♪ ♪ NARRATOR: Birds live across our entire planet.

Around 50 billion of them fill our skies, seas, and landscapes.

Each of the 11,000 species expertly adapted for the environments they inhabit.

But how did they come to be such an evolutionary triumph?

The answer lies with their ancestors.

Today's birds are dinosaurs.

They are every bit as much of a dinosaur as a T. rex is or a Brontosaurus is.

NARRATOR: But exactly how this spectacular evolution took place, is still somewhat a mystery.

Right now, we have very little data.

I mean, that's how it is with paleontology.

You have hundreds of millions of years and just a few fossils scattered around the world.

NARRATOR: Now, a new breed of scientist is making spectacular discoveries... We scanned it.

And then what we found was... Whoa.

...a lot of skeleton underneath the rock.

Holy cow!

NARRATOR: ...to fill in the gaps of this evolutionary tale... We thought it's just going to be a simple thing.

They're like dinosaurs or they're like birds.

And what we found is that, they are like neither.

NARRATOR: ...and answer one of paleontology's biggest questions: why were birds the only dinosaurs to survive the asteroid impact 66 million years ago?

FIELD: The development of new technologies has provided us with a richer picture of avian evolution than has ever previously been possible.

NARRATOR: "Dino Birds"-- right now, on "NOVA."

(animal screeching) ♪ ♪ (birds chirping) ♪ ♪ NARRATOR: Birds are full of surprises.

They lay eggs like reptiles, but have warm blood like mammals.

Only birds have feathers and it's one of their most distinctive characteristics.

NARRATOR: There are 11,000 species across the globe, dazzling in their variety-- size, shape and behavior.

We think we know them well, but beneath those colorful feathers lurks an enduring mystery.

Why are birds here?

Scientists have discovered that they are the sole surviving branch of the dinosaurs.

When an asteroid wiped out nearly all animal life 66 million years ago, birds somehow survived while all other dinosaurs disappeared.

Why?

Was it pure chance?

Or did they possess some special advantage that set them apart?

♪ ♪ Now, new discoveries in paleontology and evolutionary biology might finally be providing answers.

♪ ♪ FIELD: I think this is going to be a great place to see some birds GUILLERMO FERNANDEZ: Oh, I agree.

NARRATOR: Historically, it's fossils that have revealed the past to us.

But in the case of birds, the record is patchy.

Their fossil remains are notoriously rare and fragile.

(bird screeches, wings flapping) Hey, over there?

A Eurasian griffon vulture.

(bird screeches) FIELD: So you've got... two griffin vultures, on top and then one below.

Oh, I can see it.

Yeah.

FIELD: I think they're warming up before they take off.

Do you see it stretching its wings?

FERNANDEZ: I think so, yeah!

I think that vulture is about ready to go.

(bird screeches) Oh, one just took off!

FERNANDEZ: Oh wow, yeah.

One just took off!

Yeah!

FERNANDEZ: He's flying.

FIELD: Oh, Wow!

FERNANDEZ: Beautiful.

FIELD: That is a huge bird!

FERNANDEZ: Beautiful.

FIELD: Oh my goodness!

♪ ♪ This is such an amazing place to come bird watching.

But these birds are never going to turn into fossils, right?

It's not the sort of environment that is conducive to fossil formation.

♪ ♪ The bird fossil record is particularly sparse, and that's because birds are, by their very nature, fragile animals.

♪ ♪ And the very lightweight nature of bird skeletons, which tend to be filled with air to make them even lighter as a specialization for flight, and those are features that tend to conspire against their preservational potential in the fossil record.

NARRATOR: This left the story of bird evolution virtually unwritten for decades.

Barely a clue to tell us where today's four main groups of birds all came from.

But every now and then fossils do appear, and when that happens, a new chapter of the story is unlocked.

♪ ♪ One of the greatest of these fossil finds lies in the heart of London, in the treasure room of the Natural History Museum.

FIELD (voiceover): This is Archaeopteryx, one of the most amazing fossils in the world.

NARRATOR: Found just two years after Charles Darwin published his ideas of natural selection, Archaeopteryx was one of the many great 19th century dinosaur discoveries.

Dinosaurs took over the Earth some 220 million years ago, evolving from reptilian relatives.

They inherited from reptiles the defining features we associate with dinosaurs today-- big claws, sharp teeth-- but with one major difference.

Dinosaurs, whether they walk on four legs or two, have an upright stance-- their legs below their bodies.

In reptiles however, the legs extend horizontally from their bodies.

Over the 140 million years that dinosaurs ruled the Earth, countless species came and went.

The approximately 150 million-year-old dino bird, Archaeopteryx, has turned out to be one of the most important in the story of bird evolution.

With the discovery of Archaeopteryx and some of the first dinosaurs, even then, the earliest paleontologist looked at these things and are like birds are dinosaurs.

