Nature’s most cunning con artist

This week we pictured a giant cuckoo chick being fed by its tiny hedge sparrow ‘mother’. Meet . . .

The RIBALD whooping ‘ cuck- oo’ of the male common cuckoo will soon herald the welcome arrival of our spring.

But the remarkable photograph of a dunnock, or hedge sparrow, feeding an enormous cuckoo chick, featured in the Mail earlier this week, is a reminder that, for many other birds, the cuckoo is a signal of doom. It is nature’s most notorious cheat.

The cuckoo lays its eggs in the nests of other birds, just one egg in each. Soon after the cuckoo chick hatches, it throws the host’s eggs and young out of the nest.

every summer, thousands of small birds will have their eggs and chicks tossed overboard by young cuckoos.

The host parents are then tricked into spending their summer raising a cuckoo instead of a brood of their own chicks.

how does the cuckoo get away with such outrageous behaviour?

This puzzle has intrigued me for the past 30 years.

The cuckoo’s favourite host on Wicken Fen, a nature reserve in Cambridgeshire, is the reed warbler. Our studies here have revealed an evolutionary ‘ arms race’ involving cuckoo trickery and host defence. every year brings new surprises.

The way in which cuckoos deceive their hosts is astonishing.

The female cuckoo glides down into the reeds from a secret lookout perch in the bushes nearby. She removes one egg from the reed warbler’s clutch, lays her own in its place and then she flies off.

her visit lasts no more than ten seconds, and her egg, green with darker spots, is a beautiful match to the host’s own eggs. This egg mimicry and secrecy in laying are both vital for success.

We found that reed warblers were more likely to eject a cuckoo’s egg if it differed in appearance from their own eggs — they puncture it and throw it out! And if they see a cuckoo at their nest, they are even more attentive to their clutch and more likely to eject a foreign egg.

So why does the female cuckoo utter a loud ‘chuckle’ call, a rapid series of ‘kwick . . . kwick’ notes, just as she leaves the host nest? It sounds like a call of triumph at a trick completed, but surely this would alert the hosts?

Jenny York, my colleague from Cambridge University, and I have just discovered that this chuckle call, given only by female cuckoos, is perhaps the most remarkable cuckoo trick of all. The cuckoo’s chuckle is similar to the rapidly repeated, high-pitched call of a sparrowhawk, ‘kiii . . . kiii’, a dangerous predator of small birds.

So we wondered if the chuckle might trick the hosts into fearing for the wrong enemy. This would divert their attention away from the clutch and towards their own safety. Our experiments supported this idea. When

we broadcast calls, through a little loudspeaker, we found that reed warblers ignored the ‘cuck-oo’ call of a male cuckoo, or the ‘ coo ... coo’ call of a harmless dove.

But the chuckle call of a female cuckoo had the same effect as the call of a predatory hawk in raising the reed warbler’s vigilance for their own safety, and we found that it reduced the chances they noticed a foreign egg in their nest.

If small birds ignore even the slightest signs of danger from a hawk, they face the risk of certain death. Perhaps this gives the female cuckoo the last laugh in this battle with her hosts.

Cuckoos are similar to hawks in appearance, too, with their barred under-parts, long wings and tails.

In ancient times, it was believed that cuckoos vanished in winter because they were transformed into hawks. In fact, they migrate south to Africa, a long journey involving an incredible 60-hour non- stop flight over the Sahara. The first firm evidence for this migration came from a recovery of a cuckoo ringed as a chick in a pied wagtail nest in eton on June 23, 1928. It was shot by bow and arrow in Cameroon, West Africa, on January 30, 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 church pastor, who reported the ring number to the British Museum.

Folklore has given rise to all sorts of strange stories about cuckoos.

Farmers used to think it unlucky to hear a cuckoo before breakfast (an incentive to rise early!), but a child born on the first day a cuckoo calls in spring would be lucky all its life. The number of ‘cuck- oo’ calls was also said to predict how long you had to live, the number of children you would have, and so on. In Denmark, people believed that was why the cuckoo didn’t build its own nest — it was too busy forecasting the future.

There are distinct genetic races of cuckoo, each with a different egg type that matches the eggs of their favourite host species.

