(upbeat music)
When winter descends upon Europe,
many animals have no choice but to leave.
The migrations of many creatures
had been a mystery until now.
And yet they cross oceans and deserts,
brave intense weather,
and travel thousands of kilometers unabated.
Thanks to recent scientific progress,
we are finally able to follow their journey
and discover the secrets behind their incredible migrations.
Bats are extraordinarily gifted creatures.
They are capable of navigating in absolute darkness
and are the only flying mammals on the planet.
While most European bats spend the winter in areas
that never get too cold, a small number of bat species
opt for an alternative survival strategy.
Migration.
The most surprising of them all
is surely Nathusius' pipistrelle.
Weighing just a few grams,
this bat holds the European record
for furthest mammal migration.
Over 2,000 kilometers.
But why does Nathusius' pipistrelle migrate?
How does it manage to travel this far?
Throughout it's journey, scientists study the bat
in hopes of finally understanding
one of the greatest mysteries of the animal kingdom.
(uplifting music)
It's early August in northeast Latvia.
These wild lands are covered with vast forests,
representing an ideal habitat for Nathusius' pipistrelle.
Particularly discreet,
this small creature usually lives inside of hollow trees.
But, the regions many wooden buildings
are another perfect fit.
The 230 year old wooden church of Apšu, is home to one
of the largest Nathusius' pipistrelle colonies in Europe.
It is just before dusk and all is still calm,
because bats are nocturnal creatures.
They slept all day in one of the many hiding places
within the building's wooden frame.
But as soon as night falls, the colony comes to life.
300 adult females and their offspring
spread their wings before the night's first outing.
Strangely, the colony doesn't include a single adult male.
It's now midsummer,
and almost all of the juvenile bats have learned to fly.
Ready for takeoff, they await their mother's signals.
As soon as it's dark, the females take flight,
followed closely by their young.
First comes flying social hour,
a dance that helps them warm up their wings.
Then, they're off to the forests and ponds
where they'll spend the night hunting insects,
their favorite prey.
Averaging around eight grams in weight
and a wingspan of 25 centimeters,
Nathusius' pipistrelle is one
of the smallest bats in Europe.
It's large flexible ears give it exceptional hearing skills.
How do these acrobats dart between the trees
and manage to find their prey in total darkness?
It's not thanks to their eyes.
To orient themselves at night,
Nathusius' pipistrelles along with other bats,
use a special sense.
Echolocation.
They produce high pitched sounds from the mouth,
which are inaudible to humans.
These fast highly directional ultrasounds
bounce off the surrounding objects
and back into the bats ears as echoes.
It is these echoes that allow the pipistrelle
to construct a tri-dimensional representation
of its surrounding environment.
When it's hunting, the pipistrelle raises the frequency
of its sonar emissions,
to obtain a more precise display of its prey.
Like this nocturnal insect.
As night comes to a close, the satiated predators return
to the safety of their shelters.
The pipistrelles may be resting during the day,
but they are not entirely inactive.
Huddled together, mothers and their young
reinforce their social connections.
Or use the time to care for their wings.
(melodic music box music)
A biologist from the University of Jelgava,
Gunārs Pētersons and his students
take an annual survey of the church's colony.
Counting the colony members allows the scientists
to compare the number of bats present before
and after migration.
The census takes place just before the bat's departure,
using a strange device that makes their cries audible.
So ultrasound detectors, the equipment used by batwalkers,
people when they see bats, they don't hear anything,
but actually they produce quite loud sounds all the time.
It changes ultrasounds to audible.
Fingers make no sound although a bit of unique sound,
but, if you, now
(scratching sounds)
you'll get sound which is inaudible for us,
it is also ultrasound and such.
(bats clicking)
This evening, they recorded 285 pipistrelles.
For many students, this outing is the first time
they actually come into contact with bats.
(speaking in foreign language)
These small silky coated creatures
are much more endearing than the students expected.
Come early August in northern Europe,
nature is still incredibly productive.
So why do the pipistrelles migrate?
In just a few weeks,
temperatures in Latvia will begin to drop.
