All language subtitles for BBC.Natural.World.S28E07.Cuckoo.720p.x265

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.

The reed warbler returns
and checks her nest.

She settles down, seemingly
unaware of the rogue egg,

but then she senses something
isn't quite right.

She starts to peck the egg.

Each time she returns to the nest

she repeatedly targets the new egg,
until eventually, she punctures it.

Next, she drinks some of the
contents until it's safe to move it

without spilling something
over the other eggs.

To destroy an egg that might
hatch out into your own chick

would be a calamity.

There is enough variation in their
own eggs that they could make

a mistake, but it's worth the risk.

They have to ensure the
survival of their own offspring.

If you give reed warblers a blue egg
or a white egg or a brown egg,

very different to their own
green eggs, they throw them out,

but if you give them a green
egg matching their own eggs,

in other words mimicking what
the cuckoo actually does,

the reed warblers
tend to accept that.

If you give them a giant egg,

the reed warblers find
it very difficult to sit on

and will often desert.

So the cuckoo's egg not
only has to match the reed
warbler's eggs in colour,

but it also has to match
reasonably in size too,

if the cuckoo's got
to get it's egg accepted.

So this very simple experiment shows
that this egg mimicry by the cuckoo

is a crucial part of their trickery.

The common cuckoo species is
divided into several races, each

with a distinctive egg that matches
the colour of its particular host.

Blue cuckoo eggs to copy
redstart eggs,

speckled green cuckoo eggs
to copy warbler eggs and so on.

They are the only
species that can do this.

But each individual female cuckoo
can only lay one egg type.

So the reed warblers do have ways
of protecting their nests.

Wrong colour, size or shape
and the egg is out.

Only if the cuckoo's egg is a good
match will she outsmart her host.

A well-matched egg, though, doesn't
mean the cuckoo's work is done.

She now has to decide
exactly when to lay her egg.

The reed warbler lays a single egg
every day for four or five days.

The female cuckoo must keep watch
on the reed warblers to make sure

she lays on the same days they do.

The female cuckoo glides in.

The egg-laying is
completed in seconds.

If the cuckoo leaves it too late

and the warblers
have laid all their eggs,

then the cuckoo chick might
not hatch out in time.

But if the cuckoo lays too early,
there's a problem there, too.

If we put our model eggs in
before the hosts have begun to lay,

those model eggs
always get thrown out,

so very sensibly the female reed

warbler knows that if she hasn't
started to lay eggs yet,

that egg in the nest
can't possibly be mine.

You get a completely
different perspective down here
at the water level.

I'm normally up on the bank looking
for reed warbler's nests

and down here you really enter the
reed warbler world and you can see

the habitat from their perspective

and every 20 metres
or so there's a new territory.

I've just seen one
of the birds hopping around
in amongst the reeds there.

They don't seem to
mind us at all as we go.

And all the while, cuckoos up
in those trees behind, birdwatching.

You can almost imagine what it
must be like to be a reed warbler

in the reeds being observed,
up there.

The female cuckoo's job,
although not as laborious,

is every bit as time consuming
as that of the reed warbler's.

A cuckoo can lay 10 eggs or
more in one season,

but she only lays one egg per nest.

This means she has to stake out
dozens of reed warbler nests within

her territory, to ensure
she can lay each precious egg

in the best nest at the best time.

She may wait days or weeks
for the timing to be perfect.

When we ourselves have adopted
the strategy of the cuckoo

and have tried to do
these model egg experiments,

by the end of the day
we've convinced ourselves
that it's a crazy thing to do,

it's such hard work looking for host
nests and I think if I was a bird,

I'd just be an honest worker
and raise my own young.

Just how many eggs a cuckoo
can lay in any one season

fascinated Edgar Chance, but
not only for scientific reasons.

There's no doubt that one of
his motivations for discovering
the cuckoo's laying procedure

was so that he could collect

the most number of eggs that a
cuckoo had laid in a season.

Chance was determined to beat
a rival collector in Germany,

who claimed the world record
for the number of eggs

laid by a cuckoo in one season.

Edgar Chance did get
his world record.

He managed to get 25 eggs in one
season from his beloved Cuckoo A.

