Winter in the forests of North America.
A hushed world where billions of trees create a giant acoustic maze.
And deep snow muffles sound.
For the animals that live here, this is the stage
for nature's greatest game of hide-and-seek.
The red fox.
The woodland's most acrobatic hunter.
This young male is braving his first winter...
and that means mastering the tricky challenge
of catching prey hidden deep beneath the snow
just by listening.
He's after meadow voles, two feet under the surface.
To keep hidden,
they communicate in a secret language so high-pitched it's inaudible to us.
But with specialist microphones, we can tune in to their calls.
The voles use these eerie whistles to keep in touch through their tunnels...
...and to warn each other of danger.
Unlike our ears, the foxes are attuned to the voles' every move.
To work out exactly where they are,
he uses a technique called triangulation.
By comparing tiny differences in sound...
hitting his left and right ear...
he can pinpoint his prey to within two inches.
Hearing them is one thing,
but catching them is quite another.
In the deep snow, every footstep gives him away.
The voles sound their high-pitched alarm...
and make their escape.
For our young fox, learning this acoustic feat...
takes trial...
and error.
This time...
he's been outfoxed.
But he's not the only forest predator listening for voles beneath the snow.
The great gray owl...
the woodland's ultimate acoustic hunter.
Her hearing is even sharper than the fox's...
thanks largely to the unique shape of her face.
Her giant facial disc, the biggest of any owl,
acts like a satellite dish...
collecting and amplifying
every tiny sound.
It's a super-sense so powerful,
she can hear a vole beneath the snow from 150 feet away.
She's also the perfect stealth assassin.
Special serrated feathers on her wings allow her to approach in complete silence.
Constantly tracking her prey's position...
just through sound.
Only when she's locked on...
will she strike.
Thanks to their incredible hearing,
65% of great gray owl hunts end in a meal.
After six cold months, North America's forests begin to warm,
and otherworldly sounds fill the airwaves.
Frozen lakes shift and crack.
Microphones above and below the surface
capture the sound of each fracture.
Higher notes travel through the ice faster than the lower ones...
creating these sonic pulses.
A rare phenomenon heard only a few days a year.
It's a sign the soundscape of these forests
is about to undergo a radical transformation.
Under the strengthening sun,
snow covering half the continent of North America...
starts to melt.
Releasing trillions of gallons of fresh water...
it turns icy trickles...
into raging rivers.
Now these northern forests are filled with the sounds...
of new life.
And deep in the woods...
hidden cameras capture the debut performance
of one of America's most secretive animals.
This four-week-old wolf pup...
is emerging from the den for the very first time.
With his parents off hunting,
he starts to practice...
the iconic wolf howl.
Well, everyone has to start somewhere.
Each pup has a unique voice
that holds important information about their place in the hierarchy.
Even at this age,
the more dominant the wolf, the deeper the call.
Howling also plays a key role in keeping the pack together.
Sometimes more adventurous pups wander off.
And these woods are no place to be alone.
Bumping into a black bear...
would not end well.
To find his way home,
this little wolf needs to use his brand-new howl.
Through sound...
they're reunited.
In just a year's time, their adult howls will allow them to keep in touch...
...from seven miles away.
As spring progresses,
a more peculiar noise cuts through the forest...
produced by an unlikely little animal.
The ruffed grouse.
A plump little bird full of surprises.
This amorous male is hoping to attract a partner.
Like all ruffed grouse, he's no singer.
But he is a drummer.
He belts out this mating beat more than 12 hours a day.
But in these woods...
...broadcasting your whereabouts
at top volume is a dangerous game.
Goshawks patrol here.
And a portly grouse would make an ample meal.
He somehow needs to make himself heard by potential mates
without attracting unwanted attention.
For the first time,
super slow-motion cameras reveal how he does it.
With each wing beat, the lower feathers flick forwards,
scooping up a pocket of air...
creating a loud boom...
which is channeled directly in front of him.
Strangely, from the side, just a faint fluttering can be heard.
But from in front, it's three times louder,
allowing him to broadcast his call only in the direction of potential mates.
And his efforts are paying off.
A female attracted by his beat.
