BBC The Natural World Complete Series

CUCKOO SINGS

This is perhaps the best-known bird call in Britain.

A wandering voice, Wordsworth called it.

The harbinger of Spring.

An icon of our countryside.

Yet the owner of this call is a cheat, a thief and a killer.

Few know what it looks like, and even fewer its unique behaviour.

The cuckoo never builds a nest.

Instead, it tricks other species

into accepting its egg as one of their own.

It will steal and eat other birds' eggs.

The new-born cuckoo's first instinct

is to kill anything else in its nest.

Finally, and perhaps most remarkably of all, the monstrous cuckoo chick

manages to fool two tiny foster parents into feeding

and caring for it, for weeks.

How does this rule-breaker get away with it?

100 years of study are only now revealing the cuckoo's secrets.

Nick Davies is Professor of Behavioural Ecology

at the University of Cambridge.

He's one of the country's top scientists and like many ornithologists before him,

he is intrigued and puzzled by the cuckoo's extraordinary behaviour.

Nick has spent the last 23 years studying the cuckoo

and divides his time between college life in Cambridge

and his study site.

We're on Wicken Fen in Cambridgeshire,

right in the heart of the Fenlands and we're here

because this is a fantastic place for studying cuckoos and what makes it so

good is that one of the cuckoo's favourite hosts,

the reed warbler, nests right along this stretch here.

And the reason the cuckoos love stretches like this is that adjoining

the lode, this waterway here,

are these tall trees, from which the cuckoos can watch the hosts.

It's late April, and the cuckoo's intended host or victim, the reed

warbler, has made a long journey all the way from Africa.

The reed warbler

is just one of 20 species in Europe that the cuckoo takes advantage of.

In Britain, it has four other favourites - meadow pipits,

robins, dunnocks and pied wagtails.

Individual female cuckoos specialise in exploiting

just one particular species - here it's the reed warbler.

Nick has spotted the first reed warblers of the season.

As soon as they arrive, they busily set up territories in reed beds

along the lode and each male proclaims his territory

with a striking song.

Once he's attracted a mate, she works hard building an

intricate nest, using the old reed heads and spiders' webs.

But the small warbler's peaceful existence on the fen

is about to end.

CUCKOO SINGS

Cuckoos have arrived from Africa.

They know exactly when to turn up,

just as the warblers are building their nests.

The males arrive first and sing to announce their arrival.

Male cuckoos set up territories where there are lots of warbler nests.

They dash around at high speed, chasing off rival males.

This will continue for the ten weeks they are in Britain.

Places rich with reed warblers,

like Wicken Fen, have several male cuckoos in a small area.

HE MIMICS THE CUCKOO SONG

It's not completely understood what the cuckoo's call is all about.

It's certainly a territorial call and a "Keep Out" for rival males.

So if another male comes, or I come along and mimic another male,

very quickly the resident will approach and get cross.

Here he comes.

The male cuckoos may also be calling for females.

Courtship is a rarely seen aerial display high above the fen.

The female doesn't call like a male bird but makes a strange,

seldom heard, bubbling cry.

Several males often chase a lone female.

It's only once mating has begun that the real cunning begins.

By mid-May, the first reed warblers' nests have been completed

and eggs are about to be laid.

Unbeknownst to them, the female cuckoo,

with her distinct reddish brown breast,

is secretly watching and waiting.

Unlike the female reed warbler, she will never build a nest.

She has plans for this warbler nest.

Well, the Ancients knew all about cuckoos.

They knew they were parasites and they just couldn't work out why

a bird would bother producing young and not look after them.

Gilbert White puzzled about this and thought maybe God

had just done a bad job on cuckoos,

and he called the cuckoo's lack of maternal care a monstrous

outrage on maternal affection.

These quaint ideas seem ridiculous now but before Darwin came up with

the idea of evolution, the cuckoo's

habits really just were very odd and they made no sense at all.

Darwin correctly suggested that the cuckoo's strange instinct to lay

eggs in another bird's nest evolved from ancestors that had built nests.

By becoming parasitic,

cuckoos were freed from nest-building and parental duties,

so they could lay many more eggs.

So successful was this cheating,

that it was passed on through their young.

But how does the cuckoo deceive another species

into raising its young?

Early birdwatchers were uncertain

whether the cuckoo sneaked an egg or even a hatchling into the host nest.

It's a question that has puzzled naturalists since Aristotle.

Remarkably, no-one knew for sure until as late as the 1920s.