I mean, the skeletal similarity is enormous.

Archaeopteryx provides us with an amazing glimpse at a combination of features that we expect to see in living birds in the present day-- like large feathered wings-- as well as the kinds of features that we associate with dinosaurs: like a long, bony tail, jaws full of sharp, pointy teeth and strong claws.

NARRATOR: It bore an uncanny resemblance to a known dinosaur fossil.

♪ ♪ Hello, Dan!

Good morning Olivia!

Good to see you!

Should we go look at some fossils?

Yeah, sure!

Great, let's take a look!

♪ ♪ This is the skeleton of Compsognathus, which was a really important theropod dinosaur into the 19th century, because it helped convince Thomas Henry Huxley that there might be an evolutionary link between birds, like Archaeopteryx, and theropod dinosaurs.

♪ ♪ NARRATOR: Compsognathus and Archaeopteryx have some surprising similarities.

They would both have walked on two legs, had long arms, long bony tails, and teeth.

♪ ♪ Features that convinced Huxley that birds were likely descended from dinosaurs, a controversial theory in the 19th century.

This idea was rejected for 100 years, uh, primarily because dinosaurs did not preserve; or no known dinosaur at the time had a fossilized furcula-- that's the wishbone.

But then, you know, in the '70s the first dinosaur wishbone was found.

♪ ♪ NARRATOR: The discovery of dinosaurs with wishbones-- a feature previously only found in birds-- sealed the deal.

And we now recognize without a doubt that Compsognathus, Archaeopteryx, and modern bird all belong to theropod dinosaur, the group of three-toed, meat-eating dinosaurs like T. rex.

NARRATOR: Despite this evolutionary link, there are still mysteries.

Creatures with early feathers that don't look like modern birds, and archaic birds-- ancient species, whose evolutionary lines are now extinct-- that have bird-like beaks, but filled with teeth.

But when was the moment one evolved into the other when modern birds first appeared?

How did dinosaurs take flight?

And why did some birds survive when all other dinosaurs died out?

♪ ♪ Today, new discoveries are revealing important new clues.

FIELD: It's an amazing time to be studying bird evolution.

NARRATOR: On the border of the Netherlands and Belgium, quarry excavations have revealed the world as it was 67 million years ago, not long before an asteroid killed off 75% of animal species on Earth.

Welcome to the very last days of the dinosaur era.

This is it.

JAGT: So what we see here was basically what was deposited in a shallow sea, quite warm, during the last days of the era of dinosaurs.

We can really compare this with the Caribbean or the Indonesian Archipelago these days.

FIELD: Sounds good.

JAGT: Yeah.

FIELD: It's a good place for vacation.

NARRATOR: These shallow seas provide the perfect conditions for fossilization to occur; a wet environment where sand and mud cover remains quickly.

Waters filled with hungry mosasaurs whose fossils would come to light, millions of years later.

FIELD: So this begins somewhere in the range of about 67 million years?

JAGT: 67 and a half, something like that, and then going up.

NARRATOR: These layers of rock protected one of the most incredible bird fossils ever found; a chance discovery made by an amateur fossil hunter in the year 2000.

This is the level that produced both bird fossils.

Right.

I think it's time to unwrap the birds.

All right.

And here it is.

This is the only known specimen of Asteriornis maastrichtensis.

I still remember what my first impression of this fossil was like, when I looked at it, uh... (chuckles) and it wasn't a very positive impression.

NARRATOR: It wasn't until 18 years after its discovery, that Daniel Field began studying the specimen.

FIELD: This fossil clearly preserves a few bird bones.

And those bird bones are not well preserved.

You can see there's a broken thigh bone on top and there's a broken shinbone underneath.

But these are clearly the bones of birds.

And so even though the fossil didn't look very... beautiful at first, we thought maybe it would tell us something about what birds were like just before the asteroid wiped out the giant bird cousins, the large dinosaurs.

NARRATOR: Using a CT scanner, Daniel and his team were able to digitally remove the rock, and reveal what secrets it held within.

What he saw exceeded his wildest dreams.

♪ ♪ FIELD: When we removed the low-density rock and saw this skull staring back at us, my PhD student Juan and I could not believe our eyes.

It was a shock.

It was really exciting.

And we realized immediately that this fossil was going to be very important because it could provide us with information that no other fossil ever discovered could.

It was very exciting and, and one of the most exciting moments of my scientific career.

NARRATOR: This specimen provides a portrait of a bird that lived 66.7 million years ago.

This fossil is extraordinary because it shows several features that tell us that it is a member of the modern bird group.

And the most obvious of those features is the fact that it does not have any teeth.

NARRATOR: The remains of ancient archaic birds have beaks filled with teeth.

