Birds of prey are the hunters
of the sky
powerfully equipped carnivores
whose roots reach back to the age
of the dinosaurs
Most, like eagles and falcons
hunt during the day
But the owl can detect its prey
at night
with a body designed to home in on
a kill even in complete darkness
Vultures are hunters, too
though they prey on the dead
These voracious scavengers
work together
to strip the carcasses of
large animals
For all their variety, birds of prey
are savage and graceful
nature's monarchs of the air
BIRDS OF PREY
Scotland in April, and a late snow
covers the ground
A male golden eagle is hunting
for food
Like other birds of prey
all the parts of his anatomy combine
to create a highly efficient predator
strong talons for grasping prey
a muscular chest
and powerful wings which help him
lift it
And a sharp beak for tearing it open
These are the defining features of
birds known as raptors
The golden eagle feeds mainly on
rabbits, hares and grouse
though it's strong enough to kill
a deer calf
Golden eagles are among the largest
birds of prey
They can measure three feet
from beak to tail
with a wingspan of seven feet
As in most birds of prey
the female is larger than the male
Along with hawks, falcons
kites and buzzards
they are diurnal raptors
which means they hunt by day
High up on the sides of
a Scottish cliff
golden eagles nest in an aerie
The female is feeding her young
She tears the meat into small strips
to make it easier for them to digest
However, the mother is unable to
leave the nest to hunt
The thin down on newborn chicks
provides little insulation
and they are vulnerable to the cold
So the mother must keep them warm
by brooding
For the next ten days
the whole family will rely on
the hunting skills of the male
Birds of prey need patience
stealth and above all, keen senses
Eagles rely on their sight
Unlike many birds, raptors have eyes
which point forward
giving them binocular vision
By using both eyes to view
the same object
they can see depth and are able to
judge distances with great accuracy
They also have a third eyelid
called a nictitating membrane
which protects their eyes
particularly when they attack
For centuries, people have known that
birds of prey have good eyesight
hence the expression "eagle-eyed"
But until twenty years ago
no one had tested their
vision scientifically
Liz Raymond, an Australian
research scientist
was the first to develop an
eye test for eagles
The biggest difficulty with testing
any animal's, ah
vision is that you can't talk to
them directly
So it all has to be done by
indirect means
Liz solved this problem with a simple
but ingenious experiment
In the first stage
she trained an eagle to distinguish
between images...
projected onto two screens
"They have two choices
One is the black and white line
of equal widths
and the other one is, um
a projection of a homogenous
gray field
Uh, and what you ask the eagle to do
after a lot of training is to fly to
the black and white lines
And if the bird does that
then he would receive a food reward."
Liz then made the black and white
lines progressively thinner
The idea was to determine
when the eagle could no longer
distinguish the screen
with the lines from the gray screen
At this point, the eagle wouldn't
know which screen to fly to
"When he starts making the mistakes
or he's got fifty percent right and
fifty percent wrong
then you can say that that's the
actual limit of his resolving power."
Liz was then able to compare
the eagle's eyesight with that of humans
"The results were quite surprising
If you hold your finger out
your thumbnail out about that distance
and if across your thumbnail you draw
ah, sixty black lines of equal widths
and interdispersed with white
lines of the same width
that's about the amount of detail
that humans can resolve."
