The Time Terror Birds Invaded

About five million years ago, a new predator
arrived in America. It had made its way from the south, crossing
the still-forming Isthmus of Panama and onto the coastal plains of Texas and then Florida. The largest of these creatures stood 3 meters
tall and weighed 150 kilograms. And in its old home, it was the uncontested
apex predator, armed with all kinds of awesome adaptations that it used to kill its prey
-- sometimes in pretty weird ways.

And this invader wasnt alone. It was part of one of the biggest mass migrations
in Earths history. It seemed more than ready to hold its own
against North American predators like wild dogs and sabre-toothed cats. But one thing set this killer apart -- it
wasnt some toothy mammal or ravenous reptile.

It was a giant, flightless, carnivorous bird. And it came to be known by one of the coolest
and most richly earned nicknames in all of paleontology: the terror bird. The story of the terror bird invasion actually
begins nearly 145 million years ago, at the start of the Cretaceous Period. This is when the last remnants of the supercontinent
Gondwana -- which would later become Africa and South America -- went their separate ways.

South America became an island continent,
drifting through the ancient ocean while its inhabitants lived on in isolation. There were fantastic, gigantic versions of
todays armadillos and sloths. And there were the terror birds, which rose
to become the continents top predator after the extinction of the dinosaurs. Over the course of about 60 million years,
this group of carnivorous birds -- now known as Phorusrhacidae -- diversified into as many
as 25 different species, ranging in height from 1 to 3 meters, with a variety of body
types and lifestyles to match.

Some species may have been scavengers, but
others were definitely predators. And they were built to kill They had massive beaks tipped with sharp hooks
-- kind of like what you find on modern raptors -- adapted for delivering powerful stabs and
ripping flesh from bone. Their neck vertebrae also suggest that they
could swivel their heads quickly, which wouldve helped them track and strike at their prey. And some had strong, stout leg bones that
seemed better suited for kicking than for running.

Researchers think they could use their powerful
legs to crack open the bones of its victims -- possibly to get to the marrow. Plus, they had large, curved, compressed claws
were perfectly suited for subduing and stabbing prey. And unlike most other birds, many of the bones
in terror birds skulls were totally fused together. This allowed them to use their own heads as
weapons, by basically pecking stuff to death.

Other clues about how terror birds hunted
come from birds we know today. For example, many living predatory birds,
like the secretary bird, kill with vicious kicks. And terror birds closest living relatives
-- a pair of species that still live in South America called seriemas -- actually subdue
their prey by picking it up and smashing it against the ground over and over again. Which is not how I want to go Since these are the closest living analogues
to terror birds, paleontologists think the extinct giants might have used the same techniques.

And so, yeah: Hence the name! When fossils of these birds were first discovered
in Argentina in the late 1880s, they were given the rather obscure name Phorusrhacos,
which is thought by some to mean bearer of scars. But nearly a hundred years later, in 1978,
after having studied these things for decades, paleontologist Larry Marshall dubbed them
terror birds, which proved to be catchier and easier to say. Now, on its home turf, terror birds had plenty
of prey to choose from, because most mammals in South America were herbivores. But eventually, thanks to continental drift,
South America got a new neighbor...

North America. This process took a long time, of course,
but by at least 5 million years ago, a chain of islands had formed that linked the two
continents for the first time. And land masses werent the only things
on the move. The meeting of the Americas marked the beginning
of what scientists think may have been one of the greatest exchanges of animal life ever
between two continents.

Today its known as the Great American Biotic
Interchange, when animals from both continents were suddenly free to migrate, bringing into
contact all sorts of species that had never met before. And a lot of what we know about this phenomenon,
we know from the fossils of mammals. For example, we know that, five million years
ago, North America was home to deer, horses, cats, and bears  but there were also camels,
elephants, and tapirs. And all of these groups moved south.

Meanwhile, in South America, there were marsupials,
giant ground sloths, and huge cousins of the armadillo called glyptodons that moved north. And in general, the mammals from North America
were more successful in the south than the animals that made the reverse trip. Thats because, other than terror birds,
there werent a lot of large predators in South America. So the North American animals -- from mice
to canines -- did very well in their new home, and they diversified like crazy.

In fact, half of the mammal genera living
in South America today are descended from North American immigrants. But North America had way more big predators
than the southern migrants were used to. So, most animals that moved up from South
America didnt last very long. And this included the terror birds.

We know from fossils that at least one type
of terror bird followed its prey north -- Titanis walleri, one of the largest terror bird species The earliest evidence of Titanis in the US
has been found in Texas, in strata dating back 5 million years, to the late Paleogene
Period. Which is weird, because scientists think that
the land bridge between North and South America probably wasn't complete until about 3 million
years ago. So, even though they couldn't fly, these giant
birds mustve somehow managed to float, or swim, or walk through the shallow waters
that connected the islands between the two continents. From there, Titanis roamed the open, coastal
plains, eating anything it could chase down, kill, and swallow whole.