Losing them was a key stage in the evolution of dinosaurs into modern birds.

And the fact that this fossil has a beak very similar, uh, similar to living generalist birds suggests that it probably would have been happy eating almost anything it could find.

NARRATOR: Just like birds with similar beak shapes do today.

FIELD: In addition to that, if you look at the leg bones of this fossil, they're relatively narrow and relatively long, and those are features that we associate with predominantly living on the ground in the present day.

NARRATOR: Around seven or eight inches tall, with the beak of a chicken and the head of a duck, this creature is one of the oldest modern birds ever to be unearthed.

Scientists named the 67 million- year-old species Asteriornis after Asteria, the Greek Titan goddess of falling stars.

It confirms that modern birds existed alongside the dinosaurs before their extinction.

This small bird bears witness to a world prowled by T. rex and Triceratops.

Neither the largest nor the most dominant, its omnivorous feeding habits mean that it's satisfied with small shellfish and seeds.

It might also scavenge leftovers; perhaps even a washed-up Mosasaur carcass.

(birds screeching) With its toothless beak and variety of food options, Asteriornis is well-equipped to survive in the realm of the dinosaurs.

But with such evolved features, it's unlikely that it was the first modern bird.

♪ ♪ At Rockefeller University in New York, Erich Jarvis and his team are taking a different tactic to find the beginning of today's birds, using modern DNA to map their evolutionary past.

JARVIS: The way we dated the origin of modern birds is we took the genome scale tree, which is the alignment of DNA from one species to another.

NARRATOR: This reveals the genetic code for certain traits in modern birds that they can then look for in ancient fossils.

Then we took fossil data that's been collected from all around the world and placed those fossil dates on the tree for like 20 species.

NARRATOR: This tree allows Erich to see exactly where different species of living bird genetically diverged.

But what the team really wants to find out is when these divergences took place.

What does DNA suggest about when modern birds first appeared?

JARVIS: You look at mutations that occurred over time.

And from those mutations, you can get what's called the molecular clock.

How much time do you need to have one mutations, two, three, four, five?

NARRATOR: By estimating this rate of mutation, Erich wound back the clock and put a timestamp on when certain characteristics changed.

JARVIS: So from the combination of this molecular clock, the tree structure, and the fossil evidence, all three of those combined, we can date when modern birds originally formed.

And that was 90 million years ago.

NARRATOR: Fossils confirm that modern birds and their giant dinosaur cousins co-existed, and DNA indicates this wasn't a short-term arrangement.

They lived alongside one another for some 25 million years, perhaps even longer.

And new research is showing just how similar the lives of ancient birds would have been to birds today.

LIDA (translated): This is the most complete and best-preserved oviraptor embryo skeleton in history.

NARRATOR: This is the egg of a type of dinosaur that lived in Asia 70 million years ago.

(translated): They were arranged in a circle.

For some reason, perhaps because floodwater covered the entire nest in sand, they died quietly in this way.

STEVE BRUSATTE: And it looks like it was fossilized right before it would have hatched.

So in looking at this fossil, we are glimpsing the last moments of a dinosaur's development in the egg before it would greet the world.

What we're seeing with this fossil is that these dinosaurs would have developed in their eggs and hatched from their eggs just like birds do today.

♪ ♪ NARRATOR: It's the remains of embryonic dinosaurs and their shells that have helped scientists answer one of the great questions surrounding bird evolution.

Of all animals alive today, there are only two groups of animals that are warm-blooded: mammals and birds.

NARRATOR: It's a key feature of what makes flight possible.

You have more energy available that allows you to be more active, which allows you to move greater distances.

NARRATOR: Dinosaurs were first assumed to have cold blood.

O'CONNOR: And so we are very interested in knowing when this warm-bloodedness evolved in birds.

♪ ♪ NARRATOR: Scientists have been investigating the question for decades.

And recently, key evidence came from dinosaur eggshells.

That eggshell formed within the body of the mother dinosaur.

NARRATOR: Geochemist Robin Dawson analyzed minerals in the fossilized shells to reveal what temperature they formed at-- in other words, the mother's body temperature.

DAWSON: We looked at eggshell from the major clades of dinosaurs.

Major groups have body temperatures that are warmer than their environment, which suggests to us that they actually had the capability to have a high metabolism, raise their body temperature above their environment, like their warm-blooded relatives, like living birds.

O'CONNOR: For sure, dinosaurs were warm-blooded, but actually some dinosaurs, like Stegosaurs and duck-billed dinosaurs, actually secondarily evolved to be cold-blooded, which I think why we were having difficulty deciding if dinosaurs were warm-blooded or not, because, you know, there was a conflicting signal.

NARRATOR: With warm blood, dinosaurs would have a lot of energy to expend, raising interesting questions.

Like, would they have had enough energy for a high-intensity activity like flight?

