Original subtitles

When winter descends upon Europe,

many animals have no choice but to leave.

The migrations of many creatures

have been a mystery until now.

And yet they cross oceans and deserts, brave intense weather

and travel thousands of kilometers unabated.

Thanks to recent scientific progress

we are finally able to follow their journey

and discover the secrets behind their incredible migrations.

Gray plumage, a dark colored cap.

Some 20 grams of feathers fluttering in the bushes,

always alert.

It's the blackcap.

A common European bird

that is also one of the most intriguing.

Every autumn,

millions of these birds cross the continent by night

in search of more welcoming breeding grounds.

How do they navigate in the dark?

How do they find their way to their winter homes?

Even more mysteriously,

they seem to be taking new migratory routes.

Thanks to new technologies,

We are finally starting to understand

the secrets of their migration.

(lively music)

(serene music)

In southern Sweden,

nature begins transforming in early September.

(birds chirping)

Autumn approaches and the bushes are teeming

with small birds searching for sweet berries.

Sparrows, robins,

chickadees and also an abundance of blackcap warblers.

Only the male is really worthy of the name

as the female actually sports a brownish red cap on her head

This is Northern European Warbler territory

and this is where it all begins.

These are migratory perching birds.

They spent the summer in Sweden

actively preparing for their great journey.

In a few months, Sweden will be nothing but ice.

In order to survive,

the blackcaps only have one solution, migrating south.

(lively music)

With a wingspan around 23 centimeters

and weighing an average of only 20 grams,

blackcaps are surprisingly athletic creatures

capable of traveling thousands of kilometers every year.

As autumn approaches, the birds behavior changes.

Suzanne Akesson,

who studies animal mobility at Lund University,

is very familiar with these biological transformations.

They're the precursors to migration.

They have an internal time program

that will help them to time various important events

in the life cycle for example the migration.

The mechanism is everywhere in the body.

Every cell is sort of having their timekeeper.

(lively music)

When it's biological clock commands it,

the blackcaps behavior changes drastically.

First is a radical switch in diet.

In the summer they are insectivores,

so they feed on the insects they can find in the bushes

and then slowly they will start to forage on these berries

and other fruit.

This is elderberries.

So this is the various types of black camps are feeding on

at this time of year

and they will try to find them along the migration routes.

Sweet?

It's also some extra, I'm not sure.

In the weeks preceding migration,

the blackcap begins overeating.

An intentional fattening that leads to a 30 % weight gain.

(lively music)

These sugar-rich fruits allow the bird to rapidly accumulate

the body fat it needs for its first migratory flight.

Another important change takes place.

The blackcap begins to molt its feathers.

This is a blackcap and a young bird.

There are some feathers

that still have the cover around the base.

So it's molting actually.

This is very important to have a top flight apprentice

when you depart a long migration flights.

With new feathers in a new diet,

one essential change remains.

It's a transformation that ornithologists can only see

with help from special technology.

In these cages, we feel the activity from above

and we can see how they become restless.

A songbird like the blackcaps

during the other time of year,

it's mainly sleeping at nighttime

but the migration season they will suddenly become awake

and active at night because they use the night to migrate.

The blackcap which is active during the day

for most of the year,

gradually transforms into a nocturnal creature.

That's because they migrate at night.

Flying in the dark allows the blackcap

to limit hot sun exposure

while also avoiding its primary predators, birds of prey.

(light music)

The juvenile birds migrating for the first time,

make the journey alone without their parents.

How can they possibly know as they fly away in the dark

which direction to go?

To answer this question,

scientists have developed this peculiar device.

As night falls,

the device is placed in the center of a funnel-shaped cage

and the birds are exposed to the night sky for several hours

The birds start fluttering

in the direction they wish to migrate.

The traces left by their claws on the paper

indicate that each bird is innately programmed

to navigate in a certain direction.

The blackcap is very interesting

because they have to rely on their internal genetic program

that will tell them what direction to go

and how far to migrate.

The experiment has also shown

that new behaviors are emerging.

We have have preferred migration directions

more to south and southeast

but nowadays we also find birds that have other preferences

like in this case, a more westerly orientation.

