♪♪ ATTENBOROUGH: 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 thumping ] It is 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 program, we are 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.

[ Theme music playing ] ♪♪ ♪♪ [ Cracking, sloshing ] ATTENBOROUGH: This is the egg many of us see each morning at breakfast.

[ Crowing ] 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.

♪♪ [ Rooster crowing ] ♪♪ The very familiarity we have with this 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 color and shape that are produced by the ten thousand different species of birds that are alive in the world today.

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

Every animal species on Earth produces an egg of some kind, but bird eggs exceed all others in their variety and in their success in every climate and corner of the world.

From the smallest -- the bee hummingbird egg -- to the biggest -- the ostrich egg, that's 40,000 times heavier... birds create eggs in an astonishing array of size and shape, color 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 howling ] Antarctica -- one of the most extreme environments on Earth.

[ Penguins squawking ] 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 birds that once flew, and keeping weight to a minimum for anything that flies is of the utmost importance.

But there is also another reason...

Birds are hotter than mammals.

The internal temperature of all adults is 104 degrees.

No embryo can develop at such high heat.

But since they lay eggs, they can incubate their embryos at a 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.

♪♪ 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 are both extraordinarily skillful 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 is 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, the 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 is 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 does happen 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.

[ Birds calling ] But each dawn mating is not sparking an egg to life.

In humans, fertilization 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.

[ Waves crashing ] 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 fertilized egg.

In the past, seabird 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 it can be transformed into a chick.

And only when she has done that, will the egg be fertilized.

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

The extra sperm probably releases substances that start the embryo's development.

♪♪ Minutes after fertilization, the egg starts its 24-hour journey down the oviduct.

♪♪ First, it is 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 into the uterus, where it will be given its protective armor -- a shell.

♪♪ The shell is actually quite separate from what it contains.

To help understand this we can do a simple experiment.

♪♪ These are unfertilized 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 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 to the uterus 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 fertilized and enclosed within 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.

[ Crowing ] 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 remains so, because no one since Wickmann has been bold enough to use a pencil to find out.

[ Crowing ] ♪♪ 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 two percent 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.

♪♪ Birds' eggs have conquered every continent and climate.

[ Penguins squawking ] Their range is far beyond that of all other egg layers like crocodiles and turtles.

So how have birds become so efficient when compared to reptiles?

How are they able to lay eggs in places where other egg layers can't?

Well, because unlike them, birds' hot bodies are able to provide warmth for their eggs.

The element crucial to their success is heat.

♪♪ A Welsh churchyard viewed by a thermal camera.

♪♪ It might look like night, but actually its 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're huge.

Each one weighs 16 percent of her bodyweight.

In human terms, that would be like giving birth to eight whopping 18 pound babies.

But her tiny body can only cover two or three of her large eggs at any one time.

Bird embryos develop at around 98.6 degrees.

So, how will she keep them all of them warm?

Well, the secret of 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 behavior, 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 the naked skin, here, 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 is still producing eggs, she doesn't incubate them.

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

And that will enable her to care for her chicks as a group when they are all at their most vulnerable.

♪♪ It's now that her behavior 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 gray.

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.

♪♪ COLE: We've recently discovered that by varying the amount of time they actually spend incubating the eggs each day, they can actually manipulate their hatching date and they do this so they can find enough food for their chicks.

ATTENBOROUGH: 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.

COLE: It's quite remarkable that the tits are actually able to do this fine-scale adjusting even in the sort of late stages of incubation.

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

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.

The chicken egg has 10,000 of them, 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 are just beginning to incubate.

One threat to their 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.

And indeed, a swan's egg has a 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 special protection It is known 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 defense.

♪♪ And this is the second.

This colorless substance is one of nature's most remarkable and mysterious materials.

It's the albumen that acts as 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 defense system.

♪♪ But it's the egg's way if 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 vapor when we breathe.

The chick does something similar, and water vapor 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 color 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, color can also act as a defense against a murderer.

[ Bird calling ] 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's nest.

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

It's slightly larger, but its color 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.

DAVIES: 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.