It's the most physically demanding form of vertebrate locomotion.

So you have to be warm-blooded.

NARRATOR: But even if their metabolism supported flight, what about the equipment, like wings and feathers?

For 20 years this was the only snag in the dinosaur-bird evolution theory.

Where did feathers come from?

♪ ♪ In Liaoning, China, the subsoil of the Chaoyang Bird Fossil National Geopark reveals a dinosaur world that existed between 112 and 132 million years ago.

♪ ♪ Buried under volcanic ash, the fossils are spectacularly preserved.

The most famous are housed in a vast building.

A long, winding walkway hangs above the geological layers and the fossils they contain.

(man speaking Mandarin) MAN (translated): We've discovered many microraptors here, as well as birds like Confuciusornis.

NARRATOR: But it's not just the sheer number of fossils found here.

It's what they've revealed to the world.

(speaking Mandarin) (translated): You can see that the microraptor found here has feathers.

NARRATOR: Feathers-- a discovery that has revolutionized the world of paleontology.

Because a microraptor is not a dino-bird, but still a dinosaur-- a non-avian theropod.

In the '90s, with the discovery of feathered dinosaurs, I mean, it was one of the most compelling arguments for the "birds are dinosaurs" hypothesis.

BRUSATTE: The discoveries kept coming.

And now there are thousands of feathered dinosaur fossils that are known.

And it wasn't only small ones.

NARRATOR: More than 100 years after a link was first made, birds were finally confirmed as part of the dinosaur lineage.

And scientists were one step closer to understanding the origins of flight.

But if anything, this discovery raised as many questions as it answered.

Why did dinosaurs develop feathers in the first place?

What advantage did they provide?

♪ ♪ Paleontologist Xu Xing, from the Institute of Vertebrates and Paleontology in Beijing, discovered the fossil of a dinosaur with the answer-- Yutyrannus.

(Xu Xing speaking Mandarin) XU (translated): Its body is about the size of the famous T. Rex.

It was almost nine meters long and weighed almost 1.5 tons.

So we're talking about a giant.

♪ ♪ NARRATOR: Approximately 125 million years old, this is one of the oldest fossils that shows non-flying dinosaurs with feathers.

(translated): Here are Yutyrannus's feathers, You can see their length on these blocks of sediment.

We can see that Yutyrannus' body was covered in feathers, from head to tail.

♪ ♪ NARRATOR: But they don't look anything like bird feathers we see today.

They appear to be more like hair.

(Xu Xing speaking Mandarin) (translated): The most likely hypothesis is that there was a cold spell at the beginning of the Cretaceous period.

Yutyrannus is, in a way, the woolly mammoth of that time, with long, fine feathers that meant it could keep warm.

NARRATOR: Acting almost like down, it's been nicknamed by scientists "dinofuzz."

A stark contrast to the feathers of flying dinosaurs like Archaeopteryx and most modern-day birds.

But with a wealth of feathered dinosaur fossils found in just a few years, scientists have pieced together a timeline of feather evolution.

The earliest feathered dinosaurs, like Yutyrannus, had straight feathers.

In the next generation of dinosaurs, these long hairs separated, producing simple branches.

In many lineages, this down then evolved into more complex feathers with a central spine, barbs, and then barbules fitted with small hooks holding them together to form a sail, a more familiar symmetrical feather.

But only asymmetric feathers shaped more like an airplane wing are capable of creating pressure differences, and therefore lift, allowing for flapping flight over long distances.

We know the changes that took place, but the question remains-- why did they happen and when?

Fossils from Liaoning once again provide clues, suggesting that the ancient bird species Confuciusornis had another use for its feathers.

Confuciusornis is a really special bird.

It's represented by more than a thousand specimens.

And some of them are exquisitely preserved.

♪ ♪ NARRATOR: With so many fossils of this kind of bird, researchers around the world have come together to work on this project.

BAILLEUL (translated): We realized that there were two types of specimen within this species-- some with very long tail feathers and others with very short tail feathers.

So we asked ourselves, "Why?"

CHINSAMY-TURAN: So people postulated that one of the morphs was a male and the other one was a female, but it couldn't really be very sure about it.

NARRATOR: Paleontologist Anusuya Chinsamy- Turan looked inside the bone, taking a less-than-millimeter-thick slice to search for answers.

And one of the first specimens I sectioned actually happened to be one that had the most unusual bone tissue inside its marrow cavity, which we call medullary bone.

NARRATOR: That's the brown area on this slide.

And medullary bone is so interesting because you only find it in female birds.

And they form it at the time that they're ovulating and they use that medullary bone to calcify the egg shells.

So when I found this, I said, "Oh, my goodness."

I didn't know which specimen it had come from because I only had the little bits of bone.

