Original subtitles

In 1862, an American called Thomas Wentworth Higginson wrote,

"I think that if required on pain of death

"to name instantly the most perfect thing in the universe,

"I should risk my fate on a bird's egg."

I think, if pressed, I might do the same.

It's a remarkable structure that protects a new life

from the outside world

and at the same time allows it to breathe.

HEARTBEAT

It's strong enough to withstand the full weight of an incubating parent

and fragile enough to allow the chick to crack it.

But how is an egg constructed?

And, perhaps more importantly, why is it that way?

In this programme, we're going to follow an egg

from its creation to the moment when life breaks out of it.

Piece by piece, we will reveal

what lies behind nature's most perfect thing.

This is the egg many of us see each morning at breakfast.

ROOSTER CROWS

And it's the egg about which there is

the biggest body of scientific research,

because the poultry industry has invested millions

in finding out what makes the perfect egg.

The very familiarity we have with this very common article

that we find in our kitchens

may well blind us to the wonder and beauty of the eggs

in terms of structure and colour and shape

that are produced by the 10,000 different species of birds

that are alive in the world today.

It's time to reintroduce some wonder into this miracle of nature.

Each egg serves one purpose -

to nurture new life.

From the world's smallest, the bee hummingbird egg

that weighs less than half a gram,

to the biggest, the ostrich egg that's 40,000 times heavier.

And yet there is an astonishing diversity of design

not only in terms of size and shape but also colour and pattern.

It was the sheer beauty of the egg that prompted, in many people,

including me, questions about their biological perfection.

And, perhaps the most important question of all,

why lay an egg in the first place?

WIND WHISTLES

Antarctica - one of the most extreme environments on earth.

Emperor penguins are beginning the daunting task

of incubating their eggs in sub-zero temperatures.

So why haven't they evolved to develop young inside the body,

where it could stay warm?

Well, penguins are related to the birds that once flew.

And keeping weight to a minimum for anything that flies

is of the utmost importance.

But there's also another reason.

Birds are hotter than mammals.

The internal temperature of all adults is 40 degrees.

No embryo can develop at such high heat.

But since they lay eggs,

they can incubate their embryos at lower temperatures.

Laying eggs, however, does create a lot of problems.

The embryo must now be protected, warmed and nourished

outside the body.

And those aren't just challenges for birds that breed in polar extremes.

BIRDSONG

It's the beginning of spring in Oxfordshire.

The increasing warmth and light of the new season will transform

the woods into a riot of life.

Now is the time to produce young.

This is a female great tit.

And this is a blue tit.

They're both extraordinarily skilful in caring for their eggs.

Over the next few weeks, I will watch them both

as they lay, incubate and hatch their eggs.

Right now, it's the very beginning of the season.

In just over 24 hours,

this female will lay the first egg of her clutch.

She's busy and stressed.

Creating an egg is hard work.

It requires additional nutrients, including calcium for the shell.

And because tits lay large clutches,

they need large amounts of additional calcium.

Some birds can extract the calcium they need from their bones,

but not tits.

To create her clutch,

she will need to find more calcium than she has in her entire skeleton.

And this is what she's looking for - fragments of snail shell.

During the laying period,

a female will spend half her time picking up fragments like this,

so that when she goes to roost at night,

her gizzard is packed full of this material

with which she will make the shell for her egg.

We don't know exactly how many snail shells

she has to eat to produce an egg.

But we do know that without them,

she would lay eggs with very thin, breakable shells,

or even eggs with no shell at all.

And she's running out of time -

the egg is already forming inside her.

Calcium or no calcium, there's no stopping the egg's arrival now.

But exactly what happens when an egg is created?

To understand that, we must go back

to the very beginning of an egg's existence.

Forget everything you know about human conception.

Birds do it differently.

As day breaks over the River Thames,

great crested grebes and mute swans court in the morning light.

But each dawn mating is not sparking an egg to life.

In humans, fertilisation occurs within just hours of insemination.

In birds, there's a long delay.

The females store sperm.

Sometimes for a few days or even a week.