Back then, one Englishman

discovered more about cuckoo behaviour than anyone before him...

..finally proving just how it manages to lay its egg

in another bird's nest.

Edgar Chance was a businessman, but his passion was oology,

egg collecting, which is illegal today.

As a wealthy man, he spent much of his time

travelling the country, finding eggs to add to his collection

and he would go to any lengths to get them.

By 1915, his focus had changed from being a simple egg collector

to becoming a brilliant naturalist,

obsessed with trying to understand the mysterious habits of the cuckoo.

This is one, I think, of the greatest bits of bird watching ever done,

was by Edgar Chance, who is one of my great heroes.

And in the 1920s, he did some brilliant observations on a common

in Worcestershire in central England

and he was the one who, for the very first time,

showed how the cuckoo lays her egg.

Pound Green Common was close to Edgar Chance's home.

It had a good population of cuckoos

and one of their other favourite hosts - the meadow pipit.

Chance paid local children to scour the area to find nests.

He paid them well, because

nestling amongst the pipit eggs were highly-prized cuckoo eggs.

Look, I've found it!

Found one! Well done, children!

There we go. Thank you very much.

He carefully examined the cuckoo eggs and discovered that most

shared the same colour and spots.

He realised that they must have all been laid by the same cuckoo.

Chance named her Cuckoo A and began following her.

He found all the nests she was using and collected her eggs.

The egg of any individual cuckoo is unique to her.

It has an avian fingerprint on the surface of its shell.

By identifying individual eggs,

he was able to determine which nests she had visited and when.

Almost everything Chance learnt about the cuckoo

came from just studying her eggs.

"Many", said Chance, "have remained unaware how

"much of the bird's life-story is written upon the empty shells".

Chance made meticulous notes on each and every cuckoo he observed

and in doing so made some remarkable discoveries about their behaviour.

One of the things he learnt was the cuckoo lays every other day.

He also learnt the cuckoo lays her egg in the afternoon and

that was a big shock - most birds lay their eggs early in the morning.

And it really took many weeks of getting up at dawn and before,

to realise that the cuckoo must have laid the previous evening.

And once he then knew the timing of the egg-laying,

then he could watch the cuckoo's behaviour in more detail

and he got so good at predicting which nests

the cuckoo would choose next

that he was able to set up a hide and for the very first time

actually film the egg-laying of the cuckoo.

This remarkable footage, shot for Edgar Chance in 1921 by

cameraman Oliver Pike,

is one of the earliest wildlife films ever made.

Once Chance had decided which pipit nest the cuckoo was going to target,

he placed his hide and camera close by,

hoping to solve the mystery of how the cuckoo deposits its egg.

This is what he filmed.

The female cuckoo glides in from a distant tree to a pipit nest

she has been observing carefully for several days,

concealed in a tuft of heather.

Next, the cuckoo hops on the ground

and lays its egg directly into the nest, while the adult pipits

try to attack her.

A very determined Edwardian naturalist

had finally solved the age-old riddle.

The cuckoo lays directly into the host nest.

But though Edgar Chance had evidence of how the cuckoo lays her egg, the

question of how it actually fools the host into accepting it

wouldn't be solved for another 50 years.

The reason that I got interested in the cuckoo is that, of course, it's

one of nature's most famous cheats.

And in theory this cheating should

provoke an evolutionary arms race between the hosts and the cuckoo.

Once hosts start evolving defences, that should then provoke

improved trickery by the cuckoo,

and that in turn would provoke even better host defences and so on.

So the two sides in the arms race should improve their adaptations and

counter adaptations over evolutionary time and I wanted to try and test by

experiment whether this was going on.

I mean, Edgar Chance had shown very beautifully what the cuckoo does,

I wanted to try and understand

why does the cuckoo behave in this particular way

and have the hosts evolved counter tricks

to try and defend themselves against the cuckoo?

Nick's approach isn't just to observe, but also to

scientifically test the reasons for the cuckoo's cheating ways.

The cuckoo is a threatened species.

There are fewer than half the number of cuckoos in the UK today

than there were in Chance's day, and it's detailed knowledge like

Nick's that might help to save them.

On Wicken Fen, the cuckoos lay in reed warbler nests,

so their eggs have to look exactly like the eggs of the reed warbler.

They have to be the same colour, pattern and size.

Nick can test how important the mimicry is

by placing a wrongly-coloured egg into the reed warbler's nest

and seeing how the reed warbler reacts.