And so I contacted my colleague and I said, "Can you double check the specimen number and see whether the specimen has long tails or no tail?"

And he came back to me and he said, "Actually, the specimen doesn't have any feathers."

So we could say for sure this was a female Confuciusornis.

NARRATOR: That meant the long tail feathers belonged to male Confuciusornis.

Not needed to aid their flight, the feathers likely served a different purpose.

In these marshy landscapes of northeast China 120 million years ago, it's mating season.

Confuciusornis females have a lot of options.

And the males have to redouble their efforts to attract them.

Competition is fierce as they use their spectacular tail feathers to attract the best mate.

Ornamentation in dinosaurs potentially led to feathers that eventually supported powered flight.

Then the hypothesis is that you already have these aerofoils, these surface areas.

And so even though their primary function was ornamentation, they must have had some incipient locomotor function.

Even though they can't fly, they still use their wings for their terrestrial locomotion.

And then, eventually, as these wings evolve to be bigger and bigger and have greater and greater aerodynamic benefit, they eventually reach a point where they can be used for powered flight.

NARRATOR: Today, bird flight is spectacularly complex.

They soar with ease.

But the transition from a ground-based life to one in the skies would not have been an easy one.

It's something that scientists are still trying to understand.

Which of the newly-found feathered, and sometimes winged, dinosaurs could fly?

(cawing) How did they take that glorious first leap?

Surprisingly, the oldest missing puzzle piece, collected over a century ago, Archaeopteryx, is still filling in gaps today.

In a synchrotron particle accelerator larger than two football fields, scientists are analyzing this precious fossil.

VINCENT FERNANDEZ (translated): The synchrotron beam comes in through that little window.

It will interact with our object, in this case a fossil, and the images are recorded by our detectors.

NARRATOR: They fire a beam of X-rays more than a million times brighter than the sun at the fossil.

FERNANDEZ (translated): Because the synchrotron beam is very intense, we can get a signal strong enough to penetrate the entire width of the plate and virtually extract the fossil from the rock.

NARRATOR: This creates a 3D model with resolution down to one-thousandth of the thickness of a hair.

VOETEN: When we first saw the images, we immediately realized that the bone walls of the humerus and the ulna of Archaeopteryx look a lot like those of flying birds.

NARRATOR: The bone walls are thin and hollow, just like those of modern birds.

VOETEN: That was our first indication that Archaeopteryx must indeed have flown.

NARRATOR: As far back as 150 million years ago, dinosaurs' ornamental "wings" are already evolving for gliding.

♪ ♪ To fly, Archaeopteryx has to project itself from a high point.

It uses its clawed fingers to climb.

From its high vantage point, it can spot its prey very easily, and pounce on them simply by spreading its wings.

But to truly conquer the skies, gliding birds needed to learn to flap.

There are several ideas of how this happened.

The first, known as the "tree-down" hypothesis, suggests that tree-dwelling birds flapped their wings to cushion their landing.

The second, known as the "ground-up" hypothesis, emphasizes the use of wing flapping associated with running to escape danger or catch up with prey.

(dinosaur calling) But recently, another theory has been suggested, one that can be seen in birds today.

♪ ♪ ASHELY HEERS: So these are our performers today.

This is Olympia in gray here, this is Annie in brown, and this is Twinkle Jemima, or Twinky, for short.

NARRATOR: To analyze the movement of the wings in detail, evolutionary biologist Ashley Heers needs to see them in slow motion.

HEERS: Here I've got my high-speed video camera, and I can visualize what the camera is seeing on the laptop.

(birds squawking) And so you can see that she's really using her wings to brake as she gets ready to land there.

(voiceover): I actually spend a lot of my time working with developing birds, or baby birds, because they can also tell us a lot about the evolution of flight.

And in many ways, they look similar to some of the fossils that we see that are documenting this origin of flight.

And so, in these developing birds, we have this really interesting system where we can see in real time how a living animal goes from a flightless animal to a flight-capable animal and everything that happens in between, both anatomically and behaviorally.

NARRATOR: One of the behaviors seen is wing-assisted running on an inclined plane.

HEERS: So on the left here, we have a four-day old chukar partridge.

It is flapping its wings to really drive itself into the substrate here and increase traction with its feet, so that it can ascend this steep incline.

NARRATOR: This is not the only behavior that hints that dinosaurs were able to develop flapping flight.

HEERS: They may use their wings to leap into the air and fly, either for a very short distance, or for thousands of miles.

They also use their wings to come down out of trees or off of elevated surfaces, even to swim, either across the water or below the water.

And so living birds show us that, you know, they use lots of different habitats and they use their wings for lots of different functions.

NARRATOR: Bird flight developed as an adaptation to the environment, as well as to allow gliding down from trees or to evade predators on land.

But what exactly was this great environmental change?

And when did it allow birds to take the final leap to full flight?