He collected every pipit and
cuckoo egg that he could and

his vast collection is now held at
the Natural History Museum at Tring.

We now know that a typical cuckoo

will lay about ten
eggs in any one season.

But Chance intervened to make
sure his female could lay more.

Actually, this world record was
achieved with Edgar Chance's help.

And what Chance did was he used to

farm the meadow pipit's nests in the
sense that if incubation was already

underway and the cuckoo
had missed that nest,

Edgar Chance would collect
all the eggs and that would force

those pipits to start
a replacement nest,

and by farming nests in this way and
making more new nests available

for the cuckoo at a suitable stage,
he managed to get the world record.

Egg collecting was key to many of
Chance's discoveries about cuckoos.

It wasn't illegal as it is now,

but even back then, some
naturalists disapproved.

Chance was actually drummed out of
the British Ornithologist Union

because of his egg-collecting habits,

so even back in the 1920s,
many people regarded

egg collecting as something
which you simply shouldn't do.

There's a final twist in
this amazing story.

Chance's trick of removing eggs
to encourage the host bird

to lay another clutch is actually
just what a female cuckoo would do.

The cuckoo is the only
British bird to do this -

her behaviour is unique.

This cuckoo will eat
whole clutches of eggs.

Just like Chance,

she wants to encourage the reed
warbler to lay more clutches.

Regardless of his methods,
Chance's record stood and no one

thought that any single cuckoo would
ever lay as many eggs in one season.

65 years after Edgar Chance,
another amateur ornithologist

appeared on the scene,
but HE never took a single egg.

I first became interested in
cuckoos in June 1983.

I was doing a water rail survey and
to alleviate the boredom of sitting

there I watched some reed warblers.

When I found the nest, I was
surprised to see a cuckoo egg in it.

And I thought, "This is more
interesting than watching water
rails that aren't there,

"so I'll see if there are any more
reed warblers in the area".

And I found another three pairs

all of which contained cuckoo eggs
of the same female.

Now you might say I'd
been bitten by the bug.

Mike Bayliss proved himself to be
every bit as skilful as Edgar Chance

even though he had a full-time job

and could only search for
reed warbler nests in his time off.

It wasn't as if I was trying to
break his record, or even equal it.

It was only when I passed the 20 mark

that I realised it was attainable.

You did hear one singing
across here, didn't you?

Let's just cruise along here.

Mike observed reed warblers
in the reed beds along the Thames.

And, like Chance, he got lucky.

One female, he called Cuckoo X,

returned to the same site
year after year.

I'd say the best year was
obviously when Cuckoo X, in 1988,

achieved a world record under natural
conditions when she laid 25 eggs.

Er, this had previously been
done by Edgar Chance under

experimental conditions in 1922.

Mike had shown just how
productive one cuckoo could be

under natural conditions.

Cuckoo X returned to Oxford
for eight seasons

and laid a total of 113 eggs.

Again, like Chance, Mike used his
detailed knowledge to get amazing

footage of a cuckoo
laying her eggs.

This is Cuckoo X laying her egg
in a reed warbler's nest in 1989.

First, the female cuckoo
removes a warbler egg.

Holding it carefully in her bill,

she then slips forward
to lay her own egg, now.

It only takes a few seconds.

So why does the cuckoo
have to be so quick?

You can test this by experiment,
and what we've done is

we've put a stuffed cuckoo next
to a reed warbler nest

to simulate, if you like,
a female who's very slow at laying.

Not surprisingly, if the reed
warblers see this cuckoo they mob

it like mad - they've got this harsh
scolding crrr, crrr, noise like this.

LOUD CHIRPING

What is surprising though

is that not all reed warblers
react in the same way.

Naive birds tend to treat a cuckoo
like a dangerous bird of prey.

It looks rather similar
and so they won't get too close.

Experienced parent reed
warblers will close right in -

they know they aren't
in any danger.

This shows Nick that whilst
reed warblers instinctively

know to reject certain eggs,

they actually have to learn
to recognise the cuckoo.

The interesting result is that
when you take the cuckoo away,

the reed warblers seem much
more alert to any change

in their nest contents.