For example: green speckled eggs for the cuckoo race in marshland that targets reed warblers, brown eggs for the cuckoo race in the moorlands that goes for meadow pipits, and plain blue eggs for the cuckoo race that specialises in redstarts.

It’s thought that a female cuckoo always returns to parasitise the same host species that raised her. So the different cuckoo races remain distinct.

A female cuckoo can lay many more eggs in a season than most birds because she doesn’t have to build a nest or care for her eggs or young.

The record is 25, recorded by edgar Chance, who studied cuckoos parasitising meadow pipits in the Twenties on Pound Green Common, a small patch of heathland in the Wyre Forest, Worcestershire. Chance was an obsessive oologist, or egg-collector, and to make more pipit nests available for ‘ his’ cuckoos he collected any completed pipit-egg clutches that were already being incubated, and so which were too late for cuckoos to parasitise.

The poor pipits then immediately built a new nest, giving the cuckoo another opportunity to parasitise them. By farming completed clutches through the season, Chance increased the number of pipit nests available for cuckoos. ChAnCe

was the first to discover how the cuckoo lays her egg during the incredibly rapid visit to the host nest.

he became so skilled at predicting which nest his cuckoos would choose that he was able to film the laying for the first time.

his film, The Cuckoo’s Secret, from 1921, shocked audiences. The cuckoo was called a ‘ wrecker of homes’ who ‘ shirked a mother’s duties’.

But even Chance underestimated its cunning. he was proud of his trick of farming of host nests to increase the number of cuckoo eggs he could collect. But later studies have shown this is what a female cuckoo does herself!

If she comes across a completed egg clutch in a nest, she swallows the whole lot to force the hosts to start again. This gives her a second chance to parasitise them.

In Chance’s day, egg- collecting from common birds, which then included cuckoos, was not illegal. But Chance collected the eggs of rare, protected species, too.

In 1926, he was fined £13 and 10 shillings (about £575 today) for unlawfully taking crossbill eggs. he dedicated his book, The Truth About The Cuckoo, to his daughter, Ann Augusta Cardamine, named after the flower the lady’s smock or Cardamine pratensis.

It is also called the cuckoo-flower because its pale pink flowers first bloom towards the end of April, at about the time the cuckoo returns to Britain to breed.

It’s clear that hosts are on the look out for foreign eggs, ones that differ from their own, so cuckoos have had to evolve matching eggs to fool their hosts.

But why do the hosts then accept the cuckoo chick, which looks so different from their own chicks?

The sight of a little warbler or dunnock feeding a monstrous cuckoo chick, even as it grows to five times the size of its foster parents, has astonished human observers for centuries.

here we find another remarkable cuckoo trick.

The chicks have rapid, highpitched and insistent begging calls — ‘tsi . . . tsi . . . tsi’ — which sound like a brood of hungry host young. These cries are so stimulating to host parents that they bring the cuckoo chick as much food as they would to a brood of their own.

Since we started our studies in the eighties, cuckoos in lowland Britain have declined in numbers by about 65 per cent.

On Wicken Fen, we used to find cuckoo eggs in about a quarter of the reed warbler nests we checked. now it’s only two in a hundred.

This alarming decline has put

cuckoos on the ‘red list’ for conservation concern. The causes of the decline might be complex. For example, there are fewer hairy caterpillars, the favourite food for adult cuckoos in summer.

But recent satellite tracking by Chris Hewson and his colleagues from the British Trust For Ornithology suggests that cuckoos are also facing increasing problems on migration.

Most cuckoos from lowland Britain migrate south-west in autumn, passing through Spain to Africa. Only about a half make it across the Sahara, probably because they did not find enough food to form fat reserves for the crossing.

This would explain why cuckoos in lowland Britain have declined so precipitously.

Scottish cuckoos, by contrast, fly southeasterly in autumn, and fatten up in northern Italy. Ninety per cent of these birds make it safely across the desert.

This ties in neatly with the fact that numbers of breeding cuckoos in Scotland are not declining. Conserving cuckoos, and others of our migrant birds, will involve international efforts.

It would surely be sad if cuckoos vanished from our countryside. We would lose not only our herald of spring, but some of the most extraordinary natural history that you can find anywhere on earth.

CUCKOO — Cheating By Nature by Nick Davies (Bloomsbury, £9.99).