Ah, but the eagle could see up to
about a hundred
and thirty-two black lines
about two and a half time as well as
humans can see
One of the reasons for their powerful
vision is their large eyes
If our eyes were in the same ratio
to our body size
they'll be as big as oranges
A big eye allows a larger image
to be projected
onto the retina at the back of the eye
This image is decoded for color
and detail by light-sensitive
receptors called cones
An eagle's retina has
six hundred thousand
cones per square millimeter
four times as many as in humans
But eagles' eyes are practically fixed
in their sockets
and can only be turned a few degrees
To compensate for this
the eagle's neck is incredibly flexible
and can twist all the way around
Not all birds of prey have such
powerful eyesight
Smaller raptors like the kestrel
have smaller eyes
And their visual acuity is little
better than ours
Despite this, kestrels can see
things we can't
This is because they have
specialized cones
that can detect ultraviolet light
These help them find their
favorite food, voles
Voles feed during the day
Hidden in the grass
they are difficult to see
But they have one weakness
their bladder
They leave a trail of urine
wherever they go
Urine reflects ultraviolet light
And to a kestrel searching
for food
these trails are perfectly visible
from the air
This time, the vole escapes
As the light fades at the end
of the day
diurnal raptors like the golden
eagle give up the hunt
Their eyesight is adapted to work
best in bright light
In low light, their vision is worse
than ours
and they are unable to see their prey
When the moon rises... the nocturnal
birds of prey come out to hunt
Barn owls are completely
at home in the dark
They have all the features of
the diurnal birds of prey
from their sharp talons to
their strong beaks
But their eyes are different
In low-light conditions
they are one hundred times more
sensitive than ours
Still, there are times
when even a barn owl's eyes
are not sensitive enough for hunting
The film in these canisters was the
first photographic evidence
that owls could hunt without any
light at all
In 1957
Roger Payne of Cornell University
carried out an unusual experiment
"I knew that owls had
asymmetrical ears
The ears weren't the same on the
two sides of the head
And that was curious
That meant ears were important to owls
They had to be doing something
with them
So I put an owl on a perch in
a darkened room
and I then released a mouse into
leaves on the floor of the room
And the mouse as it moved around
made noise banging
into the leaves and crushing them
And suddenly in the darkness
the owl struck
And I turned on the lights and
it was holding the mouse
I just couldn't believe it
And later on I got hold of an
infrared viewing device
which makes it possible to
look in infrared
which the owl can't see and
we can't see
And then I could watch what
was going on
And it was just fascinating."
Throughout the winter in a small
shed on the campus
he used infrared light to watch
barn owls hunt
He recorded what he saw on
infrared film
What we got to remember is that
throughout this whole time
the owl can't see a thing
There's enough infrared to expose
the film
but the owl can't see in infrared
And here he goes
He's flying along
He brings his feet into a
pattern right there
which is exactly lined up with
the mouse's body
He then closes his eyes as,
'cause he knows he's about to
hit something
He lands and he's missed
The mouse is sitting right next
to him
not two inches from his foot
But he can't see it
So he folds his wings
Ooh, and then he looks at the mouse
because the mouse has just
made a sound
And now he leans way forward
He's six inches from it
And at this moment he does
a curious thing
He brings his feet from the floor
all the way up
and puts them out like that
'cause it's the only way he can line
them up with what he's hearing
Here he goes. Wham!
He nails it right there
"Make sure that it wasn't
what he was homing on the smell
of the mouse
or that it had actually seen the
mouse somehow
or that maybe it was looking at the
mouse in the infrared heat
that a mouse gives off
I took a small wad of paper
dragged that through leaves and
the owl struck the paper
Paper doesn't smell
It doesn't have any infrared
coming off it
It doesn't look like a mouse to
anybody, not even an owl
And the result is, it had to be sound
Sound was what they were using
At first sight, the barn owl doesn't
appear to have ears at all
unlike its cousin,
the long-eared owl
Looks, however, can be deceiving
The tufts are only for display
lts real ears, like the barn owls
are hidden under the feathers
alongside its eyes
"Everything about the way an owl looks
the reason an owl looks like an owl
is all to do with acoustic adaptations
in other words, adaptations for hearing
One of the most obvious ones is
this circle of feathers
around both sides of the head
Actually, this is one parabola to
collect sound for this ear
and this is another parabola to
collect sound for that ear
which is actually very much like
cupping your hand behind your ears
a means of collecting more sound
And you can also see another
peculiar thing
Owls sit with their bills pointed down
like this looking at the world
And the reason they do is to maximize
the surface area over
which they can collect sound
If this was a, a robin or some
other bird, songbird
the bill would be held roughly
horizontally like that
But no. They tilt it way down
And the reason they do that is
because this dimension of
the head is greater
than that dimension of the head
That whole posture is part of why
an owl looks like an owl."
The barn owl locates the direction
of a sound in two ways
When the mouse makes a noise
the sound reaches one ear a fraction
of a second
before it arrives at the other
This minute time difference
enables the owl
to work out the left-right direction
of the mouse
When the mouse is directly
in front of the owl
the sound arrives at both ears
at exactly the same time
Now it has to calculate if the
mouse is above or below
"And if you cut just that collection
of feathers in the very center here
that help the air flow by smoothly
then you get what we've done here
to this owl
which is a stuffed owl
that's had just those feathers removed
nothing else
So right here you can see
that there are two flaps of skin
And those flaps are directly
in front of the ear opening
The ear opening is back here
But they're asymmetrically placed
One is higher than the other one
This one is actually lower
than that one
These asymmetric flaps...
gather the sound from the mouse
in different ways
The noise of rustling leaves
come from below the owl
The sound passes under the high flap
which is on the left side of
the owl's head
and is reflected by the ruff
into the ear
The low flap on the right side deflects
the sound coming from below
When the sound is louder in the left
than the right ear
the owl knows it's coming from below
By combining the up-down with
the left-right information
it pinpoints the mouse with
great precision
The mouse, however
has ears of its own
If it hears the owl coming
it can simply run away
"Well, here's a fight between
owls and mice
Mice have ways to avoid owls
Owls have ways to improve their
chances of catching a mouse
and this is one of them
The very leading edge of
the feather
that is in the front on the wing
has a little series of tiny
little spikes that stick out on it."