Our experiments
show that they now

are very likely to reject even a good
matching model egg from their nest.

The arms race is very
much alive at this stage.

The warblers can fight back and
their defences can work.

For the cuckoo's trickery
to be successful,

there is a lot she has to do.

She must first remove one
or more warbler egg

to make room for her egg.

She has to a lay
a similar looking egg

so it's not recognised
and thrown out

and she has to do this quickly so
she doesn't alert the warblers.

If the cuckoo gets all this
right, the trap is set.

Warblers who have been tricked can
have no idea of what is to come.

Those whose nests have
remained safe from the cuckoo

are ready for a brood of their
own hungry chicks to emerge.

The long days of summer,
with endless supplies of insects,

are bountiful for the
warblers of Wicken Fen.

In a good year, a pair of warblers
can raise two broods each

of up to four or five chicks.

Those that have been tricked
by the cuckoo will have

no time for a second brood.

It takes as much effort
to raise one cuckoo

as ten of their own chicks.

The cuckoo chick has just hatched.

And now the reed warblers
have lost everything.

Their lives will be totally
dominated by this imposter,

and there is nothing they
can do about it.

Just 24 hours old
and still naked and blind,

the cuckoo chick
instinctively pushes out

any other eggs in the nest.

So why is it left
to the new-born hatchling

to take on this Herculean task?

You might think that one
of the things the female cuckoo

could do is simply remove all of the
host eggs and leave her egg instead.

Well, the host will always desert
a single egg, so she can't do that.

And that explains very nicely why
it's the young cuckoo that has to

take on this task of
rejecting the host eggs,

because although the host
will always desert a single egg,

they never desert a single chick.

The cuckoo chick is
astonishingly strong and has

a distinctive hollow back

that helps balance
the host's egg or chick

before throwing it out of the nest.

Nothing the little ogre does
alarms the foster parents.

Even when their own eggs are
being forced out of the nest

from right beneath them.

The simple fact is that a
warbler nest won't be big enough

to hold both reed warbler chicks
and the growing cuckoo chick.

The imposter will need all the food
that its adopted parents can bring.

Sometimes, the reed warbler's eggs

are more advanced and both warbler
and cuckoo chicks hatch together.

Again, it falls to the blind cuckoo
chick to deal with the situation.

You might think the cuckoo's cruel
and of course in a way it is,

but it's no more cruel
than the reed warbler.

If I was a fanatic of
damselflies and dragonflies,

I might get very upset

when I see a reed warbler murder a
damselfly and feed it to its chicks,

just as I would when I see
a cuckoo chick eject reed warbler

eggs or reed warbler young.

In nature, individuals
are cruel, they're all

out for what they can get, exploiting
others as food or hosts or whatever.

Those reed warbler pairs who
managed to escape the attention

of the cuckoo, or spotted the
egg and ejected it, are now busy

looking after their own brood.

These reed warbler chicks are

nine days old and demand to be
fed whenever there's daylight.

In the nest nearby, the cuckoo
chick is about eight days old.

Having cleared the nest of all
competition, there's now just one

hungry mouth devouring all the food
the warblers can bring to the nest.

Just two days later and the
cuckoo chick has grown massively.

It's about half-way through
it's time in the nest now

and can barely still fit inside.

When I was a young student I saw,
in the fens here actually,

a little reed warbler feeding
an enormous cuckoo chick

and this just amazed me. I think this
is one of the most astonishing things

you can see in the whole of nature.

Its foster parents are lavishing

as much care and attention upon it
as they would for their own brood,

instinctively caring for
whatever hatches from an egg

they assume is their own.

The cuckoo chick is well fed

and it's huge in comparison to
reed warbler chicks of the same age.

It's a very strange-looking
youngster and bears no resemblance

at all to a reed warbler chick.

It has fooled a pair of adult
warblers into working

as hard as they possibly can,
16 hours a day.

An extraordinary feat
of manipulation.

Reed warblers are programmed
to treat any chick in their nest

as one of their own,
but why do they feed it so well?

It's a question that has puzzled
bird watchers and academics

since it was first observed.

The cuckoo chick does have a

problem and the problem is how
on earth does it stimulate the

reed warblers to bring as much food

as they would to a whole brood
of their own hungry young?

