Magical Appearances David Attenborough's Natural Curiosities

The natural world is full of extraordinary animals

with amazing life histories.

Yet certain stories are more intriguing than most...

..the mysteries of a butterfly's lifecycle,

or the strange biology of the Emperor Penguin...

Some of these creatures

were surrounded by myth and misunderstandings for a very long time.

And some have only recently revealed their secrets.

These are the animals that stand out from the crowd,

the curiosities I find...

most fascinating of all.

Some of our most familiar animals

puzzled scientific minds for a surprisingly long time.

The mysterious comings and goings of Barn Swallows

led to some far-fetched ideas,

while the life-cycle of the Painted Lady butterfly

took centuries to unravel.

Swallows have successfully nested

and raised their young in this barn for several years.

These chicks will soon leave the nest

and make their first exploratory flights around the farm.

But in a few weeks' time, they will suddenly vanish.

Where do they go to?

In the past, that gave rise to some extraordinary speculations.

In fact, in the 18th century

it became a very long-running debate

headed by some well-known church figures.

And swallows are not the only birds

that appear and disappear with the changing seasons.

For centuries, people speculated about where such birds go.

One explanation was that some birds changed into others

by growing different adult plumage.

Perhaps the Redstart turned into a Robin,

or the Garden Warbler into a Blackcap.

Since these species were seldom present at the same time,

the explanation seemed entirely plausible.

The Barnacle goose was another mystery.

Each winter, huge noisy flocks of them appear on European shores,

apparently from out of nowhere.

No-one had ever seen them build a nest

or raise young.

The Barnacle goose gave rise to some extraordinary folklore,

as this medieval illustration shows.

It was thought that the geese grew on underwater trees,

starting life as small marine creatures

called goose barnacles.

Goose barnacles do of course exist.

They're small-shelled marine organisms

with what looks like the head,

which is in fact enclosed by a shell,

attached by a stalk,

which was thought to resemble the neck of a bird,

to a bit of wood or a rock.

The confusion about the nature of the Barnacle goose was put to good use by some.

Since it was unclear whether it was a bird, a fish or some other creature,

you could surely be allowed to eat it

on days when meat was forbidden by the church.

But the most commonly held belief

was that birds disappear in winter because they hibernated.

Swallows and their close relatives, the swifts and martins,

were thought to do so in mud at the bottom of ponds and rivers.

And it's easy to see how this idea originated,

because the birds spend much of their time near water,

skimming low over the surface, hunting for insects or taking a drink.

It wasn't until the Middle Ages that another theory was proposed -

that some birds may migrate.

And one of its strongest proponents was an influential religious leader.

Frederick II of Hohenstaufen

was a powerful Holy Roman Emperor and known for his unorthodox views.

He ignored the philosophy of the church

and based his knowledge of natural history on direct observation,

rather than what was ordained.

Frederick was also a keen falconer

and he wrote this book, The Art of Falconry,

and in it, surprisingly,

there are entire chapters on the migration of birds.

His confidence came from the fact that,

unlike his contemporaries and those before him,

he had actually observed birds in the field for himself.

He had no doubt about the migration,

and so little patience for the myths surrounding the Barnacle goose.

He considered the story to be quite ridiculous

and argued that the birds simply breed in distant lands.

His views started a debate that split people into two camps -

those believing in the old hibernation theory

and those who supported the idea that birds migrate.

This was the start of a new era,

which was to sweep away myths

and focus instead on facts and careful observation.

Across Europe, the evidence for bird migration

started to accumulate.

In Germany, a 12th-century monk is said to have taken a swallow from its nest

and attached a parchment note to its leg that read,

"Oh, swallow, where do you live in winter?"

The following spring, the bird returned with a note saying,

"In Asia, in the home of Petrus" - that is Israel.

The story may not have been true,

but it certainly gave the right hint.

In the early 16th century,

a bishop from Sweden called Olaus Magnus

reignited the debate about swallows

with this picture...

He claimed that in winter,

fishermen often drew up swallows in their nets,

hanging together in a mass.

This astonishing assertion provided ample fuel for the anti-migration lobby

and, unlikely as it was,

the view that swallows spent their winters underwater

became increasingly entrenched.

By the 18th century, the debate about migration versus hibernation had come to a head,

and across the continent opinions were divided.

But new evidence was about to come from an unusual source.

Edward Jenner was an English country doctor

who also had a deep interest in natural history.

He noted that although swallows often splash in water as they skim across it,

they never immerse themselves.