There's some large variation between individuals

but it's also recent evolution of migration.

That's very exciting with the blackcaps.

The direction the blackcap takes

for its migration differs from one bird to another

yet each of them instinctively knows which course to set.

Historically, they used to leave Sweden and head south

but for the past few years

more blackcaps are choosing new directions

some of which seemed unusual.

Before being released,

these birds are tagged with a small ring

displaying a unique number for each individual.

If this blackcap is recaptured elsewhere in Europe,

its ring will record the path that it took.

(lively music)

The time has finally come for the blackcaps to depart.

This evening, thousands of them will fly south.

As they leave Sweden,

the birds crossed Denmark in a single stretch.

400 kilometers later after eight long hours of traveling,

they finally reached Germany.

This first leg seems to follow an almost perfect trajectory

but how did they orient themselves so precisely?

Researchers have proven that blackcaps navigate

by observing the rotation of the celestial hemisphere.

The stars trace concentric circles in the sky

that are precisely centered around the geographic North Pole

It is this stationary reference

that allows the blackcaps to orient themselves.

But they aren't born

with a fully functioning internal compass.

Young birds need at least 21 nights of studying the stars

before being able to correctly orient themselves.

So what happens if a cloudy night sky

interferes with their celestial compass?

One scientist has spent years working on a device

to help the birds fine-tune their navigation.

At the University of Oldenburg in Germany,

Henrik Mouritsen and his team have built an apparatus

that allows them to alter the magnetic field.

Well, this is a (mumbles) system

which can be used to change the direction

and intensity of the Earth's magnetic field.

Out here, the magnetic nose is pointing north

and this is actual north

and if I put this compass inside the coil system here

where I have set it to change the north-south component,

you will see now the red arrow points towards South.

So you can put these birds a little orientation cages,

they will jump in the direction which they wanna fly

and then when you turn the direction of the field,

they started jumping on different directions.

So they they directly show you but they have now detected

that the magnetic field has changed.

The iron core found at the center of the earth

generates a magnetic field

that goes from the South Pole to the North Pole.

The birds don't learn direction

based on the orientation of this magnetic field

but rather on another of its characteristics, its tilt.

The lines of the Earth's magnetic field

form an angle in relation to its surface

which varies according to latitude.

That angle was 74 degrees at their departure point in Sweden

but it is only 65 degrees at their arrival point.

The nine degree difference between the two angles

tells the birds they are now 350 kilometers further south.

But which part of the bird's body

contains this particularly efficient GPS?

Henrik Mouritsen thinks it's based on a visual perception

of the magnetic field lines.

Basically the idea is

that you have a light-sensitive molecule in the bird's eye,

this absorbs light

and due to complicated quantum mechanical phenomenon,

you can generate a chemical composts.

And to the birds, it would mean that they would actually see

the magnetic field.

This chemical compass

is apparently generated by proteins

located on the bird's retina.

On this model which seems like a torture device,

imagine each nail as one of these proteins

and the basketball as the retina.

This is the half ball shape

would mean that there would be molecules oriented

in all possible axial directions.

Then imagine you look along the magnetic field lines

and let's say the magnetic field lines are running here,

then this molecule here would be parallel to the field lines

and let's say that you would see a light pixels

around the edge of the eye, you would get dark pixels

and you can now imagine that

if you now move your eye like this

you would maybe do a head scan.

You would scan your head 90 degrees to this side,

90 degrees to that side.

Now that light point will kind of move over the retina,

and this would tell you where the magnetic field lines are

so that maybe how they could perceive it.

This mechanism is the very first

to highlight a quantum physics phenomenon in animal biology.

It opens up an entirely new investigative field,

quantum biology.

Simultaneously guided by the stars

and the angle of the Earth's magnetic field,

the blackcaps begin the next leg of their journey Southwest

along the shore of the North Sea.

After a long night of flying,

the exhausted birds must imperatively rest and recuperate.

The Sun rises over the Flemish coast,

slowly revealing a green oasis in the distance.

The Zwin Nature Park.