Now he must make sure he looks as impressive...
as he sounds.
It seemed to be going well.
But at the first hint of danger...
he chickens out.
As the forests continue to heat up...
even the trees start making bizarre noises.
Microphones beneath the bark capture strange gurgles...
as cottonwood trees slurp up water from their roots.
But as summer draws to a close...
the airwaves are filled...
with the unmistakable sound of the elk.
Their unusual call is known as a bugle,
and it means it's almost time for them to breed.
This majestic old bull is the boss around here.
He guards a large herd of females known as a harem.
But at ten years old, this could be his last season at the top.
For the next month,
keeping rivals away will all come down to his ability to bugle.
It's his way of telling other males to back off.
And it's more complex than you might think.
The elk's strange call is made up of two different sounds
produced simultaneously, known as biphonation.
By slowing down and separating these sounds...
we can hear a high, piercing whistle...
as well as a deep, bellowing roar.
The low notes give an accurate measure of size.
The bigger the male, the deeper the bellow.
But these don't travel far through the forest.
The high notes give less information about size but carry far further.
They can be heard from a mile away...
by rival males.
The high whistle alone is usually enough to scare off the competition.
But this interloper has other ideas.
Calling back is a direct challenge...
and an unwelcome sound for the old bull.
As the intruder closes in,
both males try and intimidate each other with their deepest roars.
But these two are evenly matched.
To hold on to his harem,
the old bull now has no option but to fight.
Lasting up to 30 minutes,
the aim is to wear down and exhaust the opponent.
The old bull sends the interloper packing.
Now he has one last sound trick to play.
A victory bugle.
Elk are the only mammals besides humans thought to use sound to celebrate victory.
In just four weeks...
these noisy bugling contests will be over.
It won't be long before the temperature starts to drop
and the volume fades down once again
in the forests of North America.
To capture the spectacular drumming solos of the ruffed grouse
like never before, the Earthsounds team traveled
deep into the forests of Minnesota
where they searched out fallen logs,
the sound stage from which the grouse perform.
For sound recordist, Nick McMahan,
recording the audio of this behavior should be relatively straightforward.
I'm very fortunate that the ruffed grouse will be drumming right on the log here.
So, it's-- it's much more accessible than the birds that are up,
in the trees singing up above us.
Male ruffed grouse move from log to log in the forest,
and on each one, they can drum for up to three hours.
As the grouse does its drumming performance,
it's moving its feathers very quickly.
And so, I'm hoping that some of the feather motion
and the-- and the movement of that will be captured with these microphones.
Nick sets up a remote camera to record
when the grouse displays and leaves his microphones running.
But for cameraman, Dawson Dunning,
capturing the visual display is more of a challenge.
You can't necessarily just walk up to these birds and film them.
They're actually quite skittish.
They'll tolerate you kind of being around, but they won't perform.
To make sure he doesn't give the grouse stage fright,
Dawson sets up a camera he can operate remotely.
But to show how the sound is produced,
he also wants to capture it in super slow motion.
And that means using a specialist camera from a hide a distance away.
I think what this slow-motion camera will really allow us to do is see the path
of the wings and, kind of, how they're actually pushing the air
to create this sonic boom sound.
So, this is really the only way we can slow it down enough
to see how the feathers are moving.
Otherwise, it's just a blur to our eye.
With everything set up, the waiting game begins.
Sometimes he'll, like-- he won't come here for hours
because he's stuck somewhere else on another log
and I'm just sitting here doing nothing because, you know,
I just have to wait for him to show up.
After days of trying and failing,
Dawson's luck starts to change.
Can hear him coming on the leaves.
Yeah. There he is.
At first, the bird seems a little shy.
But then his confidence builds...
allowing Dawson to finally film the perfect drum solo.
That's so cool.
I can just feel that sound just hit your chest.
Look at that.
That was amazing.
Using this super slow-motion camera,
you can see the path of their wings and really how they're creating
this amazing sound.
They're doing, like, 50 of these wing beats in one little quick session.
After three weeks in the forest,
the team manages to capture the drumming shows like never before
and reveal how these plucky birds produce,
surely, the most unusual sounds in the woodlands of North America.