But in its new home, for the first time, Titanis
had to deal with competition from other big predators -- like sabre-toothed cats and the
ancestors of modern wolves. Then, around the beginning of the Pleistocene
Period, the outlook for the terror birds got even worse, when the climate began to change. Temperatures grew colder, and glaciers began
their march south. Soon, beset by advancing winters and bigger
predators, the last North American Titanis met its end around 2 million years ago, as
the most recent Ice Age started to set in.

So, the invasion of the terror birds turned
out to be brief -- less than three million years. And because their time here was so short,
they didnt leave much evidence behind. The entire fossil record of Titanis in North
America consists of just a few dozen bones and bone fragments -- mainly of the neck,
legs, feet and toes -- found at only four sites in Florida and one in Texas. But their migration was just one small wave
of the Great American Biotic Interchange, which turned out to be a crucial chapter in
the history of the Americas that changed life on both continents forever.

In the end, the time when terror birds came
to North America is an important reminder of how big changes can create lots of awesome new
opportunities for some of us  while also creating tremendous pressure to either adapt. Or disappear. Thanks for joining me this terror-ific episode
today. Now, what do you want to know about the story
of life on Earth? Let us know in the comments.

And dont forget to go to youtube.Com/eons
and subscribe! But dont stop exploring now! Do yourself a favor and check out some of
our sister channels from PBS Digital Studios..

The Real Reason Birds Fly In A V-Formation

Look! Up in the air! Its a bird! Its a plane! No, its a bird. Hey there flappy birds, Jules here for Dnews! Youve probably seen birds migrating, or
just making short trips for feeding, and youve probably noticed that birds tend to fly in
a V-shape. Theres usually one bird up at the front
leading the way, and each successive bird lines up back and to the right or left of
the bird in front of it. There are other basic shapes, such as J-formations,
and inverted versions of both, and these groups are actually types of echelon formations,
meaning that they line up linearly.

V-formations have been used extensively, not
just by birds, but by military generals. From the earliest days of war, allegedly stemming
back to when the Thebans were fighting the Spartans in 4th century BC, all the way to
today, the formation is used in sea battles and aerial warfare. Birds, planes, boats, and people all have
similar reasons for doing this. Lining up in a V gives each member a clear
line of sight ahead of them.

In some cases, it also takes less energy to
travel in this formation. See, when a bird flaps its wings, a vortex
of air directly behind it is pushed downward, called downwash. The air further back and to the sides responds
by pushing up, called upwash. Any bird situated in another birds upwash
has to expend less energy to stay aloft, since theyre already being pushed upwards, and
a V-formation situates each member back and to the side, directly in its neighbors
upwash.

In 2001, researchers at the French National
Center for Scientific Research put heart-rate monitors on pelicans flying in V-shapes, and
they found that the ones farther back had slower heart rates and did not have to flap
their wings as often to stay afloat. In fact, there was an 11-14% total energy
savings for the birds. Another study from the Proceedings of the
National Academy of Sciences also found that the flock also tries to makes sure that the
bird up front doesnt get too tired. Since it doesnt receive any benefits from
the communal upwash.

The leader bird rotates among the flock, although
scientists still arent sure if there is a hierarchy of who gets to be the leader. Of course, all this mess could be avoided
if we taught birds how to order plane tickets. One bird, the arctic tern, flies roughly 56,000
miles round-trip and is able to sleep while in flight. But how does the tern, and plenty of other
birds pull off the ability to sleep mid-air? Find out in this video.

And do you have any animal questions for us? Ask us in the comments, don't forget to subscribe,
and come back here for more DNews every day..

The Origin of BirdsHHMI BioInteractive Video

[Footsteps] [chime plays] [music plays] [CLARKE (narration):] The animal kingdom
is made up of major groups, recognized by key traits. Fish have fins. Some land animals
have four legs, others six, and several different groups have wings. Biologists have long sought
to discover how groups of animals, and their key features, evolved.

And one of the greatest mysteries
has been the origin of birds. Our world has more than 10,000 species
of birds with feathered wings. Where did birds come from, and how did wings
and feathers first arise? To find out, scientists
have scoured the fossil record... And they have uncovered
surprising twists in the evolution of birds
from their flightless ancestors.

[CLARKE (to camera):] In the past 30
years we've found a treasure trove of new fossil discoveries. They've made the origin of birds one of the best-documented transitions
in the history of life. [Music plays, birds call] [walking through grass] [CLARKE (narration):] I am fascinated
by birds. And as a paleontologist,
I've spent my career chasing their evolutionary origins
in the fossil record.