And on the other side of the world, one bird stores it for longer still.

A female albatross flying across the vast southern seas.

The albatross and its close relatives store sperm

for far longer than any other bird.

It can be two months between mating and laying a fertilised egg.

In the past, sea bird biologists used to have a rather romantic idea

about this period spent away from the colony.

For a start, they called it the honeymoon period.

But that rather missed the point,

because the female goes away on her own.

So why does she fly hundreds of miles away from home

with her mate's sperm, still unused, inside her?

Because she's busy building up this...

Yolk.

This is the bird's reproductive tract.

Here in the ovary, one of the ova is filling with yolk.

On the yolk's surface sits a tiny disc

that contains all the female genetic material needed to create an embryo.

The albatross has now to collect enough food

to enable her to amass a yolk so big

that it can be transformed into a chick.

And only when she's done that will the egg be fertilised.

Remarkably, it takes more than one sperm to start new life.

The extra sperm probably releases substances

that start the embryo's development.

Minutes after fertilisation,

the egg starts its 24-hour journey down the oviduct.

First, it's swathed with albumen, the egg white,

that contains the water needed by the growing chick.

That done and enclosed within a membrane,

it travels on to the uterus,

where it will be given its protective armour, a shell.

The shell is actually quite separate from what it contains.

To help understand this, we can do a simple experiment.

These are unfertilised quail eggs.

And this is vinegar.

What the vinegar will do is

to reverse the process of shell formation

by eating away the shell from the outside.

These thousands of tiny bubbles are carbon dioxide.

They're the result of the acetic acid in the vinegar

They're the result of the acetic acid in the vinegar

reacting with the calcium carbonate of the shell.

In 24 hours, the shell has dissolved.

And this is the egg as it would have been

when it first arrived at the uterus -

a yolk surrounded by a thin layer of albumen

all contained and supported by a loose, soft bag.

And unexpectedly, it's this bag, the membrane, not the shell,

that gives the egg its shape.

So now, back inside the uterus, the egg is almost complete.

Calcium carbonate, carried by blood vessels, is deposited

on the soft egg membrane, where

it will harden and set, forming the shell.

Then other cells begin to discharge pigment,

like paint being squirted from hundreds of tiny paint guns.

As the egg slowly revolves,

yet more cells spray out spots and streaks.

It's taken just under 24 hours for the egg to be fertilised

and enclosed in a hard shell.

And now, within the dark uterus, it waits like an actor in the wings,

ready to make its appearance on life's stage.

But which end will emerge first from the bird - big end or little end?

Well, let's start with the chicken and a study conducted in 1896.

A German scientist called Heinrich Wickmann

poked a pencil up a chicken's bottom.

ROOSTER CROWS

Using eight very tame chickens,

Wickmann used a pencil to mark

a cross on the end of the egg

that he could see just inside the hen's oviduct.

This enabled him to establish

that an hour or so before the egg is laid,

the pointed end is pointing outwards

and then immediately before it's ejected,

it turns round, like that,

and comes out blunt end first.

But this isn't the way all birds lay their eggs.

Some lay pointed end first.

But do they also turn their eggs inside them as a chicken does?

That is still something of a mystery.

And perhaps it will remain so

because no-one since Wickmann has been bold enough

to use a pencil to find out.

Working out what happens inside wild birds

before they lay is, understandably, a rather more tricky business.

But we can observe what happens afterwards.

We've seen that eggs come in different sizes,

but is their size always directly proportional

to the size of the bird that laid them?

And what about the number of eggs that birds lay?

Some, like the nocturnal kiwi, lay just one huge egg

that weighs a fifth as much as the bird that produced it.

Others, like the male ostrich, incubate enormous clutches

to which several females have contributed.

These are the largest eggs laid by any living bird...

..but they're tiny when compared to the bird which produced them.

Each egg weighs just 2% of the adult.

Eggs are perfect in so many different ways.

And they have to be,

because they have to be laid in so many different conditions,

from the poles to the tropics,

wet and dry, in nests and without.

But they all need heat.

A Welsh churchyard viewed by a thermal camera.