Were they to do so, he suggested,

their wings would become so wet that they would be unable to fly.

To test his idea, Jenner reportedly held a swift underwater for two minutes.

Not surprisingly, it died.

Jenner went on to devise another experiment

to discover where the birds go.

He took 12 swifts from their nests

and marked them by taking off two of their claws.

The following year, some of the birds he'd marked

were caught again in exactly the same spot.

Although Jenner could not discover where his swifts had been over the winter,

he was the first to show that they return to use the same breeding sites

in the following years.

And we now know that this is true for swallows, as well.

About the same time across the Channel,

a German bird enthusiast had come up with a similar idea.

Johann Frisch caught several birds near his house

and attached to their legs woollen threads like this,

which he'd dipped in red watercolour.

He predicted that if swallows really did spend the winter at the bottom of lakes,

the red colour would be washed off.

The following spring, Frisch's swallows returned and the threads were unchanged.

It was a very simple but very effective experiment.

Evidence against the hibernation theory continued to mount

and eventually, a new technique put the final nail in its coffin -

systematic bird ringing.

This bird has just been fitted with its own individual marker,

a small metal ring on its leg, with a unique code of numbers.

It's part of a national scheme that's been running for over a 100 years

and provides scientists with invaluable data on bird movements.

Early in the 20th century,

the study of migration really took off.

Birds were recovered on their breeding and wintering grounds

and often en route, too.

600 years after Frederick von Hohenstaufen had first started the debate,

real evidence was beginning to accumulate.

In the summer of 1911, a metal ring, just like this one,

was clipped onto the leg of a young swallow in Staffordshire.

The number on the ring was B830.

18 months later, the same bird was caught by a farmer in South Africa.

Here, at last, was the indisputable proof

that swallows migrate

and spend the winter thousands of miles away.

Off you go. There we are.

Today, of course, we know that the swallow's migration

is one of the most impressive in all the animal kingdom.

It takes it across the largest desert in the world - the Sahara.

It's a gruelling and dangerous journey,

and many die on the way from exhaustion or starvation.

They travel for nearly four months,

covering nearly 6,000 miles,

and eventually reach southern Africa.

And bird ringing also helped to dispel the myth of the Barnacle goose.

In the 1960s, a Norwegian expedition ringed geese

nesting on the Arctic island of Spitsbergen.

That autumn, some of the same birds were sighted on the west coast of Scotland,

over 1,000 miles away.

Frederick von Hohenstaufen had been proved to be absolutely correct.

It took centuries to discover the truth

behind the swallow's seasonal movements.

But in that time, they baffled the minds of many great naturalists

and started one of the longest-running of all scientific debates.

But in the end, the true story proved to be even more extraordinary

than the fantastic myths that were invented to explain it.

Just like the swallow,

the Painted Lady butterfly seems to appear magically out of nowhere,

and that started some extraordinary ideas and controversies.

The Painted Lady is one of our largest butterflies

and a familiar summer visitor to our gardens,

and yet its appearance and disappearance each year has puzzled us for centuries.

It's only now that we are beginning to understand this extraordinary lifecycle

and discover where it vanishes each year.

Early naturalists were confused by sudden appearance of Painted Ladies each spring

because they were unaware of the connection

between butterflies and caterpillars.

For a very long time, it was widely believed

that butterflies arise from rotting material

by what was called "spontaneous generation".

In the 1830s,

a German scientist named Renous was arrested for heresy

for claiming that he could change caterpillars into butterflies.

Arresting someone for something now known to be common knowledge may seem rather extreme,

but many still believed that caterpillars and butterflies were completely different creatures,

created by the hand of God.

Needless to say, people had been well aware of the existence

of both butterflies and caterpillars

since the earliest times.

But the thought that any two were related -

let alone the same species - seemed impossible,

and it's easy to see why.

Not only do caterpillars and butterflies

look like very different types of animals,

but the colours and patterns of a caterpillar

don't match up with those of its adult form.

The only way to know which larva and which butterfly go together

is to keep caterpillars and watch them turn into butterflies.

But it wasn't until the 17th century

that anyone left a record of doing that.

One of the first was a remarkable woman named Maria Sibylla Merian.

Merian was born in Germany

at a time when women still had little formal education

and no role in the scientific world.

But she was an accomplished artist

and painted plants and insects she saw around her.

To do that, she kept caterpillars, fed them on leaves,

and watched them turn into butterflies.