(lively music)

Last night was quite favorable for migration

and a great number of blackcap are landing here this morning

This forest is lush with protective shrubbery

and an abundance of elderberries

making it an ideal place for a layover.

(serene music)

All is going well until.

(dramatic music)

Luckily the birds time in this net won't be very long.

Didier Vangeluwe and his fellow Zwin Park Rangers

quickly free them up.

Every year this scientist

from the Royal Belgian Institute of Natural Sciences

tags thousands of blackcaps with identification rings.

Here's another blackcap.

It's been mostly blackcaps today

but this one's already tagged.

It says Rix Museum, Stockholm

which is the Swedish Royal Museum.

So this bird was tagged by our Swedish colleagues a few days

or possibly even a few years ago.

Today we'll measure him, determine his age

and update his fact sheet.

That document is how little by little, we tracked the bird

and try to understand how their migration patterns

evolve over time.

Let's put him in this bag so we can take a look at him later

To raise the number of successful catches,

Didier Vangeluwe uses a sonar trap

to lure the birds into the net.

Sonar emits bird calls and songs.

It boosts a natural stimulus

that may be a little surprising to many people.

But it pertains to the fact that

the birds usually sing at autumn

even though we are well outside of the reproductive period.

We think the songs are a natural mechanism

that helps migrating birds

understand if the area they fly over is appropriate.

It can offer them enough food for a few days.

So we imitate nature in order to increase our odds

of catching more birds.

To avoid subjecting the captured birds

to too much stress, they must be tagged quickly.

The researchers take advantage of the moment

to measure and weigh them.

Male wing length, 76.

Their weight is a precious clue.

This one weighs 17.8 grams

which tells us that it arrived last night.

Such light weight

means this bird used up all of its fat reserves.

Now it will take a break for a few days

to replenish its energy.

It will likely give back up to 22 grams

which means it will gain about five grams here Zwin.

That's why it's so important

to manage the natural areas efficiently

and why it's essential to limit pesticides

and encourage biodiversity.

These birds have to eat

and if they don't have a varied biotope

with fruit bushes like they do here,

they won't be able to replenish their energy reserves

which in turn mean they won't be able to migrate efficiently

or might not even survive their migration.

You have to understand that young blackcaps like this one

typically have an annual mortality rate that's around 70%.

The law of the jungle makes it clear.

If the birds are not in the best shape possible,

they will die, there's no other way.

On the other hand,

the survivors will be those that are fully developed,

know how to feed efficiently, understand the weather

and know when they've reached peak physical fitness

so they can choose the right night

to travel those 200, 250 kilometers.

This is what affects population numbers.

Where is this one from?

No come on,

I'm listening but alas, I don't speak Swedish.

As soon as they are freed,

the blackcaps resumed their elderberry feast.

They only have a few days

to replenish their body fat reserves.

For a human weighing 70 kilos,

that's the equivalent of gaining 21 kilos

in less than a week.

The tagging team is happy with their results,

they've tagged 85 blackcaps today.

Each time a tagged bird is recaptured,

that data is meticulously entered in an open database.

Decade's worth of information is stored here

and it's the foundation of everything we know

about blackcap migration.

This is the recapture data map

which shows every blackcap capture in Belgium since 1927.

Each purple dot corresponds to a place in Belgium

where a blackcap was found.

We see that most blackcaps crossing over Belgium

hibernate around the western Mediterranean basin.

While most of the birds

follow a logical path heading south,

some of them migrate in another direction

that may seem absurd, north.

For example, here's a blackcap

that was ringed in Greenburgh in Belgium on September 9th

and this bird was found in Iceland

on November 8th the same year.

So instead of going south east, it went northeast.

It's likely this bird won't survive

but this is an example of the type of mistake

a migrating bird can make.

It can be fatal

but it also allows them to discover new territories.

The tagged blackcaps

now have all the energy they'll need to resume their trip.

As soon as night falls,

they'll begin their journey to their next destination,

France.

Over a million blackcaps have been tagged

in the past century and thanks to the numerous recaptures,

we know there are two primary groups of migratory blackcaps.