[CLARKE (to camera):] Above all else,
what makes birds unique are their wings. They're made of feathers
that are stiff, yet flexible. And bird wings are even more remarkable
than airplane wings, because they can flap, which allows them to maneuver rapidly
and ultimately defy gravity. [CLARKE (narration):] The quest
to understand the origin of birds and other animals began in earnest
over 150 years ago.

When Charles Darwin
wrote "The Origin of Species," he argued that every major group of animals
evolved from a pre-existing one. He predicted that we would find fossils with features
that linked one major group to another. In fact,
he staked his theory of evolution on the existence of these intermediates. But no fossils were yet known
that revealed these transitions.

Then, just two years later, a marvelous creature was unearthed
from a limestone quarry in Germany. The 150-million-year-old fossil,
named Archaeopteryx, rocked the scientific world. [CLARKE (to camera):] This Archaeopteryx
fossil is truly remarkable. It preserves in fine detail feathers
along the wing-- just like those we see in living birds-- and feathers along the tail.

But the bony features
tell a very different story. We look closely,
we'll see see teeth in the jaw, tiny claws preserved in a hand, and a long bony tail,
lacking in living birds, but present in things
we think of as traditionally reptilian. For Darwin, it must have been
an incredible vindication. He predicted that we would find forms
like these.

[CLARKE (narration):] Archaeopteryx
pointed to a close link between birds and reptiles. But which group of reptiles? Flying pterosaurs had been discovered
with light hollow bones. But their wings are constructed
very differently than the wings of Archaeopteryx and birds. [CLARKE (to camera):]
Here is a tiny pterosaur, and if we take a closer look at its arm,
we'll make out 3 small digits, and a fourth,
which is really, really long.

[CLARKE (narration):] The membrane
of a pterosaur's wing attaches to this fourth digit
and along its body and hind limb. In contrast, the wings of Archaeopteryx
and birds have only three digits. And their feathers attach individually
along their arm and hand bones. These differences tell us
that pterosaurs and Archaeopteryx
evolved flight independently.

Archaeopteryx must have descended
from different reptiles. Thomas Huxley, Darwin's champion, was astonished
by Archaeopteryx's resemblance to a turkey-sized dinosaur
called Compsognathus. Compsognathus' hand
also had three digits. It had hollow bones
and stood on two legs.

Similarities like these led Huxley
to propose that birds are related to the branch
of reptiles called dinosaurs. But other scientists
questioned this conclusion. Birds appeared so different
from dinosaurs, and some characteristic features
of birds--like wishbones-- seemed to be missing from dinosaurs,
but were present in other reptiles. [HORNER:]
We found an articulated foot...

[CLARKE (narration):]
When paleontologist Jack Horner began his career, few thought that birds
could have come from dinosaurs. [CLARKE:] So Jack, why was it so hard to believe that birds
and dinosaurs were related? [HORNER:] Most of the dinosaurs that the public
knew about were really big. Like, you know,
this is a shoulder blade of a Sauropod. And Sauropods were gigantic.

[CLARKE (narration):] Scientists thought
that dinosaurs were cold-blooded and slow-moving, like other reptiles. [HORNER:] People
couldn't imagine dinosaurs being agile and hopping around. They look at these big giant things
and they lumber. There's no way to relate them to birds.

[Music plays] [CLARKE (narration):] Then,
in 1963, John Ostrom discovered a fossil in the badlands
of Montana that challenged that view. [HORNER:] What John Ostrom
first found was was this claw. Obviously goes to a foot. It was not a claw for walking on.

This dinosaur actually used that claw
for slashing. [CLARKE (narration):] Deinonychus
was small with a delicate build. It ran upright on two legs. It had a long, stiff tail for balance.

Not all dinosaurs
were big and lumbering. [HORNER:] Ostrom hypothesized
that the animal would scale its prey and start using its slashing claw and probably eating the animal
while it was alive. [CLARKE (to camera):] Ostrom's discovery
set off a revolution. What if dinosaurs weren't slow, but warm-blooded and fast-moving,
like birds? [CLARKE (narration):] When Ostrom
compared Deinonychus to Archaeopteryx, he saw that they both had
lightly-built, hollow bones.

And they shared even more features, including long arms
and similar hip and shoulder bones. Ostrom concluded that birds
did descend from dinosaurs as Huxley had argued. Not from lumbering sauropods, but from another lineage called
therapods that walked on two legs, and included T. Rex, and agile predators like Deinonychus.

While some scientists
did not accept this idea at first, supporting evidence
continued to accumulate, including the discovery
that theropods had a feature of birds not previously found: a wishbone. [HORNER:] People
had sort of looked for them, and really didn't know
what it was going to look like. And then all of a sudden
we started finding them. Here is the wishbone
of Tyrannosaurus rex.