Nick believes he's
discovered the answer.

It's a trick the cuckoo chick uses
from the very moment it hatches,

to make sure it gets
as much food as it needs.

If you listen to the cuckoo's begging
call, it is absolutely remarkable.

Most chicks when they're hungry
just go cheep cheep,

but the cuckoo's got
the most incredibly rapid call.

It goes cheep cheep
cheep cheep cheep - very fast.

When it's a week of age actually it
sounds like a whole brood of hungry

reed warbler chicks, and by
two or three weeks of age

it sounds like two broods
of hungry reed warbler chicks,

so we thought maybe it's this very
rapid begging which simulates lots

of hungry young, which spurs
the reed warbler foster parents

on to extra work.

Nick and his colleague
Becky Kilner tested this idea

with an ingenious experiment.

First, they carefully borrowed a
blackbird chick the same size

as a cuckoo chick and
temporarily swapped them around.

So now a blackbird chick

is in the nest that was occupied
by a similar-sized cuckoo chick.

Next to this nest,

they placed a tiny loudspeaker
connected to a tape player.

They measured how much food
the warblers brought to the

blackbird chick on its own
and compared this with how

much food the warblers brought in
when they played cuckoo chick

begging calls through the speaker.

Their results were astonishing.

I think the reed warblers are coming,
I can see the reeds moving.

OK, tape's on. Here we go.
Right, blackbird begging now.

OK.

Still begging.

OK, feeding now. The size
of the chick is not important.

A big chick alone doesn't
encourage the reed warbler

parents to bring more food.

A cuckoo's begging call is vital.

And now she's gone. Stop begging.
Stopped begging.

The warblers will feed any
chick in their nest,

but only with the cuckoo's begging
calls will they bring enough food

to satisfy a chick this big.

The female cuckoo uses the visual
trickery to get her egg accepted,

and the cuckoo chick uses vocal
trickery to get enough food.

It's now the beginning of July -
just ten weeks since the adult

cuckoos arrived in Britain and
they are already leaving for home.

With no more new reed warbler nests
being built and no new opportunities

for the cuckoo, they set off.

HE IMITATES CUCKOO CALL

They have the shortest breeding
season of any British migrant

and the birds can be
back under African skies

before the last of their
offspring has even left the nest.

The giant cuckoo
chick is 20 days old.

The nest can hardly
hold it any longer.

Soon it will have to leave the nest,

but still the reed warblers will

continue to feed it for another two
weeks, until it becomes independent.

This reed warbler's season
has been wasted

raising another species' offspring.

In a way, it's surprising
there are not more cheats

to exploit honest workers.

The big question is why?

There's only one parasitic bird
in Britain - that's the cuckoo.

In fact, only about 100 birds out of
the 10,000 species in the world are

professional cheats like the cuckoo.

And I think the reason
is simply that cheating seems

a wonderful thing to do until those
who are duped begin to fight back.

And I think that it's the fact that
the hosts fight back which

really limits the cuckoo's options

and is the reason why cheating
doesn't really prosper

so well in the very long term.

It's hard to believe that in three
to four weeks this clumsy chick

will begin its very first journey -
a 3,000 mile flight to Africa.

If it survives the long and testing
flight, it will return to the fen

next year, ready and able to trick
the reed warblers yet again.

I think people often like
the idea of individuals who make

a living in a rather unusual way -
don't follow the crowd.

I think they admire cuckoos cos

they wonder how on earth
they can get away with it.

They equate the cuckoo's behaviour

with tremendous cunning
and wit, if you like.

There's an old saying,
I think by Edward Topsell,

who says, "The grand creator has
given the cuckoo extra wit

"to make up for the fact that
it lacks maternal affection".

The cuckoo will need all
the wit it can find,

for its future is uncertain.

Nick's research will
be vital for saving it.

For not only is the cuckoo in an
arms race with all the host species,

but the cuckoo has also had to cope

with huge changes
in our countryside.

We should treasure the brief
summer visit of the cuckoo

and listen out
for that delightful call.

I, for one,

hope that it continues to announce
spring for years to come.