In the middle of winter,
they can get as low as negative 20 degrees.
While most bats take shelter in caves
where they hibernate to avoid freezing,
Nathusius' pipistrelle adopts
an entirely different strategy.
It chooses to migrate south towards latitudes
where it never freezes,
even in the coldest months of winter.
One night in August, upon nightfall,
the pipistrelles leave the church tower a final time.
(dramatic music)
The mothers and their young
will be flying for a very long time.
This mass migration takes place all over Northern Europe,
as Nathusius' pipistrelles begin their migration
across the continent.
The church colony flies in total darkness
towards the Baltic Sea.
With an average of 100 kilometers per night,
it will take them four days to reach the shoreline.
Gunārs Pētersons is already waiting for them.
He joins a team of European scientists
in the Pape Nature Park, who have all come for the purpose
of studying the bat's migration.
A giant funnel-shaped net has been strung across the forest.
At 15 meters high and 50 meters wide, this gigantic trap is
one of the biggest bat capture nets ever built.
(laughter)
Much like bird monitors, Gunārs wants to tag
as many bats as possible with identification rings.
But the number of catches will directly depend
on the weather.
We are now in the middle of August,
and it's smaller or less bigger migration.
This night, wind direction will change from western wind,
what we have now, to southern wind or front wind for bats.
And at such winds they tend to fly lower,
then we can catch them.
Everyone is on high alert.
After days of waiting, the team of scientists
can only hope tonight will bring a wave of migrating bats.
During migration, the pipistrelles often fly
over 20 meters in altitude.
Only moderate headwinds will send the bats into the net.
The pipistrelles are now migrating from forests
all over the Latvian Coast.
Gliding along the canopy, they emit powerful ultrasounds,
allowing them to visualize their surroundings
despite the darkness.
In Pape, the ultrasound detectors begin to pick up signals.
The first bats appear shortly after.
With a few wing flaps, they fly to the back of the net.
But their echolocation system quickly tells them
that they've fallen into a trap,
so they turn right back around.
Luckily, volunteers are prepared to catch the bats
as they retreat.
The number of catches multiply,
and the cages begin to fill with bats.
The pipistrelles are quickly taken to Gunārs Pētersons
and he immediately begins the next step in the process.
I have one Nathusius' bat, and quite calm animals,
don't bite very much.
Thanks to ringing, we know the bat's covering
the longest migration flights more than 2,000 kilometers
from here to Spain.
Unlike with birds,
rings for bats are not attached to their legs,
but rather to the edge of their wings on their forearms.
Each ring has a unique code.
If a bat is recaptured elsewhere in Europe,
the ring will help figure out the distance covered
by the bat.
In the past 30 years, Gunārs and his team
have ringed tens of thousands of migrating pipistrelles.
Unsurprisingly, this years captures
prove that male adults are rare during peak migration.
The majority of the bats flying south
are females or juveniles.
After being ringed,
the pipistrelles are immediately released.
(light classical music)
At sunrise, a new experiment is put into place.
Jens Koblitz is a bioacoustician
from the Max Planck Institute in Konstanz.
Specialized in echolocation,
he sets up an ultrasound recording device,
that will allow him to measure the bat's exact flying speed
as well as the type of echolocation
they use during migration.
There are eight microphones on this part,
and eight more on the part over there.
This high number of microphones allows us
to precisely identify where the sound is coming from.
We know the bats use echolocation
to orient themselves in small spaces
and to detect obstacles in their path.
We also know they use echolocation for hunting.
They emit sound at regular intervals.
They go beep, beep, beep.
And then, as they get close to their prey,
they go beep, beep, bzzt, and they usually catch their prey.
But we don't really know how bats use echolocation
when they undertake long distance migrations.
Like when they migrate south for the winter.
Wind always favors migration.
The sun has barely set, and the first bats
are already flying over the 16 microphones.
Wow, that's crazy!
Look at that, five at once!
There's another!
Wow!
Christian Voigt, a biologist
from the Leibniz Institute for Wildlife Research,
joins Jens.