ATTENBOROUGH: Nick tests how important color is by placing a wrongly colored egg into the reed warbler's nest and seeing how the reed warbler reacts.

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

She starts to destroy the new egg.

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

So this very simple experiment shows that this egg mimicry by the cuckoo is a crucial part of their trickery.

ATTENBOROUGH: There are several races of cuckoos in Britain, each with a distinctive egg that matches the color of its particular host's eggs.

But new research has shown that some cuckoo's forgery skills are increasingly being put to the test.

♪♪ Thousands of miles from the fens, on this side of the Atlantic, is 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 analyzes colors and patterns on an egg in the same way as a bird might see them.

♪♪ STODDARD: 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 colored world than we humans are.

ATTENBOROUGH: Cassie's computer program has revealed that the duel between the cuckoo and its victims is much more sophisticated than anyone previously thought.

STODDARD: We were astonished to find that some hosts have evolved highly recognizable pattern signatures on their eggs in response to cuckoo mimicry.

So it's not just that cuckoos have evolved the ability to match the color and pattern of host eggs, hosts are also fighting back against cuckoo mimicry by evolving patterns they can easily recognize on their own eggs.

ATTENBOROUGH: Some hosts are evolving ever more complex patterns on their own eggs to make mimicking them much harder.

STODDARD: This is similar to the way in which a bank might insert watermarks on their dollar bills to make life more challenging for counterfeiters.

♪♪ ATTENBOROUGH: 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 an egg that we have not yet examined... Its shape.

Describe something as being egg-shaped and it is usually a chicken's egg that we have in mind.

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

Each has doubtless been developed for a particular reason... although it's not always clear what that is.

And there is one egg with a shape so extreme that it has long been one of ornithology's great mysteries.

♪♪ It is 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.

Its 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.

BIRKHEAD: Guillemots lay these fabulous eggs.

The guillemot egg shape has for years been recognized as the most extreme -- it's more pointed, more extreme than any other species, and that's been a puzzle for a long time.

ATTENBOROUGH: 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.

BIRKHEAD: 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 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 it could be for why the eggs were this shape.

[ Waves crashing ] ATTENBOROUGH: 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.

♪♪ BIRKHEAD: 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 meter.

ATTENBOROUGH: Such crowding, Tim noticed, meant the birds were inevitably covering their eggs, and those of their neighbors, in something that may look like dirt but is actually rather nastier.

BIRKHEAD: Because guillemots don't make any nests, their ledges become covered in what's technically known as feces.

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... ATTENBOROUGH: Now, there are, as we've seen, at least two defenses 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.

BIRKHEAD: 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 on the ledge in this 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.

ATTENBOROUGH: Who would have thought that the most likely explanation for 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 is 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 egg.

She is exploring how egg shape has evolved among birds world-wide.

♪♪ STODDARD: We analyzed the shapes of almost 50,000 eggs from digital images... ♪♪ And these eggs represented 1,400 species, about 14 percent of all birds.

ATTENBOROUGH: 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?

STODDARD: 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 because 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, because then a bird can still pack a large volume into an egg without it disrupting the birds' body plan.

ATTENBOROUGH: The bigger an egg, the more nutriments it can hold for the chick.

But if the bird 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.

STODDARD: 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.

ATTENBOROUGH: 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.

BIRKHEAD: Although people like the idea of a single factor explaining a phenomenon, it might be that several different factors might all have worked together to have helped 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, will find a different kind of truth.

♪♪ ♪♪ ATTENBOROUGH: 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 the chick the 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 defenseless 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.

♪♪ 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 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.

[ Chicks chirping ] 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 behavior -- 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 ten thousand 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.

♪♪ The ostrich, the emu, and the rhea, together with the kiwi and the cassowary -- a family with a remarkable success story -- despite having never flown a day in their lives.

They're lineage can be traced back to the time when dinosaurs walked the Earth.

It's one of the natural world's great mysteries: why did these birds abandon flight?

♪♪ ♪♪ ♪♪ ♪♪ ♪♪ ♪♪ To learn more about what you've seen on this "Nature" program, visit pbs.org.

♪♪ ♪♪