Those born in Western Europe

that migrate west around the Alps and head toward Spain,

those born in Eastern Europe avoid the Alps

and migrate east towards the eastern Mediterranean.

These two population groups are separated

by an invisible barrier

that ornithologist call the migration divide.

Although we've known about migration divides for a long time

it was only in the 1970s

that work by Peter Berthold and Andreas Helbig

gave us of better understanding of the phenomenon.

It is at the Max Planck Institute

in Radolfzell, Southern Germany

that Peter Berthold now 78 years old carries out his studies

after the astonishing discovery he made

thanks to the blackcap.

We went to collect blackcaps

at Lake Neusiedl in Austria.

And then we tested them in emmalin funnels

right here in our lab.

Where did the birds want to go?

Obviously east towards Israel in Eastern Africa

and then we took our own birds from around here

that normally want to fly towards Burgundy

and France and Spain, which is what they did.

The next step was to place the birds in an aviary

as mixed couples and then to observe them.

The mixed couples produced hybrid birds

that wanted to migrate south the perfect in-between.

It was incredible.

That's how we proved that migration orientation

is genetically determined or in other words innate

and inherited from the parents.

When migratory directions combine

from two different parent birds,

it leads their offspring to an entirely new destination.

The experiment proved that the birds migratory routes

are clearly based on genetics

and could also change very quickly.

Yet several questions remain.

Do the hybrids survived their migration heading south?

And what happens to the innumerable blackcaps

that follow new migratory routes?

In the heart of Vienna

on a protected island of the Danube River,

Miriam Liedvogel from the Max Planck Institute

For Evolutionary Biology in Plon

is carrying out a large international study.

Her goal is to precisely understand

the blackcaps migratory routes

by following each of them individually.

We walk around on site

and listen carefully for the blackcaps call.

Once we have a general idea of where they're located,

that's where we hang our net.

We're really in the heart of Vienna

in the very center of the city.

The number of blackcaps found on this island

in the middle of the Danube is incredible.

If you walk along that path, you'll hear them everywhere.

They nest very close to here.

Our experiments take advantage of that.

We've already caught a blackcap in our net.

Let's go pick it up.

There.

Hey you, hello.

A blackcap

only weighs about 20 grams on average.

Scientists must hence used tools

adapted to that body weight.

Miriam Liedvogel and her team

have embraced a new piece of technology, a geo locator.

This ultra lightweight receptor only does one thing,

record the date and intensity of sunlight.

That's the light sensor.

It's outside the feathers.

It records the length of the day

and the maximum height of the sun.

Using those two coordinates,

we can calculate the longitude and latitude.

That information allows us to follow their entire journey.

We know when the bird has left here,

where it makes it stops, where it spends the winter.

The device will tell us all of that

when we recaptured the bird and download its data.

The downside of the Geo locator

is that it doesn't transmit data remotely.

The device will have to be retrieved the following year

when this blackcap comes back to Austria.

Luckily, the males are territorial

so they have a tendency to nest

in the same location year after year.

But before any of that,

they first need to survive their journey.

The mini backpack must be precisely placed

so as to not encumber the bird in any way.

So our main objective

is to attach these backpacks

so that they're comfortable for the birds

so that they have enough space to move around

and they're gonna put some body mass on to migrate

and we want to make sure that there's enough leeway

for them to fly comfortably with these backpacks.

One of the goals of this experiment

is to prove that the hybrid birds

which are born in the heart of the migratory divide

are able to migrate south and return to their birthplace.

This is why Austria was chosen

as the site for the experiment.

Austria is ideal for this research.

This is where most of the geo locators

are placed on the birds.

The first year we installed about 200 geo locators in total,

the following year we must have placed another 150 or so.

In Austria,

two groups of blackcap populations intermingle.

Those who migrated West and of those heading east.

To ensure a well-rounded experiment,

the 350 geo locators were set in distinct geographic areas.

Some were placed in the area

that's home to blackcaps migrating West,

others on easterly migrating blackcaps

and others still were placed

in the heart of the migration divide

where the hybrid blackcaps migrate south.

But the southern migration route is highly dangerous.