No other bird of prey has these spikes
The leading edge of an
eagle's wing is smooth
As the eagle flies
its wings cut through the air
creating turbulence
This turbulence creates large
sound waves
making it noisy in flight
When an owl flies
the tiny spike on its wings
break down the air
into smaller spirals called
microturbulence
This makes the flight of
the owl quieter
Sharp hearing also helps an
owl hunting in daylight
The great gray owl lives
in northern latitudes
and in the winter months its prey
is hidden beneath a blanket of snow
But the snow offers no real protection
Even through this covering
the owl can detect the sound of
a mouse eating
The great gray owl
like all birds of prey
has senses finely adapted
to its environment
Turkey vultures ranging over the rain
forests of South America
can't see their food either
The dense canopy of trees covering
the ground hides animal
carcasses from view
So how these scavengers were able to
locate food remained a mystery
until the late 1930s
In California, the Union Oil Company
was plagued
by a leak in a forty-two mile
long natural gas pipeline
The leak proved impossible to
find in difficult terrain
But company engineers had noticed
that vultures often gathered
while they were repairing holes
in other pipelines
So on a hunch, the engineers
from the gas company traveled
along the pipeline
Eventually they saw turkey vultures
circling in the distance
When they reached the spot
they started digging
To help find pipeline leaks
a gas with a distinctive smell of
rotting meat
is always added to natural gas
It was this that attracted the vultures
At the bottom of the hole the engineers
found the broken pipe
They had also discovered
that turkey vultures could hunt
just by smell
Turkey vultures have nostrils so
large you can see right through them
Smells are processed by a very large
olfactory bulb
at the front of the brain
The bulb is so sensitive
it enables the bird to detect the
direction a smell is coming from
The turkey vulture follows the
concentration of odors
in the air to the source of the smell
This acute sense provides a service
to other vultures
While the turkey vulture flies
low over the forest canopy
the other vultures soar high above
waiting and watching
These birds have a less-developed
sense of smell
and rely on the turkey vulture to
guide them to the food
When it descends, they follow
Although first to the prey
the turkey vulture isn't strong
enough to rip
through the skin of this
large rodent
It is soon joined by black vultures
which are also unable to
pierce the hide
They must wait for the arrival
of the king vulture
Like the black vultures
the king vulture has a weaker
sense of smell
But it does have a strong, sharp beak
It repays the favor of being led
to the meal
by tearing through the skin of the
carcass to provide food for all
Back in Scotland, it is May
The newborn golden eagle chicks are
now seven weeks old
They still need feeding twice a day
But now they can be left alone on
the aerie
They've now acquired a thick layer
of downy feathers
and no longer depend on their mother
for warmth
The feathers to come, however
will take them much further
A typical bird of prey has over
seven thousand feathers
each with its own role
Some insulate the bird from the cold
Others waterproof it from the rain
And then there are those that provide
lift for flying
Before a bird flies
it raises and untangles its feathers
putting each one into place ready
for flight
Flight is a bird's ultimate
evolutionary achievement
The evolution of birds is still the
subject of fierce scientific debate
The earliest ancestors of birds
probably emerged two hundred million
years ago as reptiles
Some that climbed trees grew elongated
scales to form primitive wings
These allowed them to glide
from the treetops
Fifty million years later
archaeopteryx appeared
It still had teeth and solid bones
just like a reptile
But what made this animal so different
was that it had feathers
Like scales, feathers are
made from keratin
But they are light and flexible
Archaeopteryx did more than glide
It could fly
Flight improved over the next
seventy-five million years
as every possible ounce was trimmed
from what were now the birds
To save weight
they even lost their teeth
Bones became hollow, strengthened
with a honeycomb of trusses
About fifty million years ago
as the number of rodents and small
mammals began to increase
birds appeared that could hunt this
rich food supply
The modern bird of prey was born
Eagle zero-one 
Wind one-six-zero-five
Cleared for takeoff
Change to your departure
Eagle zero-one, cleared for takeoff
Changing to departure
Air bases throughout the world
have a problem
The short grass around an airfield
attracts flocks of hungry birds
Just one bird flying into the engine
of a jet can cause it to crash
The most dangerous time is when
planes are coming in to land
"Wart hog three-one
clear to land runway two-four
Wheels down."
