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\u2019s 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\u2019s magnetic field. Let\u2019s hope some will make it safely back to our shores one day, as heralds of a new spring.<\/div>\n\n

Speculative theories<\/strong><\/div>\n
\n
It\u2019s a crime that has captivated nature lovers for centuries. In the 18th century, Edward Jenner first recorded a newly hatched cuckoo\u2019s murderous rampage. Puzzled by the migratory cuckoo\u2019s parasitical existence, he suggested that the adult birds simply didn\u2019t have time to raise their young, needing to nip back to Africa as soon as possible. Around the same time, the famed naturalist Gilbert White guessed that the cuckoo\u2019s substantial stomach meant that it couldn\u2019t incubate its eggs. To White, the desertion of the eggs was a \u201cmonstrous outrage on maternal affection\u201d.<\/p>\n
But it was Charles Darwin who cut to the chase in 1859\u2019s On the Origin of Species. He identified the immense benefit of being a brood parasite. Freed from parental duties, the cuckoo was able to lay many more eggs than its victims. However, Darwin\u2019s observation prompts one question: if the cuckoo\u2019s con is so effective, why aren\u2019t more birds at it? Of all of British birds, only the common cuckoo (Cuculus canorus) goes to such extraordinary lengths, and worldwide only around 1 percent of all birds share its methods.<\/p>\n
After 23 years studying the cuckoo\u2019s sinister plans in Wicken Fen, Britain\u2019s foremost expert on cuckoo behaviour, Dr Nicholas Davies of the University of Cambridge, is able to shed some light on the matter.<\/p>\n
\u201cFoisting your parental duties on somebody else may seem to be a wonderful thing to do,\u201d he explains. \u201cBut over evolutionary time, the hosts fight back so that the poor cuckoo has to work incredibly hard to be lazy, simply because it has to overcome all of these defences. What we witness is a fantastic arms race between parasite and host.\u201d<\/p>\n
This titanic battle commences with that famous bird call. In May, the blue-grey male cuckoo arrives on our shores from Africa and booms out his distinctive \u2018cuc-coo\u2019, thereby establishing himself as God\u2019s gift to the slightly browner female. Nature takes its course and the female\u2019s work begins.<\/p>\n
Watching and waiting<\/h3>\n
If you think you\u2019re a good birdwatcher, you\u2019ve nothing on the female cuckoo. Perching motionless in a tree she will lay in wait, monitoring the nests of her potential victims. When the host is away feeding, the cuckoo strikes, silently swooping into the empty nest and gobbling down one of the host\u2019s eggs so that her own, which she speedily lays, is offered better incubation. Her job done she zooms off, never to see her offspring again. The entire operation takes just 10 seconds.<\/p>\n
But why does she have to be so quick? Davies\u2019s experiments hold the answer. With his team, he planted stuffed cuckoos near reed warblers\u2019 nests. When the warblers spotted this threat they attacked the fake bird, and after it was removed they were more likely to check their clutch, rejecting eggs that they didn\u2019t believe were theirs. It\u2019s the same as when you suddenly decide to install a burglar alarm after you\u2019ve heard about robberies in the area. If the hosts are alerted to the presence of a cuckoo they are more suspicious of the eggs in their nest. But if they are unaware, they will incubate the interloper in blissful ignorance. Recent research has also shown that if the cuckoo is discovered and mobbed by its victims, the resulting hullabaloo is more likely to bring the nest to the attention of predators, thereby putting the cuckoo\u2019s precious egg in jeopardy. To ensure that they can trick the hosts, the cuckoo has evolved to produce eggs that perfectly mimic the host\u2019s own. They specialise in one species and pass the ability down to their young.<\/p>\n
Evolution in action<\/h3>\n
Over a period of two years, Davies added painted wooden eggs of various sizes and colours to reed warblers\u2019 nests. If they were even slightly different to the bird\u2019s own clutch they were pushed out of the nest. The experiment also highlighted another aspect of this battle. \u201cHosts that are targeted by cuckoos evolve less variation in their own eggs,\u201d he explains.<\/p>\n
\u201cThis makes very good sense. It\u2019s easier to spot a strange egg if all of your eggs look the same. So the cuckoo had to develop an even better match.\u201d<\/p>\n
While this mimicry is fantastic, the real jaw-dropping moment comes when the chick is born. The cuckoo\u2019s egg has a head start, requiring half-a-day\u2019s less incubation than the host\u2019s clutch, possibly due to the fact that newly laid cuckoo eggs contain partly developed embryos. The chick\u2019s homicidal tendencies are just as developed, and within hours of hatching the blind and naked infant pushes any remaining eggs from the nest. If any other chicks have had the misfortune to have already hatched, they\u2019re also barged out to fall to their death.<\/p>\n
Alone in the nest, the cuckoo now has the sole attention of its foster parents, who will dart around to feed it, leaving them no time to breed again for the entire season.<\/p>\n
But even if the host had been hoodwinked into incubating a perfectly matched egg, surely they notice that their baby is twice their size? Early in his research, Nicholas wondered if the chick\u2019s immense proportions were the reason that the hosts were confused into feeding the chick, which eats about the same amount as four ravenous reed warblers. Yet when he replaced a cuckoo chick with a similarly sized blackbird, the reed warblers cut back on feeding. Then he twigged that while the mother cuckoo uses visual trickery, the baby uses aural.<\/p>\n
\u201cThe cuckoo chick has this amazing begging call,\u201d he says. \u201cIt sounds like a whole brood of hungry chicks. So we repeated the blackbird experiment, giving it a helping hand in the form of a little loudspeaker next to nest. Every time the blackbird begged, we played the cuckoo begging call through the speaker.\u201d The effect was instantaneous; the reed warblers doubled their efforts to feed the chick.<\/p>\n
As the chick gets older the intensity of its begging cry increases to make the hosts work harder, victims of what Darwin called \u201cmistaken instinct\u201d. The host is hard-wired to feed its hungry young, and so the con works. After 19 days the cuckoo is literally bursting from its nest and will still be supported by its poor foster parents for a further two weeks, before abandoning them to head to Africa.<\/p>\n
So, we know how the cuckoo pulls off its dastardly plan, but still haven\u2019t answered the question of why. Is it a cruel or evil bird? Of course not. This is just nature at work, and perhaps one of the best examples of Darwin\u2019s survival of the fittest. The arms race will continue, with both sides evolving to protect themselves or deceive the other, but our fascination with the cuckoo will remain. After all, everyone loves a rogue.<\/p>\n<\/div>\n
Where to see cuckoos in Britain<\/h2>\n
$\"""\"$ <\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>