world. What has caused this precipitous decline? Loss of natural habitat and scarcer insect food are likely to have played a part, but cuckoos are now also facing increasing problems on migration to Africa.
Only in the past decade have we learnt exactly where our cuckoos go in winter. Ninety years of ringing had produced just one recovery south of the Sahara; a cuckoo ringed as a chick in Eton on 23 June 1928 was felled by bow and arrow in Cameroon, West Africa, on 30 January 1930. We know this because the hunter gave the ring to his wife to wear as an ornament in her nose. Thus it came to the notice of the local pastor at church, who reported the ring number to the British Museum.
Cuckoos in the Congo
Now, satellite tracking by the British Trust for Ornithology, led by Chris Hewson and Phil Atkinson, has revealed that our cuckoos winter in the forests of the Congo. Cuckoos migrate mainly by night, and often at high altitude, between three and five kilometres above the ground, perhaps to find strong following winds to help them on their way. They cross the Sahara in one 50-60 hour continuous flight. Imagine them launching into the night skies at dusk. They then fly nonstop through that night and all the following day, then another night and day, and finally a third night, before they have the chance to feed once more.
The migration route varies, and this provides the clue to the varying fortunes of our summer cuckoos. In autumn, most English cuckoos go southwest through Spain, but increasing droughts in SW Europe have made it difficult for them to put on sufficient fat reserves and only half of the tracked birds survive the long desert crossing. This heavy mortality on migration matches the dramatic decline in our English breeding cuckoos; we have lost two-thirds of them in the last 30 years. By contrast, most Scottish and Welsh cuckoos take a different autumnal route, south-east through Italy. Here, feeding conditions seem much better, because more than 95% of these tracked cuckoos make it safely to winter quarters.
This is reflected in the better fate of their breeding populations; Welsh cuckoos have declined much less than in England, and in Scotland, numbers have remained stable.
Why does the cuckoo lay its eggs in another bird's nest?
As cuckoos disappear from the English countryside, we lose part of our cultural heritage. But the loss of an evolutionary treasure is surely more heartbreaking, for the cuckoo’s breeding behaviour is one of the wonders of the natural world. The cuckoo never raises its own offspring. Instead, it lays its eggs in the nests of other birds; just one egg in each host nest. Soon after the cuckoo chick hatches, it balances each of the host’s eggs on its back, one by one, and heaves them out of the nest. Any host chicks will get thrown out too. Once the cuckoo chick has claimed the nest to itself, the host parents are then tricked into raising a young cuckoo instead of a brood of their own.
The sight of a little warbler or pipit feeding an enormous cuckoo fledgling, six times the size of the foster parent, has astonished human observers for centuries. How does the cuckoo get away with such outrageous behaviour? In fact, cuckoos need extraordinary trickery
to get past host defences, for the hosts are on the lookout for cuckoo eggs and if they detect one, they puncture it and eject it from the nest.
Two favourite hosts in Britain are reed warblers in marshland and meadow pipits in moorland. Individual female cuckoos specialise on one host species and there are genetically distinct cuckoo races. Reed-warbler-specialist cuckoos lay a greenish spotted egg, just like those of reed warblers, while meadow-pipit-specialist cuckoos lay a brownish spotted egg, just like those of meadow pipits. Both these hosts reject eggs unlike their own, so the specialised cuckoo-egg mimicry is essential to fool them.
The female cuckoo also needs secrecy to succeed, because if the hosts see her at their nest they are alerted to inspect their clutch more closely. She glides down to the host nest from a hidden lookout perch, removes a host egg, lays her own in its place, and is off – all within a 10-second visit. As she departs, she often gives a chuckle call, as if in triumph. This is perhaps the most remarkable trick of all. The chuckle is similar to the rapid call notes of a sparrowhawk, and it diverts the hosts’ attention away from noticing changes in their clutch and towards their own safety instead. So the female cuckoo has the last laugh as she flies away.
Given that hosts are on the lookout for odd eggs, it seems strange that they accept a cuckoo chick so different from their own. But the cuckoo chick has a special trick, too. Its loud and rapid begging calls sound like a whole brood of hungry host young, and this fools the foster parents into bringing as much food to a cuckoo chick as to a brood of their own.
Cuckoo chicks fledge from the host nest at around three weeks of age and are then fed for two more weeks before they become independent. They will never see their real parents; indeed, most adult cuckoos will already have left the UK by late June. With no more new host clutches to parasitise, their summer’s job is done and they set off on their long journey south to Africa. The young cuckoos will follow them a few weeks later and all alone, guided on their incredible journey by the night skies and the Earth’s magnetic field. Let’s hope some will make it safely back to our shores one day, as heralds of a new spring.
Where to see cuckoos in Britain
With long, pointed wings, a long tail and barring underneath, the common cuckoo looks rather like a bird of prey. Only male cuckoos call “cuck-oo”. The bill is opened for the “cuck” and closed to form a sound chamber for the “oo”.
Good places to look out for them are: the wonderful RSPB reserve Lakenheath Fen in Suffolk, the New Forest, Dartmoor, Cannock Chase in Staffordshire, the North York Moors, Brecon Beacons, Central Wales around Tregaron, and throughout the western Highlands of Scotland, the Isle of Skye and the Hebrides.
Main image: Cuckoo sits on a perch in woodland on Thursley Common on May 28, 2017 in Thursley, England. The United Kingdom has seen a 71 percent decline in the breeding population of Cuckoos over the last 25 years. Credit: Dan Kitwood for Getty Images
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