It might look like night, but actually, it's day.

What the camera is recording is heat.

There's a bird here that has a surprising incubation strategy

that's never been filmed before.

We need the thermal camera

to give us an understanding of what she does.

The bird is a goldcrest. It's the smallest bird in Europe.

She weighs no more than a teaspoon of sugar.

Her eight eggs may look tiny too,

but in relation to her they are huge.

Each one weighs 16% of her body weight.

In human terms, that would be like giving birth

to eight whopping 18lb babies.

But her tiny body can only cover two or three of her large eggs

at any one time.

Bird embryos develop at around 37 degrees,

so how will she keep them all warm?

Well, the secret to her success is revealed

when we turn our standard camera off

and our thermal camera on...

She has hot legs.

She's pumping extra blood through them to radiate heat.

And now her actions have been caught on film for the first time.

No other bird on earth is known to do this.

As scientists continue to closely observe nesting behaviour,

they're discovering there's much more to incubation

than was previously thought.

In the woods of Oxfordshire, the tits are about to lay.

How exactly do they care for their eggs?

Like most birds about to start incubating,

this female great tit has shed the feathers

from a patch of skin on her abdomen.

It's known as a brood patch.

By controlling the flow of the blood in its naked skin

she can regulate the amount of heat she gives to her eggs.

The next stage of her incubation strategy is,

perhaps surprisingly, to stop doing so.

One egg a day - that's how most birds lay.

So this clutch has taken eight days to produce

from the first egg to the last.

The cool temperatures of the woods is not high enough

to start their development.

So while she's still producing eggs, she doesn't incubate them.

If she starts them off together, then they will hatch together.

And that will enable her to care for her chicks as a group

when they're all at their most vulnerable.

It's now that her behaviour becomes more complex

than researchers first thought.

Scientists working in these woods have recently made a discovery -

great tits are controlling the speed at which their eggs develop

in response to the weather.

As we all know, there's nothing less reliable than a British spring.

Some days are cold and grey.

That is when we might expect that her eggs need to be kept warm.

Other days, it might be sunny.

Perhaps a chance for the birds to spend less time on the nest.

But new research is revealing

that exactly the opposite is what happens.

On warm days, perhaps surprisingly, parents incubate for longer

than they do on cold days.

So when it's warm, development of the egg speeds up,

when it's cold, it slows down.

The research into why the birds are, apparently, being so contrary,

is being led by Dr Ella Cole from the University of Oxford.

We've recently discovered that by varying the amount of time

they actually spend incubating eggs each day,

they can manipulate their hatching date,

and they do this so they can find enough food for their chicks.

This is the caterpillar of a winter moth,

and each chick will need to eat about a thousand of these

in the first two weeks of its life.

That means, in turn, that the parents must time

the incubation of the eggs so that when the eggs hatch

there will be a glut of these caterpillars around.

That caterpillar peak lasts just two short weeks.

And warmer weather starts it earlier.

So by taking their cues from the weather,

the birds ensure that their eggs will hatch at exactly the same time

as their food appears.

It's quite remarkable that the tits are actually able

to do this fine-scale adjusting

even in the late stages of incubation.

Laying an egg enables the birds to do something a mammal can't do.

The parents, in fact, have some control

over when their eggs will hatch.

Now let's see what happens inside the egg when incubation begins.

The hard shell certainly provides excellent protection.

But the embryo within must be connected to the outside world

so that it can breathe.

Minute pores lead from the surface to the embryo's blood supply.

This chicken egg has 10,000 of them. HEARTBEAT

And they enable the developing embryo to take in oxygen

and expel carbon dioxide.

But a porous egg is inevitably a vulnerable one.

What else might be able to get in?

Well, some birds lay their eggs in rather strange places.

Here in Dorset,

hundreds of mute swans gather each year to breed.

For the last few weeks, pairs have built nests in reed beds

on the edge of a tidal lagoon.

Now, they're just beginning to incubate.

One threat to the eggs is hard to avoid.

Most nests are likely to be flooded at least once during the season,

so it's important that the eggs should be waterproof.

so it's important that the eggs should be waterproof.