This eminent European expert on bats
is trying to find a correlation between flight speed data
and the animal's energy needs.
What we're really interested in
is the bat's flight speed.
How long it takes them to travel 2,000 kilometers
and understanding when they take breaks.
In particular, I'm interested in learning
what energy sources they use to power that.
It's an incredible performance.
Do they cover the entire 2,000 kilometer migration
by burning fat or by metabolizing the energy directly
from the prey they consume?
In just a few hours,
the scientists microphones record hundreds
of passing pipistrelles.
The following morning,
they start analyzing the data they've collected.
Can you show me last night's data?
Our recordings show that the average speed
was between six and nine meters per second.
That corresponds to about 20 to 25
or up to 35 kilometers per hour for bats flying through
this part of Latvia.
Keep in mind that in the early evening,
we recorded a few hunting flights.
In the early evening,
we observed they were hunting for food.
But in the middle of the night,
only regular sound waves were recorded.
That means they were no longer hunting,
they were really flying to migrate.
So they hunt early in the evening,
and then at some point during the night
we believe they fly up to higher altitudes
and then from there they fly non-stop for many kilometers.
(bats clicking)
Unlike migratory birds that spends several days
rebuilding their fat reserves in between each leg
of their journey, Nathusius' pipistrelles prefer to feed
just before every nocturnal flight.
Why have they adopted such a different strategy?
At the CAnMove Institute in Sweden,
one scientist is studying bat flight
in an attempt to answer this very question.
Shannon Currie is an animal psychologist specialist
at the Leibniz Institute in Berlin.
She is studying the pipistrelle's metabolism in mid flight.
This is our little pipistrelle, Natoosie.
When they're sitting around resting and relaxing,
they're heart rate is about around 400 beats per minute.
And then when they fly,
it almost triples to over a 1,000 beats per minute.
Really tiny bats like these pipistrelles,
they're shooting through the sky,
over 1,000 beats per minute in their heart.
Their metabolism is 15 times what it is
when they're resting.
Everything is going crazy.
The experiment first measures the amount
of carbon gas the pipistrelle exhales when it rests.
This determines the amount of energy used
when it's not flying.
The bat is then placed in a machine
that allows it to fly in place, a wind tunnel.
This device allows the pipistrelle to truly show off
the incredible gifts that nature has bestowed upon it.
(dramatic music)
Got a bat!
From an evolutionary standpoint,
bats are truly phenomenal animals.
They're the only mammals capable of flying.
After one minute of flying,
Shannon needs to quickly measure the amount
of carbon dioxide the bat exhales after the effort.
The results prove that the bats spends
an enormous amount of energy to fly.
It's flight uses more energy than a bird of the same weight.
This could explain why the bats must feed
before each flight.
Luckily, the pipistrelle has a unique biological mechanism
that allows it to compensate for it's intense energy use.
Torpor.
They're fueling that flight on the wing
by eating while they're going.
And when they land, they need to compensate
for all of the energy that they've used,
so they slow everything down on purpose
and then they go into torpor.
When bats are torpid or sleeping,
their heart rate slows down a lot.
And their heart beats only three times every minute.
When the bat is in torpor,
it uses only the tiniest amount of energy.
Despite the high energy needed to fly,
the bat is incredibly acrobatic.
The secret lies in the bat's wing structure.
This bone here, is the same as your forearm bone.
This is their thumb.
This is the pinky finger, this is the next finger,
and then the top two fingers are together.
And you can see that in between,
you have a mixture of blood vessels and muscles.
That enable the bat to then maneuver their wings
when they're flying.
So unlike a bird that generally tends to go up and down,
up and down, bats have a lot more maneuverability.
They have to be that maneuverable
because they need to catch insects while they're flying.
By alternating between resting in torpor
and acrobatic flight,
Nathusius' pipistrelles can accomplish the feat
of flying thousands of kilometers over Europe each year.
The migrants leave the Baltic countries
as they continue their journeys.
In constant search for aquatic insects,
they usually fly along waterways.
The presence of wet zones is crucial
to the success of their migration.