"And wart hog three-one
expect possible go-round short final
We have a small flock of birds
crossing the runway left to right
approach end
Bird control one
I'd like you to leave ground."
"Control squadron, go ahead, please."
The US 48th fighter wing based in
England keeps peregrines
and other falcons on stand by
"Bird control one
we need you to proceed out to runway
two-four at the approach end
We have a small flock of birds
sitting on the runway approach end."
Peregrines are the natural enemy
of the rooks
and pigeons that feed near the runways
A tape recording of a distressed bird
is often not enough to scare the flock
But the release of peregrine has
a dramatic effect
First it climbs to gain altitude
then it launches its attack with
a steep dive called a stoop
A peregrine falcon in a stoop is
the fastest animal in the world
A team in Germany has measured
its speed
at two hundred seventeen miles
per hour
The point is only to scare the birds
But this time it catches a
passing pheasant
A peregrine's strong wings are
designed for maneuverability
But it depends on speed to catch
its prey in the air
The wings power the bird forward
at over four beats a second
An eagle's wings are not designed
for speed
Their shape and seven-foot span
enable them
to soar over vast areas in search
of prey on the ground
But one thing the eagle and
the peregrine have in common
with other birds of prey...
is that they strike with their feet
Each foot is armed with four talons
which pierce skin like daggers
On the eagle, they can be over
two inches long
When they grasp their prey or
a branch
raptors have a special locking
mechanism in their legs
that prevents their grip
from slipping
It works like a ratchet
As their toes grasp
ridges on the tendons lock the talons
into place
When it wants to let go
the bird pulls the outer sheath
away from the tendon
and the grip is released
In New Mexico
the talons of the red-tailed hawk
will find an easy target
As the sun goes down
Mexican free-tail bats stream out
of their cave
to start a night of feeding
The red-tailed hawk normally
hunts rabbits
But it will also take bats
And it doesn't pay to be choosy
Raptors eating their prey have the
skills of an experienced butcher
A hungry eagle can strip the bones
of a rabbit in fifteen minutes
Birds of prey are fast eaters
On the ground, the peregrine is
vulnerable to predators
so it needs to eat quickly
It swiftly plucks the feathers of this
pheasant to get at the flesh below
Each species has a beak
that is specialized
and finely adapted to the
prey it feeds on
An eagle's beak has a sharp hook
at the end
able to cut into meat like
a knife point
And the sides of the beak have
razor-sharp edges
to tear flesh away from the carcass
In the Florida Everglades
one species of kite has a beak adapted
to only one food source
The snail kite feeds on apple snails
lts thin, curved beak is
so well-designed
to prize a small snail
out of its shell.
That a snail kite doesn't
eat anything else
While larger birds of prey eat
their food where they find it
smaller raptors like owls carry
their catch to somewhere safe
A raptor's digestive system
with its specialized gastric juices
is designed to deal with its
exclusive diet of meat
Even as young chicks
barn owls swallow their prey whole
Then the owls use an internal organ
to separate the meat
from the bones and fur
Instead of teeth to break up the food
owls have a gizzard
a muscular stomach
that grinds up the food
The meat is digested by
acids and the bones
and fur are molded into pellets
and regurgitated
Dissecting a pellet reveals
the owl's diet
In a typical week
it will eat twelve voles
fourteen mice and eight shrews
In Scotland, it is now eleven weeks
since the young eagles hatched
This down feather is all that's left
of the bird's juvenile coat
Their black and white flight feathers
have grown
and the birds are just mature
enough to fly
The parents are starting to leave the
young birds to fend for themselves
If they don't fly soon
they'll starve
These first flaps are essential
preparation for flight
They build up muscles and
strengthen the wings
The eldest chick finally gets up
the courage to take to the air
This maiden flight ends with
an ungainly landing.