Merian produced hundreds of beautiful paintings of butterflies in their stages of development,

along with the plants on which they feed.

Her drawings are so exquisite and detailed

that they still rank among the best in the world.

Among the things she observed with great care were things like this -

a curious, yet strangely beautiful object.

It's a chrysalis,

the intermediate stage between a caterpillar and a butterfly.

She was one of the first to record the remarkable change

that takes place in the chrysalis.

It's one of nature's most extraordinary transformations.

At the age of 52, she sailed from Europe to South America

on a two-year expedition to study insects in the tropical jungles of Surinam.

It was an exceptional journey for any naturalist at the time

and particularly for a woman.

When she returned, she produced this beautiful book.

It turned out to be popular

because it was one of the few to be published not in the scientific language of Latin,

but in Dutch.

Because of this, her work was largely dismissed by scientists of the time.

But Merian was one of the first naturalists

to correctly connect the caterpillar with its pupa and the adult form.

Today, Merian's book is widely recognised

as a pioneering work of scientific observation,

and it put an end to the idea of "spontaneous generation".

Around the same time,

further evidence for the connection between butterflies and caterpillars

came from a different source.

In 1669, a Dutch scientist by the name of Jan Swammerdam

published the results of experiments which would finally prove

that the caterpillar and butterfly are one and the same animal.

Swammerdam was a master of the miniature

and dissected the caterpillars and pupae of butterflies and moths under a microscope.

With a steady hand and endless patience,

he carefully cut into the layers of skin with tiny scissors

and what he discovered was truly astonishing.

He found some of the body parts of a butterfly.

The structures were fragile and not complete,

but Swammerdam had proved that caterpillar and butterfly

are indeed one and the same animal.

We now know that without the caterpillar there can be no butterfly.

Yet for a very long time, the Painted Lady seemed to be an exception.

Every spring, the adult butterflies would appear across Britain

without any sightings of their caterpillars.

While some butterflies hibernate in Britain,

there was no sign of Painted Ladies doing so.

Some speculated that they flew to warmer climates, as birds do.

But how could a tiny insect cross the English Channel?

In the 20th century,

swarms of butterflies moving across Europe

finally provided evidence

that Painted Ladies do indeed cross the sea.

They were found to fly all the way from North Africa to Britain.

But there were almost no records of Painted Ladies making the reverse trip south

so, for years, it was thought that Britain must be a dead-end

for the most northerly stragglers.

And then, in 2009, the public was asked to help solve the mystery.

Among 12,000 sightings,

there were reports of Painted Ladies flying out to sea in the autumn.

And a radar station detected them flying south

at heights of 500 metres,

way beyond the sight of human eyes.

We now know that the Painted Lady's migration

is a round trip of over 12,000km.

But it's not made by any one individual.

Each only flies part of the way,

passing on the migratory baton to the next generation.

It's like a relay race

with up to six generations of butterflies involved.

The Painted Lady's epic journey from one continent to the next

would be a truly astonishing feat for any animal,

but for a tiny creature like this it seems really extraordinary.

How does it battle the wind and the weather

and navigate across vast bodies of water?

And with no single individual ever undertaking the whole migration,

how do they find the way?

It seems that Painted Ladies are pre-programmed

to either fly north or south.

This is determined whilst they are still caterpillars,

possibly by temperature, day length, and also by the plants they feed on.

But how does this information get passed on

from caterpillar to butterfly?

The answer may be hidden within the chrysalis.

Recently, CT scanners have allowed us to look inside a pupa.

They reveal that some organs remain intact

during the transformation.

A one-day-old pupa clearly shows the gut and breathing tubes,

which only change slightly as the chrysalis develops.

Could it be that the brain or nerves also remain intact

and that memories are passed on?

Recent experiments in the lab appear to support this idea.

Scientists taught caterpillars to avoid specific smells

by linking them with an unpleasant reaction.

Later on, as adults,

the same individuals remembered these smells

and chose to keep away from them.

If the experiences of a caterpillar can be carried over to the adult,

then maybe cues for migration can also be passed on.

Although we've unravelled much of the Painted Lady's life cycle,

many questions remain...

How far does each individual travel,

and do offspring follow similar routes to their ancestors?

One day, we may know the answers

but, for now, they remain some of the unsolved mysteries of nature.

The arrival each spring of our Painted Lady butterflies and our swallows

never ceases to delight us.

But now we also understand the extraordinary journeys they undertake

when they disappear again at the end of summer.