♪ ♪ One site in particular can transport us back to a time when birds were taking to the skies.

FIELD: It's amazing to be driving to Las Hoyas, one of the world's most important early Cretaceous fossil sites, with some of the most important early evidence of birds from the age of dinosaurs.

♪ ♪ NARRATOR: After 30 years of excavations, an entire prehistoric landscape has been uncovered.

(translated): We're in what was once a wetland, where there is an enormous diversity of species of both plants and animals.

And we know that this wetland was freshwater because it was completely isolated from the sea.

NARRATOR: Paleontologist Jesús Marugán and his team have uncovered a vast array of plants, animals and insects, all part of a complex and thriving ecosystem.

Digs here have revealed thousands of fossils, all between 125 and 129 million years old.

(dinosaur roaring) MARUGÁN (in English): You see all those crabs and small crayfish?

FIELD: Ah, each one of those blue marks represent a crayfish!

MARUGÁN: Yeah.

FIELD: Amazing how many they are.

MARUGÁN: It's more than 500.

We have the whole family, from the larvae-- see how small they are?

Yeah.

To the juveniles, to the adults.

Yeah, okay.

See, with all the pincers and everything?

Yes, that's crazy!

You can really tell they're crayfish!

NARRATOR: To process this huge amount of data, Jesús is combining traditional observation with the very latest technology.

(Marugán speaking Spanish) MARUGÁN (translated): Smile for the camera!

NARRATOR: They use a drone to photograph the site and geotags to help recreate a 3D model of the environment.

Each colored sphere represents a species or family of species.

The connections between these spheres indicate interactions.

(speaking Spanish) (translated): The birds of Las Hoyas are found here in this orange section; the largest ecosystem.

It tells us that the birds were not just passing through.

They were an integral part of this ecosystem.

♪ ♪ NARRATOR: Given the rarity of bird fossils, the sheer number found suggests that birds were thriving here.

But that's not all.

These fossils paint a picture of archaic birds that are adapting to geologic change.

Around 175 million years ago, the super-continent known as Pangea broke up.

New continents emerged and oceans and seas rushed in through the fault lines.

These new territories, with their temperate climate, gave rise to flowering plants, which provided a new food source.

BRUSATTE: Those plants diversified, all kinds of insects diversified alongside them, to pollinate the flowers.

And so birds were probably part of this general diversification of life that started with plants and bugs and reached all the way to the dinosaurs.

NARRATOR: Las Hoyas shows archaic birds evolving right before our eyes, and a class of ancient bird called the Enantiornithes is a perfect example.

Wow, so this is Eoalulavis.

That's it, the original.

A beautifully preserved skeleton.

FERNANDEZ: Three-dimensionally preserved.

You can see all the bones of the forelimbs.

NARRATOR: Eoalulavis is one of these Enantiornithes, and it's exceptional in more ways than one.

In particular, it has developed a highly sophisticated wing.

FIELD: Eoalulavis.

This complicated name has some important connotations.

FERNANDEZ: Yeah, exactly.

These fossil bird, back in the day when he was described, was the first primitive bird that preserved the alula.

NARRATOR: The alula is a small mini-wing on the leading edge of the main wing.

It allows the bird to increase lift at will.

This appendage gives the bird great maneuverability in flight, and for take-off and landing.

FIELD: It's incredible that this alula, which of course helps birds be more maneuverable in flight, was present in Eoalulavis, because this fossil is 129 million years old.

Yeah.

So that, of course, tells us that this structure has been around in bird evolutionary history for at least that long.

NARRATOR: Thanks to this dexterity in flight, Eoalulavis is better able to spot its prey and swoop down upon them.

This is not the case for all winged creatures that populate the Eden of Las Hoyas.

Europejara, a gigantic pterosaur with a wingspan of two meters, needs plenty of space on the ground to take off and land.

And when danger strikes, that can be the difference between life and death.

(dinosaur roaring) JARVIS: We call this convergent evolution.

Each time flying evolved in animals-- whether it be birds, bats, ancient flying dinosaurs-- the wings evolved in the upper limb on either side, not one on the head, one on the tail or the foot or whatever.

They evolved using the upper limbs for flight.

NARRATOR: In this world 130 million years ago, dinosaurs, flying reptiles, archaic birds and ancestors of early modern birds all shared the skies together.

This was bird life at its peak.

A plethora of archaic species living in trees, perfectly adapted to their environments.

Birds were taking over the skies.

But it wasn't to last.

A cataclysmic event shook the planet.

(whooshing) A gigantic asteroid six miles in diameter collided violently with Earth.

(explosion) Forests burned and volcanic ash soon filled the skies, blocking out the sun.

This was an extreme survival situation, one that 75% of Earth's species weren't ready for.