For any bird of prey the first year
is the toughest in its life
Over half will die in their
first twelve months
mainly due to starvation
To survive, these birds will
now need to find sites
where they can hunt and breed
Zoologist Mike McGrady has spent
five years studying eagles
in the western highlands of Scotland
He's used tracking techniques to
build the most detailed picture yet
of the movements a young eagle makes
once it has left the nest
For the first few months after
it leaves the nest
it's, it, ah, is resident on its
its natal territory
But after, in usually, well
November through January they
they move on and start to
wander quite widely
These, these wanderings can be over
hundreds of, of kilometers
And they don't seem to have
any direction
To follow these wanderings
Mike fits the eagles with
radio transmitters
But he has to catch one first
Eighty yards from the hide
Mike has baited the trap with
a dead rabbit
Trapping requires endless patience
"lt, it's kind of a dynamic tension
between being comfortable
and being uncomfortable because
you have to be
you have to be awake all the time
pretty much
but you, you, and ready to move
But you also, um, you're gonna ha.
Have to spend eleven hours
in this hide
when it's cold or raining or
something like that
And it, you're also trying to
make yourself comfortable."
This is the sixth day of watching
and waiting
When he does spot an eagle
flying overhead
there's no guarantee it will
be tempted by the bait
And even if it is, Mike's problems
may have just begun
"We then have to wait until
the eagle's... uh
exactly in the right position
because one is always concerned
that the
the eagle will be injured by our
trapping methods
Um, but once the eagle is in, in,
in position
we just fire the trap by, ah
radio control
Unfortunately, sometimes the eagles
spend half an hour in
the wrong position
but in the trap
and we never catch them."
Although the bird is in position,
it isn't safe to spring the trap
while its wings are open
Finally, it folds its wings
"Okay, the green number is ZZ 0832."
"Oh-eight-three-two."
"Yeah. Let, let me just get a bill
length on it."
The first job is to tag the bird
and measure it
The beak and talon lengths will
tell Mike
whether this is a male or a female
"Okay. Forty-three-point two?"
Although trapped, it is sill a
formidable animal
"Forty-nine point four."
This bird has a short beak and talons
and so Mike knows it's a male
"All right. We're ready to put the
the tag on?"
He fits the lightweight
radio transmitter
that will eventually detach
itself from the bird
"Um, this is a
tail-monitor transmitter
so it's life expectancy is
really dependent, um
upon where the, the tails, uh
how long that tail feather
going to stay in
I've tried to choose a
feather that's
that's relatively new because
eagle don't
don't molt through all their feathers
in one particular year
So hopefully this bird will
be able to
to keep the transmitter on at least
as long as the life of the transmitter
which might be about two years."
There are about four hundred fifty
breeding pairs of eagles in Scotland
To be successful, a young golden eagle
needs to claim a territory
But what if all the good sites
are already taken?
Today, Mike's picked up the signal
of a young male eagle
and followed it to its source
The territory of this golden eagle
is seven miles long
and five miles wide
The eagle has defended it
against all comers
"Just like any other
territorial animal
they, they maintain this territory
um, at great, ah, energetic expense
And the reason they have to do
that is because the resources
within that
that territory are quite important
to it breeding
Those resources include food resources
but also might include nesting
resources or a mate
They're maintaining an area
which they can survive and
and reproduce in
The other eagles are
are also interested in the same thing
And you, if you have an eagle
that's, um
non-territorial and is looking to
enter the breeding pop... population
it has to, um, establish itself on a
on a territory
And at least from our work
some of these eagles are, are
ah, seem to be aggressive about that
that, uh, territory establishment."
Mike has recently discovered just
how aggressive eagles can be
In 1992, he tagged the male of
a pair of breeding eagles
He tracked the bird for a year and half
until he eventually lost contact
"Some months later we
we picked up a very strong
signal of the
on this bird and followed
the signal in to
to find the bird dead and,
and quite decomposed."
What he discovered next came
as a complete surprise
"Once we cleaned up the, the skull
and the whole skeleton
we, we, we sort of realized what
what had caused this
this bird to die
Um, it had, ah, obviously received
a wound from probably another eagle
It must be another eagle
I can imagine anything else
inflicting a wound
like that on a adult, ah
golden eagle."
This eagle was killed for
its territory
How often this occurs no one knows
But a young male eagle has
now taken over the territory
Chances are, this was the killer
The new young male will mate with
a resident female
He will learn from her
where the best...
hunting sites are and benefit
from her experience
When she dies, he will stay
in the territory
and acquire a new mate
In this way, territories have been
handed down in an unbroken lineage
stretching back centuries
All birds of prey share an
evolutionary legacy
Whether they're hunting or scavenging
flying or gliding
they are the ultimate aerial predators
THE END