The birds are forced to brave the Alps,

cross the Mediterranean Sea at its widest part

and tackle the ultimate obstacle, the Sahara Desert.

Are the hybrid blackcaps capable of surviving

such a trying ordeal?

Most scientists wonder.

But Miriam Liedvogel is the only one

who actually has a way to find out.

This young male is ready and his geo locator is in place.

But before beginning the great journey

males and females gather for one last grooming session.

Their feathers need to be in optimal condition

for the upcoming long-distance flight.

This small pond on the edges of the Danube

serves as a peaceful place where many different bird species

that normally keep their distance gather for a bath.

As the Sun sets

the blackcaps embark on their long night of flying.

They join the immense cohort of nocturnal migrants,

a gathering of hundreds of thousands of other blackcaps

already on their way to their winter destination.

After migrating for a month and a half,

the Scandinavian blackcaps heading west

enter the Rhine Valley.

(light music)

Today's pit stop treat, grapes.

Tomorrow, it will be apples.

These sugar heavy fruits are key

as the blackcaps remarkable metabolism

transforms sugar into fat in record time.

A few days later, the migrating birds come face-to-face

with an immense mountain chain blocking their path,

the Pyrenees.

After flying overnight for nine hours,

they once again need to refuel.

How do the blackcaps keep up such an intense effort

week after week?

In the matter of long-distance runners,

the blackcaps dig deep into their reserves

on each leg of their migration.

But they do so much more efficiently than mankind.

The blackcaps remarkable metabolism

allows it to transform lipids into energy ten times faster

than most other vertebrates.

During its journey it powers its strong pectoral muscles

with a constant flow of energy from its body fat reserves.

This is how the blackcap is able to fly

for hours on end without foraging.

Unlike this runner.

It's late November

and the first winter sleet hits the Pyrenees.

The blackcaps make their way across

either by going around or over the top.

In two weeks, these passerines cover 1200 kilometers

to get to southern Spain.

For most of the blackcaps coming from Western Europe

arriving in Andalusia, marks the end of their journey.

(light music)

Having left Northern Europe more than two months ago,

the birds finally reached their winter destination.

Located on the southernmost tip of the Iberian Peninsula,

the Lowes Alcornocolles massive

is in El Dorado for migrating blackcaps.

(serene music)

The young birds that survived their first journey,

discover their new surroundings.

A world of abundant food

and where the temperature rarely drops

below 10 degrees Celsius.

If blackcaps could speak

the native languages of their homelands,

this forest would sound like the Tower of Babel.

One would hear English, German,

French and Scandinavian languages.

But another language would also be present, Spanish.

That's because these mountains are home

to another group of blackcaps that haven't moved all year.

These blackcaps don't migrate at all.

A researcher at the Complutense University of Madrid,

Javier Perez Tris studies the relationship

between these sedentary blackcaps

and the thousands of migrating birds

that meet here each winter.

Even though they're all part of the same species,

their cohabitation is not always peaceful.

(mumbles)

So they arrived already.

This must be the migratory blackcaps

because they serve it at seldom use by the residents.

If you look at the forest,

this is the habitat of the resident population of blackcaps

in the Jakarta area.

The migrants are arriving to the place like in high numbers

so the amount of blackcaps is 20 times as much

as they are in Summer.

The interesting thing is that the juvenile migrants

they are apparently pushed outside the forest

to the shrub lands like these ones that you see here.

So this is an open habitat and they have to move in the open

between the bushes and this is risky for them.

So they get exposed to (mumbles) and all the radiators

that can attack them.

While the youngest migratory blackcaps

are pushed to the edge of the forest,

the adults claimed territory beneath the cork oaks.

Javier Perez Tris can spot morphological differences

that exist between the migratory birds

and the sedentary ones in a single glance.

12.5, 1.5, 5.

This is a migrant for sure.

So the main difference between the two birds,

between the migrant and and the resident

is the wings of the migrants are much longer,

they are more pointy and the tail is comparatively shorter.

The legs are typically shorter also.

I mean this is very good at migrating

and then the migrants typically have a lighter structure

so you don't carry so much load.