And indeed a swan's egg has an outer layer

that waterproofs it without suffocating the chick.

But water isn't the biggest danger.

It's what's carried in the water.

Eggs can be infected by bacteria, and bacteria can travel in water

and so get in through the pores.

Adult birds have an immune system that can fight off microbes

that might invade their nest,

but the developing embryos don't.

It's a battle of bug versus bird.

Microbes might get inside the egg through its pores

and consume the developing embryo within.

But the eggs have a special protection.

It's know as S-A-M, SAM.

The letters stand for Shell Accessory Material

and it's a microscopic protective layer that all eggs have,

whether or not they are laid near water.

This is the egg's first line of defence.

And this is the second.

This colourless substance is

one of nature's most remarkable and mysterious materials.

It's the albumen, that acts as a barrier,

both biological and physical.

To a microbe, travelling through the albumen to the yolk is like

a human trying to walk across a desert -

there's nothing to sustain life.

But albumen contains lots of other things.

Over 100 antimicrobial proteins have been identified in it so far

and it seems likely that many more remain to be discovered.

It might be hard for us to grasp that the white we see

in our chicken eggs at breakfast is such a miraculous defence system.

But it's the egg's way of defending itself against microbes

that would, given half a chance, consume the developing embryo.

Protected by albumen, and nourished by the yolk,

the embryo continues to grow.

As it does so, it generates water.

We do this too when we eat,

and we get rid of at least some of such water

as vapour when we breathe.

The chick does something similar

and water vapour diffuses through the pores in the shell.

The loss of this water creates a space at the blunt end of the egg.

As the embryo develops, so the air space increases,

and the oxygen it contains will help to give

the chick the energy it needs to help it crack the shell

when it starts to hatch.

The shells of eggs can be extraordinarily beautiful.

Some are almost jewel-like.

But colour isn't mere decoration.

It can play a crucial part in the egg's survival system.

Sometimes, it serves as camouflage.

Sometimes, it prevents overheating.

And, remarkably, colour can also act as a defence against a murderer.

A cuckoo...

..in the Fens of East Anglia.

The cuckoo never builds a nest or cares for its young.

Instead, it tricks other species into accepting its egg

and then raising its baby instead of their own.

A female cuckoo takes an egg from a reed warbler nest.

Within seconds, she's laid her own egg in its place.

It's slightly larger,

but its colour exactly matches that of the reed warbler's.

Professor Nick Davies from the University of Cambridge is

the world's leading cuckoo expert.

And he knows it isn't easy being a killer and a thief.

It's actually a crazy thing to do,

it's such hard work looking for host nests.

I think if I was a bird, I'd just be an honest worker

and raise my own young.

Nick tests how important colour is by placing a wrongly coloured egg

into the reed warbler's nest and seeing how the reed warbler reacts.

The reed warbler immediately senses something isn't quite right.

She starts to destroy the new egg.

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

very different from 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.

So the cuckoo's egg has to match the reed warbler's eggs in colour

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

So these very simple experiments show that this egg mimicry

by the cuckoo is a crucial part of their trickery.

There are several races of cuckoos in Britain,

each with a distinctive egg

that matches the colour of its particular host's eggs.

But new research has shown that some cuckoos' forgery skills

are increasingly being put to the test.

Thousands of miles from the Fens,

across the Atlantic, Princeton University.

Here, one of Nick's former colleagues,

Dr Mary Caswell Stoddard, is continuing cuckoo egg research.

To really understand what's going on,

Cassie and her team created a computer program

that analyses colours and patterns on an egg

in the same way as a bird might see them.

It's very important to take a bird's-eye view

when asking a question about egg mimicry,

and that's because birds have very different vision than humans do.

Birds are seeing a much more richly coloured world

than we humans are.

Cassie's computer program has revealed that the duel

between a cuckoo and its victims is much more sophisticated

than anyone previously thought.

We were astonished to find that some hosts have evolved

highly recognisable pattern signatures on their eggs

in response to cuckoo mimicry.