[CLARKE (narration):] When scientists
analyzed the skeletons of theropods and birds, they found too many similarities
for any explanation but common ancestry. Jack's collection
at the Museum of the Rockies offers an opportunity
to compare their features. [HORNER:] Here is an Albertosaur tibia, and as you can see, it's hollow,
just like a modern bird. [CLARKE (to camera):]
This is a T.

Rex foot. What we see here
are three forward-facing digits that bear the weight of the animal, and in the back, a much smaller digit. If we take a look at this chicken foot,
we'll see the same pattern. We've got three forward-facing digits
and on the back, a much smaller one.

All dinosaurs share an S-shaped neck. You can see it here and in living birds
like this chicken. [CLARKE (narration):] New kinds
of evidence also emerged. In 1978, Jack made the surprising discovery
of a vast dinosaur nesting ground.

[HORNER:] We discovered
that dinosaurs nested in colonies, cared for their young,
brought food to their babies. We also had evidence that they
came back, probably over and over again, for many years to the same site. [CLARKE (narration):] In a radical
shift, by the 1980s, a consensus was finally building that birds
descended from theropod dinosaurs-- from active predators
that walked on two legs. But scientists were about to discover
the most startling evidence of all.

In the mid-1990s,
farmers in northeast China began unearthing dinosaurs
120 million years old. And these fossils
preserved astonishing detail. [CLARKE (to camera):] In 1996,
I was a first year graduate student at my first scientific meeting. They were passing around pictures
of this dinosaur.

[CLARKE (narration):] This chicken-sized
theropod, named Sinosauropteryx, did not have scales. It was covered
in some primitive kind of feather. [CLARKE (to camera):] To see
those photos of a tiny, fuzzy dinosaur... It just blew everybody's minds.

[CLARKE (narration):] This dinosaur
was just the first of many fuzzy and feathered theropods to be uncovered. Another, called Caudipteryx, had feathers identical to living birds
on its tail and hands, but lacked wings. With the discovery
of these spectacular feathered finds, there was no longer any doubt
that birds were related to theropods. But while feathered dinosaurs
settled one question, they raised a new one: These animals could not fly.

Why were they feathered? [CLARKE (to camera):] It was long
assumed that feathers evolved for flight. But what we found
was that clearly feathers predate flight and arose for some other purpose. [CLARKE (narration):] So why did
the first feathers evolve? That's hard to tell
from just the fossil evidence. But living birds may offer answers.

Feathers provide insulation. So the first feathers
might have helped keep dinosaurs warm. Birds also use colorful feathers
in communication, in courtship and in territorial displays. Dinosaurs may have used feathers
in the same way.

Feathers likely played different roles
at first, and then were modified for flight. The modification
of an existing structure for a new use is called co-option. It is a common way that new structures
and abilities evolve. Bird wings are modified forelimbs
once used for grabbing and feeding.

Just as the walking limbs
of land animals are modified fins. And the turtle's shell
is a modified ribcage. So the co-option of feathers for flight enabled Archaeopteryx
and its relatives to take to the air. And other features also evolved.

[CLARKE (to camera):] When we look
at evolution after the origin of flight, we see a lot of characteristics
of living birds gradually accruing. [CLARKE (narration):]
But not in a simple linear sequence. Like other dinosaurs, this crow-sized
bird had large claws on its hand, but like living birds, it had a toothless beak
and a short bony tail. While this species had teeth, its hand bones were partially fused
to form a stronger wing.

And this bird had a large breastbone
for well-developed flight muscles, like living birds. But it also had teeth. [CLARKE (to camera):]
We don't find forms that are somehow lock-step intermediate
between Archaeopteryx and living birds... We find a diversity of forms, forms
we could not have predicted.

[CLARKE (narration):] For tens
of millions of years, an assortment
of scaly dinosaurs, feathered dinosaurs, and many types of birds lived together. Then, 66 million years ago, almost all of these creatures died out. [Rumble] A six-mile wide asteroid
slammed into the planet... [Explosion] ...And triggered a global mass
extinction.

[Music plays] Only a small group
of toothless birds survived... And they evolved into the 10,000 species
of birds we see today. [Bird calls, music] We once might have said
the dinosaurs all died out, but now we know that living birds are a lineage of theropod dinosaurs
in the same way that we are a lineage of primates. [HORNER:] Have dinosaurs gone extinct? Absolutely not.

We separate dinosaurs
into two groups now: the non-avian dinosaurs
fortunately have gone extinct, and the avian dinosaurs are still alive,
making it a beautiful world. [Music plays] [CLARKE (to camera):] Dinosaurs
are still with us. We just call them birds. [Music plays] [bird calls] [music plays].