The residents, they have long legs, bigger bills,

rounded wings better for maneuvering in those vegetation.

At the same time by being bigger,

we think that they can afford confrontation (mumbles)

with the migrants that come from the north

and exploit the same territories,

they use for the whole year.

Probably they are not very (laughing)

I mean very fond of each other.

The birds that coexist

in this Andalusian forest

prove that different behaviors can be present

in creatures that are technically the same species.

Some of the blackcaps decide to go a lot further.

These tireless travelers cross the Strait of Gibraltar

and continue until they reach the Magreb countries

or even Western Africa.

All these birds chose to leave at the right time

as winter has now taken over all of northern Europe.

(lively music)

In southern England,

morning frost or more and more frequent.

The forests and hedges now have little to offer

to passerines.

A blackcaps chances of survival are very low

in such hostile environments.

Yet against all logic, some blackcaps take the risk

of hibernating in these latitudes.

In Chilbolton in Hampshire,

one man is lucky enough to have witnessed this himself.

A bird lover since he was a boy,

Glynne Evans has noticed this mysterious phenomenon

growing in scale over the years.

50 years ago we never used to see blackcaps in gardens

in winter in England and that's comparing with 25 blackcaps

that I've encountered in the garden this winter.

I mean always these things are very fascinating.

You get these strange changes that take place

and one wonders why.

The presence of these wintering blackcaps

is a mystery.

Why are there numbers rising?

How did they survive the winter?

Glynne is part of a vast network of voluntary observers

that work in collaboration with scientists from the BTO,

the British Trust for Ornithology.

Greg Conway is the Program Coordinator

and he has several explanations for the birds behavior.

The wings distribution of blackcap

is increased by it was 77%.

We know that the climate has changed

quite considerably since the 1980s.

We've got substantial warming

across pretty much the whole the British Isles.

That could be at a point now

where that makes it more suitable for the birds.

They can survive better in these areas

but there are other factors as well.

I mean in the British Isles people really love the birds

so we find between 48 maybe 55 percent of households

actually put food out for birds

during the course of the winter.

So that's a substantial resource for many wintering birds.

(lively music)

Great Britain

is currently Europe's biggest bird food consumer by far.

The sale of bird food

is a multi-million Euro annual business

that shows no signs of slowing.

It's encouraged by the country's garden stores

and supermarkets.

This winter, some 60, 000 tons of seeds, fat balls,

dried insects and fruit were offered to the birds.

But while these industrial food products

are a joy for the passerines.

Ornithologist wishing to observe

specific types of birds in their gardens

are forced to cook up something of their own.

That's just what Glyne Evans does,

having perfected his very own recipe for blackcaps.

Blackcaps are insectivores birds

and the commercial fat balls are rather hard for them.

So I try and make a match of it a bit softer.

Rather like this which appears quite hard.

But if I stick a spoon in

you see that it's much softer than the hard things

that we get from commercial suppliers.

With an increasingly mild weather

and an abundance of available food,

the blackcap is now able to survive the English winter.

Yet one question remains,

are these blackcaps the same ones

we see in England in summer?

Or as some believe, are they from somewhere else?

Benjamin Van Doren from Oxford University

hopes to have the answer to that question soon.

Last winter, he fit about 30 blackcaps with geo locators

to find out where they spend the summer.

It's crucial that the young scientist

retrieves as many of his geo locators as possible.

He meets up with Glyne Evans,

who rigged his garden with capture nets for the occasion.

You have information from ringing blackcaps

but there have only been a handful of recoveries

of these birds that have spent the winter in Britain

and have been captured elsewhere.

So these blackcaps could be going elsewhere

or could be staying in the UK

or could be doing other things that we would have no idea

without tracking them individually with geo locators.

Last winter we deployed 36 geo locators

we'll hope to get back at least 12%, 15% if we can.

Of the 36 blackcaps ringed

with geo locators in this garden,

only three have returned this winter.

But Benjamin Van Doren can finally smile.

The data he's collected reveals something extraordinary.