So it's not just that cuckoos have evolved the ability to match

the colour and pattern of host eggs,

hosts are also fighting back against cuckoo mimicry

by evolving patterns they can easily recognise on their own eggs.

Some hosts are evolving ever more complex patterns on their own eggs

to make mimicking them much harder.

to make mimicking them much harder.

This is similar to the way in which a bank might insert watermarks

on their dollar bills to make life more challenging for counterfeiters.

So, egg patterns, it seems, can change.

And as new technology reveals ever more ways

in which we can understand how birds see their eggs,

who knows what more will be discovered?

And the same is true of the one important characteristic of the egg

that we have not yet examined...

..its shape.

Describe something as being egg-shaped

and it's usually a chicken's egg that we have in mind.

But eggs are extremely varied both in size, shape and colour.

Each has doubtless been developed for a particular reason.

Although it's not always clear what that is.

And there's one egg with a shape so extreme

that it has long been one of ornithologists' great mysteries.

It's produced by a bird called the guillemot.

And if we want to solve the puzzle of this curiously conical egg,

then here is a good place to start.

The Welsh island of Skomer.

It's home to one of the largest guillemot colonies in Britain.

Professor Tim Birkhead is a leading ornithologist

who has been studying the birds that breed here for over four decades.

And in the last few years, he's turned his attention to their eggs.

Guillemots lay these fabulous eggs.

The guillemot egg shape has, for years, been recognised

as the most extreme.

It's more pointed, more extreme than any other species

and that's been a puzzle for a long time.

Many people have put forward theories to explain it,

and one egg collector came up with a suggestion

that was beguilingly simple.

It's based on the fact

that guillemots don't lay their eggs in nests,

but instead balance them precariously on cliff ledges.

This is a fake guillemot egg.

When it's spun, it rotates like a top.

This, the egg collector's theory went, ensures

that if an egg is accidentally knocked

it will spin on its axis rather than roll off a narrow cliff ledge.

The egg, he said, would rotate in the breeze,

you know, which is just ludicrous.

You know, the egg is so heavy it couldn't possibly rotate.

What he was doing was using a museum egg which was empty,

but, you know, a real guillemot egg is full of yolk or an embryo.

It can't do that.

If you watch guillemots,

not that many eggs actually roll off the ledges

so we started to think about what other possible explanations

there could be for why the eggs are this shape.

To investigate the mystery of the guillemot egg shape,

Tim had to get his hands dirty and investigate

another aspect of the guillemot's breeding environment.

So this is a rather typical guillemot colony -

almost sheer cliffs, birds breeding on tiny ledges,

often, though, at incredible densities,

up to 20 pairs per square metre.

Such crowding, Tim noticed, meant that the birds were

inevitably covering their eggs, and those of their neighbours,

in something that may look like dirt but is actually rather nastier.

Because guillemots don't make any nests,

their ledges become covered in what is technically known as faeces.

And when it's wet, this turns into an absolute pig farm,

I mean, you can see my hands are covered now.

And, you know, the birds are incubating,

alternating wet and dry weather,

so that muck is continually being heated up, cooled down

and, you can see here, covering the egg.

Now, there are, as we've seen, at least two defences

that should prevent microbes in the muck

from infecting the developing chick -

first the shell and then the albumen.

But could nature also have evolved an unusually shaped egg

as an additional protection?

I kind of started thinking about this

and noticing that it's this pointed end of the egg

that becomes the most covered in this muck

because that's the one that's lying, like that, on the ledge in the muck.

That made me think that maybe the pointed shape is actually

an adaptation for coping with that filth on the ledge.

Because, as I say, when the egg is sitting there,

it's this big end here, you can see, is relatively free,

and that's where the airspace is

and that's where the chick is going to be breathing from.

So I think this shape of the egg may be related to the fact

that these birds breed in such an unusually dirty environment.

Who would have thought that the most likely explanation

of the guillemot's pointed egg was its droppings?

But that's what Tim's research has shown,

that this shape helps the blunt end, where the chick's head is,

to remain relatively free of filth

so that it's easier for the chick inside to breathe.