(dinosaur calling) Among the mammals, the smallest, some weighing less than one pound, managed to escape the catastrophe, as well as some reptiles like turtles and crocodiles.

Birds were also hit hard.

Only the smallest, such as Asterionis, survived the chaos.

How is it that all but one group of birds were wiped out?

What did they have that no other archaic bird and no other dinosaur had?

Janavis is a bird that did not make it.

Its fossil dates back 67 million years, to a time just before the asteroid hit.

Close analysis is helping us to better understand the reasons for its disappearance.

JUAN BENITO MORENO: Here you can see this skeleton of Janavis.

We have some of the bones, particularly the arm bones and part of the vertebral column.

NARRATOR: Equipped with teeth, Janavis has an imposing stature.

Is this why it didn't survive the asteroid strike?

The key difference between Janavis and Asteriornis is size.

Asteriornis is much smaller, a trait that was key to its survival post-asteroid strike.

So larger bodied animals, through this mass extinction event, didn't do very well at all.

So a really big bird like Janavis, probably would have been at a major disadvantage compared to Asteriornis.

After the extinction event, getting a meal for an animal the size of Janavis probably would not have been a very easy thing to do.

MORENO: They also had very different ecologies.

That's true.

Janavis was basically a sea bird.

Like one would have been fishing and eating fish and squid.

Mm-hmm.

Asteriornis was more like a generalist, ground-dwelling bird eating whatever he found at the beach.

Mm-hmm.

NARRATOR: With the trees gone, a large number of tree dwelling birds could not survive.

Instead it was ground-dwellers like Asteriornis that held the advantage.

(fire crackling) Archaic birds, although more numerous, were devastated by the asteroid.

Their ecosystems were destroyed.

In this hostile environment, their imposing size, life in the trees and specialized diets led to their extinction.

Only modern ground-nesting birds, with more flexible diets, were resilient enough to survive a decimated landscape.

The sun starts to shine again, the dust has cleared, and it illuminates a world that's devastated, a world that's almost empty.

There's no T. rexes anymore, no Triceratopses anymore.

Most of the other birds were gone as well.

And so, this would have been a world of abundant opportunity, open frontiers.

NARRATOR: In the million years that follow this apocalypse, new ecosystems emerge.

Out of the ashes of the dinosaur world, plant life re-appears.

And a whole system of coevolution, between flora and fauna erupts.

With little competition, the surviving birds enjoy unprecedented evolutionary success.

JARVIS: Those few survivors then exploded into new species with the opening up of new environments to become most of the 10,000 species we see today.

♪ ♪ NARRATOR: A site in Denmark bears witness to this key moment in bird evolution.

On an inland island to the north of the Jutland peninsula lies a geological formation dating back 55 million years.

Made up of more than 180 layers of marine deposits and volcanic ash, these mineral strata form the cliffs of the small island of Fur.

Hundreds of bird fossils have been unearthed in this geological jumble.

Many of them are birds related to today's cranes and rails.

They show that birds had begun to thrive once more, at least on the ground.

But were birds back in the trees by this time?

To understand that, we need to find birds with more complex feet, specialized for grasping and perching in trees.

NARRATOR: There is one that indicates this change in behavior.

This one actually looks very interesting, and it seems like it's got a more complex foot that might be specialized for grasping.

So it's not like a modern perching birds foot where you have one, two, three forward and one backwards.

It looks like it's probably got two toes pointing forwards, and two pointing backwards.

And that is the condition that you see in a few modern groups of birds, including woodpeckers, parrots, and cuckoos.

The fact that this bird seems to show pretty clear specializations for perching in trees, tells us that by 55 million years ago, this lineage of birds was probably already experimenting with a tree-dwelling lifestyle.

NARRATOR: Ten million years after the asteroid hit, the surviving modern birds have diversified.

New species have appeared.

They have adapted to colonize trees once more.

They diversified like crazy.

(birds cawing) And most of the birds we know today, everything from ostriches to emus to hawks to hummingbirds to owls, to songbirds and everything in between emerged during those manic years, decades, centuries, millennia, after the asteroid impact cleared the deck.

NARRATOR: In certain regions of this reborn world, and because of their geographical isolation, some small surviving birds grow impressively large.

Reaching gigantic proportions, they evoke the ghosts of carnivorous theropods.

One in particular appears to be especially intimidating.

♪ ♪ The huge bird, named Gastornis, was discovered in the Paris basin in 1855.

It was so big, people assumed it was a fierce predator.

But was it?

If so, why doesn't it rule the roost today?

(translated): The reason we wanted to do this research was that in the '90s, two American studies came to two diametrically-opposed conclusions, one team said it was herbivorous, and the other team said it was carnivorous.

With the advancements of paleontological tools, we thought we'd finally be able to answer this question definitively.