The first bird that we captured left the UK on March 29

and then we can see it spent the summer months

in eastern France and worked its way back to the UK

by early November.

Is very weird for a bird to do.

I don't know any other examples of birds

that in the northern hemisphere migrated south

for the summer and north for the winter.

The second bird, it went all the way to southern Germany

potentially the Western Czech Republic.

The third bird we caught

it seems to have gone to a similar location,

potentially a little bit further south towards Italy.

The data collected from the three birds

is baffling.

In autumn when the majority of birds headed south,

these three blackcaps migrated towards the Northwest.

Global warming and an abundance of food from humans

allowed them to survive the English winter.

It seems as though England

is becoming a new migratory destination.

(dramatic music)

As soon as winter ends,

the blackcaps prepare for the return trip.

The first to go are those that spent winter

in the United Kingdom.

In early March,

the blackcaps that spent winter in western Africa

rejoin the birds from the Iberian Peninsula

that are already in route towards the north.

Three weeks later,

these birds reach the foothills of the Pyrenees Mountains.

Their arrival coincides

with the flowering of the willow trees and wild cherries.

With few insects around,

the blackcaps adapt once again filling up on nectar.

Less than six weeks after leaving Andalusia,

the first migratory birds arrive at their destination.

The blackcaps that nest in France, Belgium and Switzerland

are already home.

In the (mumbles) Valley in the heart of the Austrian Alps,

blackcaps returned from every possible direction.

Miriam Leidvogel is waiting for them here in the forest.

This valley is in the heart of the migratory divide

making it a great location for recapturing hybrid blackcaps.

Birds from the southwest, the southeast

and maybe even from England all come here for mating season.

(birds chirping)

It's now the male's turn to sing.

(birds chirping)

New couples form every day.

Soon the first eggs will appear.

Since returning the blackcaps have once again become diurnal

and taken up a high protein insect based diet.

This is exactly what their fledglings need to grow quickly.

Males and females must also regularly remove

the excrements produced by their chicks

which is conveniently wrapped in small airtight packets.

(light music)

Joseph Hemburger, the valleys leading ornithologist,

is already hard at work.

The night before Miriam Leidvogel spotted a male

that seemed to have a Geo locator.

Capturing the bird and retrieving the data

from its geo locator is crucial for the ornithologists.

To be successful, they'll need every bit of their know-how

and also a little luck.

[Female translator] How's it going?

There's a male in the net,

I'll go, get him.

[Female Translator] Wonderful.

Yes, this one was ringed with this geo locator a year ago.

Maybe he went to Africa or something,

we're about to find out.

Let's remove the geo locator.

The bird is okay.

We can release him.

Now to visualize exactly where he was located,

we need to download the data to the computer.

This data, the very latest of the season,

is being added to 66 other findings taken all over Europe.

The blackcaps migratory map is slowly starting to change.

[Female Translator] The data seems okay.

The information we need

to trace this migratory blackcaps itinerary is all here.

What we can see is exactly what we wanted to know

which is that the hybrid birds

do indeed follow intermediate itineraries.

They don't follow the east or west trajectories

around the Mediterranean Sea,

they go straight down the middle.

What the data proves with certainty

is that it's possible for a bird to fly over the Alps,

the Mediterranean Sea and the Sahara Desert

before returning unharmed.

That was not obvious before we had this data.

So for us, the blackcap is particularly interesting

because it has an immense repertoire

of possible migratory routes.

It easily adapts to different environmental conditions

and it can overcome numerous barriers.

This is a very robust bird

that quickly adapts to any new territory for example,

those that survive winter on the British Isles.

I wholeheartedly believe this is the bird of the future.

These dedicated researchers

have learned that the blackcap shows complex behavior

and evolving migratory skills.

Its incredible ability to adapt

allows it to modify its itinerary, change its diet

and succeed where many other species are failing.

It's one of the few migratory passerines

whose population is growing.

By migrating in the opposite direction

and spending winter in England,

it demonstrates an amazing defiance

in the face of human activity.

If the future of biodiversity belongs to the animals

with the best capacity for adaptation,

the blackcap is certain to be among them.

(lively music)