But could there be other factors at work as well?

Back in the Princeton University labs,

Cassie Stoddard is also pondering on the mysteries of egg shape.

But not just that of the guillemot's egg.

She is exploring how egg shape has evolved among birds worldwide.

We analysed the shapes of almost 50,000 eggs from digital images.

And these eggs represented 1,400 species,

about 14% of all birds.

Eggs can be round or conical, or anything in-between.

And Cassie was intrigued to discover that there is a link

between egg shape and a bird's flying ability.

The stronger a bird's flight,

the more elliptical and pointed its egg will be.

But why?

Well, our best guess at the moment is that

in order for a bird to maintain a streamlined body plan

it can't lay an egg that's too wide across

cos this would disrupt the streamlined nature

of a flying bird's body.

And so one solution to this potential problem may have been

to lay an egg that's more pointy, more elliptical,

cos then a bird can still pack a large volume into an egg

without it disrupting the birds' body plan.

The bigger an egg, the more nutrients it can hold for the chick.

But if the birds are to fly efficiently,

their bodies can't accommodate a wide, bulky egg.

A long, pointed egg reduces that problem.

Case closed, you might say, on the mystery of the avian egg shape.

But you'd be wrong.

Well, we're certainly not closing the book on egg shape.

There is still so much to discover.

There's much more to be done to really understand

why birds lay eggs that come in such a variety of shape.

For Tim, too, the explanation of egg shape is far from complete.

He thinks that future research will show

that the demands of incubation are also major influences.

Although people like the idea of a single factor

explaining a phenomenon, it might be that several different factors

might all work together to help to shape the evolution of eggs.

As any good scientist knows,

what you understand is going on in the world

is what we call the truth for now

because, probably, somebody will come along later

with some new evidence, we'll find a different kind of truth.

An egg, whatever its shape, is an excellent life-support system.

But paradoxically, its success will ultimately depend

on the ease with which it can be broken.

The time comes when a chick must break free.

Some species invested time building up large yolks.

Their chicks will emerge fully feathered

and ready to search for food.

Others have not made that investment.

They will have to spend their energies

over the next few weeks feeding naked and defenceless chicks.

But how do the chicks break out from the cramped confines of the egg?

How can the shell that's been strong enough to protect the chick

from the outside world be also weak enough

to allow the chick to break it?

The first breath of fresh air outside the egg.

A captive-bred jungle fowl chick emerges.

It's the climax of the egg's existence.

CHICK TWEETS

The shell may look the same as when the egg was laid,

but out of sight it's been changing.

It's been getting thinner.

The chick has been absorbing calcium from the shell

into its own bones, making itself stronger and the shell weaker.

Not only that, but it also used the shell's calcium to create

a tool to help it break free.

A hard, jagged tip on the end of its beak, an egg tooth.

The chick couldn't have broken free without it.

Even so, it can still take hours, sometimes days,

to hammer its way out of a shell.

This egg and this newly hatched little chick are part of a clutch

that was laid on the ground between 21 and 26 days ago

and they are just now hatching.

This one is only about half an hour old.

And this one is just beginning to peck its way out.

And as they do, they communicate with one another.

And the sound of this little chick encourages that unhatched chick

to break its way out of the egg

so that within an hour or so the whole clutch is hatched

and then they can run away as a little group and find safety.

But what of the other woodland birds I was watching?

The tits are breaking out of their shells

into the British spring.

They are naked, blind and hungry.

But outside, the woods are filled with food.

And the parents' careful timing has paid off.

They took account of the weather and changed their behaviour and won.

Each element of the egg combined to create new life -

from the nutritious yolk

to the defensive albumen

to the protective shell.

Nature's most perfect life-support system has served its purpose,

broken by the life that it sustained.

Every new arrival is a confirmation of the complex efficiency

of a seemingly simple egg.

As with all forms of life,

what we see are the success stories -

the adaptations that work.

So it's little wonder that we think of eggs as being perfect.

But of the 10,000 different species of birds

that exist in the world today,

there are still hundreds whose eggs have never even been described.