NARRATOR: After crushing the fossil Gastornis bones, Paleornithologist Delphine Angst extracted carbon-13 so that it could be measured using a mass spectrometer.

We quite literally are what we eat, and levels of carbon-13 correspond to the amount, or lack of, protein in our diets.

ANGST (translated): That's it, everything's weighed, we can start the manipulation.

ARNAUD (translated): Great, the spectro is up and running, so we're ready to go.

ANGST (translated): Perfect.

The carbon-13 analysis of the Gastornis bones showed us without any ambiguity that it was clearly a herbivore.

Combined with other results we already had, it showed us that Gastornis was 100% a herbivore, and there's no possible doubt about it.

NARRATOR: Gastornis was a peaceful herbivore.

It didn't run, but walked.

Despite being nearly seven feet tall and weighing more than 400 pounds, it was not a terrifying predator.

ANGST (translated): It disappointed a lot of people that were sad that the image of Gastornis chasing little horses through the forest was wrong.

My husband's still mad.

NARRATOR: For several million years, it walked its clumsy way across Europe and North America.

But as with any great story, the fate of birds was to change again.

40 million years ago, the continents collided, allowing species to move into new landscapes.

Previously isolated territories were populated by new arrivals, including newly evolved, predatory mammals; a terrible threat to the peaceful giant Gastornis.

(leaves rustling) Faced with these faster creatures, and despite their imposing size, these large birds are not sufficiently armed to compete.

They are doomed to extinction.

A recent discovery indicates that other giant birds suffered the same fate as Gastornis.

Found in New Zealand, Kumimanu was a giant penguin, as revealed by its flipper bones.

You can see here a Humboldt penguin.

That's your average-sized penguin today.

An Emperor penguin, which is the largest species alive today.

And then this behemoth here is Kumimanu.

So it's quite an impressive bird.

By looking at the proportions, the thickness, the length, the width of this specimen, we can estimate the body size.

And so we believe Kumimanu probably weighed about 150 kilograms.

NARRATOR: That's 330 pounds.

In the first 15 million years after the asteroid hit, birds are triumphant.

They reclaim the reborn forests and become rulers of the skies and seas.

However, many of the giants, like the colossal penguin and the giant Gastornis, eventually disappear.

Competition with mammals is sounding the death knell for these great birds.

Resistant to the many climatic changes that the Earth would subsequently undergo, today there are four groups of birds.

JARVIS: Land, higher-land birds, waterbirds, the group that includes the Columbiformes, that's pigeons and so forth, and flamingos and a group that includes the hummingbirds.

NARRATOR: And one subgroup in particular has achieved astonishing evolutionary success in recent times.

This is the great family of passerines.

Nesting in our towns, parks and gardens, passerines are part of our daily lives.

Singing birds that perch in trees, they demonstrate extraordinary adaptability.

Contrary to the saying, sparrows' brains are actually quite powerful.

FIELD: Pretty good diversity of birds around, Lizzie.

Yeah, I think most of them are probably passerines.

Yeah, that makes sense, I guess passerines are the most diverse group of birds in the world, right?

STEELL: Mm-hm.

FIELD: How many species are there?

STEELL: There are more than 6,000, which is more than half of living bird diversity today.

FIELD: That's a great view of Eurasian magpie up there.

Yeah, that's a really nice representative of a big group of passerine birds called Corvides.

FIELD: And Corvids in general, I guess, are probably most famous for having some of the very largest brains, compared to body size, of all birds.

So these are things like crows, jays, ravens and magpies just like this one, which make them some of the most intelligent birds in the world.

So it's easy to think about magpies and their relatives as the hominids of the bird world.

They're the smart ones.

Exactly.

NARRATOR: Passerine intelligence is illustrated by their behavior: the construction of their nests, their exceptional memory, and their ability to communicate through song.

A skill they share with us humans, despite our evolutionary paths diverging some 300 million years ago.

Something happened in nature to get this convergence to be similar between humans and these modern birds.

So what we and others had found is that in our forebrain area evolved this new circuit, and that circuit evolved in a similar way with a direct connection to the voice circuit in songbirds, humans, parrots, and hummingbirds.

Why only these few species?

If you evolve vocal learning, you're more likely to be eaten, and you won't survive.

Only if you're at the top of the food chain or near the top of the food chain, you can.

We found that songbirds and parrots evolved from apex predators.

(birds cawing) NARRATOR: Birds are beautiful examples of evolution's power to create variety.

Today, there are more species of birds than there are mammals or any other terrestrial vertebrates.

They thrive around the world, perfectly adapted to the tremendous diversity of environments and landscapes.

Birds are incredibly successful today.

And I think it is a really neat fact of life that there are double the number of bird species than mammal species.

And by that measure, the age of dinosaurs still continues today.

NARRATOR: Long live the dinosaurs.

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