When it comes to the most perfect thing in the universe,

there's still much magic and mystery to explore.

Collecting birds' eggs, of course, is now illegal,

but it wasn't when I was a boy, 80 years ago,

and I too had a collection of birds' eggs.

And I blew them simply by taking a pin and making a hole in one end

and then one in the other and then blowing it.

But professional egg collectors were more thorough and neat about it.

They started with a little needle

which they created a small hole like that

and then they used one of these and enlarged the hole.

And then you take a little glass pipette like this...

And there it is, the egg is blown.

And this is what the collectors collected.

The empty, dead, lifeless outer cover of an egg.

By discarding the most essential part of an egg,

collectors were able to preserve their fragile shells.

Here, at the Natural History Museum in Tring,

there are more than a million eggs from historic collections.

It's quite right that amateur egg collecting has been made illegal.

But that doesn't mean that we shouldn't ask questions

of egg collections that are already in museums.

And some of the answers they provide are very surprising.

It was by studying museum collections

that scientists discovered exactly why some populations of birds,

like the song thrush,

had been in decline ever since the Industrial Revolution.

They found that the eggshells had been getting thinner.

The birds had been unable to get the calcium they needed.

And that was because they normally got that calcium from snail shells,

and acid rain had killed the snails.

Again, in the 1960s, historic eggs exposed

another calcium-related crisis,

this time when populations of top predators

like raptors, owls and herons suddenly crashed.

It was measurements made on eggshells in museum collections

that showed only too clearly

that the reduction in thickness of the eggshell coincided exactly

with the introduction of DDT.

Half a millimetre of calcium,

that's the difference between life and death.

So what more can we learn from eggs?

Well, deep in the basement of the Natural History Museum in London,

one of the most important eggs in the world is going to be

gently interrogated to see if it will surrender any of its secrets.

This is the oldest egg in the museum's collection,

and it's about to be placed within one of the most advanced

and latest pieces of technical equipment here.

What makes this egg so special is that it was laid by a great auk,

a giant sea bird that became extinct in the 19th century.

There are only 70 of these eggs left in the world.

This is a 3D X-ray microscope

and it will scan the entire structure of the egg.

Whoever it was who collected this egg back in the 1700s

could never have imagined something like this.

The research is being led by Professor Birkhead.

It's a unique opportunity for us

to use this fantastic technology here to explore the egg's structure

so we can infer something about the biology of the great auk

that we don't know. It's very exciting.

We know the great auk was a relative of the guillemot.

But what no-one knows is whether they bred in a similar way -

packed tightly together in faecal squalor

like the birds we saw on Skomer.

The guillemot's egg when it's resting on its cliff ledge,

which is where it's incubated,

the blunt end is raised up because of the shape of the egg.

It's raised up out of the muck that guillemots typically breed in.

And we think that the additional pores there is an adaptation

to allow the embryo to breathe under these rather dirty conditions.

The egg is a fragile link back to a vanished past.

As you can see it's very beautiful, the patterning is just exquisite.

The shape of the egg is similar to that of the guillemot's.

But what about the pores?

And now we can take a slice through the shell.

So this is now a cross-section.

You can see a pore here running from the outside surface of the egg...

The results are a complete surprise.

Interestingly, it turns out that great auk eggs

are much more like the eggs of a razorbill

than they are of a guillemot, so strikingly different.

The razorbill is another sea bird relative of the great auk,

but it breeds in clean nests that are spaced out.

Their eggs don't need a high density of pores at the blunt end,

unlike the guillemot egg that's adapted to dirty conditions.

Razorbills don't have that problem,

because they breed in very tidy nests,

they defecate outside the nest,

whereas guillemots breed in very messy circumstances.

Amazing, amazing.

So while the egg's shape suggests that the auk bred like a guillemot,

its pore structure reveals that, in fact,

it's likely to have bred like a razorbill.

One ancient egg and one modern machine have revealed

a tantalising glimpse into the life of an extinct bird.

And as more technological innovations come along,

who knows what other mysteries egg collections of the past

could help us solve?