It's been two weeks since these bats left Latvia,
and they've now arrived in Berlin, Germany.
One would expect that they would avoid
this urbanized landscape.
But Germany's capital is also
one of the greenest cities in Europe.
The many green spaces are a welcoming rest stop
for the bats.
One of their favorite spots is Tierpark,
the zoo in East Berlin.
It's the largest animal park in Europe.
160 hectares of forest, prairies and canals,
in the very center of the city.
At dusk, once the visitors have left
and the gates have been closed behind them,
the animals in the park finally enjoy some peace and quiet.
For the pipistrelles, it's time to hunt.
Christian Voigt is particularly interested in bats
that stop in urban settings,
because they have to deal with something they really hate.
Artificial light.
We're at the zoo in East Berlin.
It's a magnificent place in the center of the city,
which I personally find fascinating,
because it has very little light.
Bats like darkness.
That's why there are multiple bat species here.
They have adapted to these nocturnal nests.
They clearly avoid daylight,
because that's when they are most at risk from predators
like birds of prey or owls.
The cover of night is like a safety net
while it's incredibly dangerous for them to fly at dawn.
For the sake of our city's biodiversity,
it's really important that we preserve dark spaces
such as this one.
That means natural parks, zoos, but also cemeteries.
We have to avoid installing lights
just to please the public.
We must also think about the many nocturnal creatures
that need spaces like these.
And we should do our part to preserve them.
No one fully knows
how artificial light impacts the bats.
But some even take advantage of it.
We've now left the protected dark area
and are back in the light.
You can see that the animals and insects here
are disoriented,
they fly in circles around the light until they die.
For the bats,
this has both positive and negative repercussions.
The negative is obvious.
The insects die, and as a result, the biomass of insects
as a whole gets smaller in well lit areas.
On the positive side some species have figured out
how to use this to their advantage,
and they hunt insects around the lamp posts.
Nathusius' pipistrelles behave kind of in the middle.
Traditionally they avoid artificial light,
but occasionally we see them hunting near the lamp posts.
Insects are only attracted
by certain colors of lights.
The violet and ultraviolet wavelengths
in public light figures are mostly responsible
for the mass dying of insects.
Yet there is a type of light
that they find a lot less attractive.
There is a simple solution.
We can replace the streetlamp
with an alternative light source.
If we put in an LED light instead for example,
these insects would all disappear.
That light is less attractive to them,
but that wouldn't be great for the bats
that take advantage of these lights.
If there are no insects, then the bats won't come here.
On the other hand, it would be beneficial for the insects
in the area and by extension, to the bats as well.
Because more insects means more bats.
The bats' time in the Berlin zoo
will soon come to an end,
but before they begin traveling again,
the pipistrelles stock up on food.
Hunting insects that fly over pools of water.
(bats clicking and flapping)
The presence of these wet zones on every leg
of their journey is crucial for a successful migration.
As soon as they have eaten enough,
the pipistrelles continue their migratory journey south.
60 kilometers away from the German capital,
the group stops once again to hunt near a forest pond.
Specially designed nesting boxes for the bats
have been hung in these trees.
It's a perfect place for them to rest.
But these nesting boxes are not empty.
(louder clicking)
Strange sounds are coming out of them.
Intrigued, the female bats investigate more closely.
It is midnight.
Silke Voigt-Heucke, a bioacoustician
from the Natural History Museum of Berlin,
knows that something very important is taking place.
What we're hearing right now
are the echolocation sounds of the female bats
fluttering around this area.
One of the reasons the females may have chosen
this area in particular,
is that there might be male bats around here.
They obviously attract the females.
(loud clicking)
Now, that's the sound of a male bat.
I hope you heard it.
The male is trying to seduce the females
and get them to enter his nest.
This is part of his nuptial parade
that he uses to indicate that he's strong
and would make a good mate for them.
What we believe is happening is that the males sing,
and that's a sign to the females
that this is a very good place for them to stop
during their migratory journey.
This migratory itinerary could hence
be considered a love highway for the bat.
Migration and reproduction are closely connected
when it comes to Nathusius' pipistrelles.
It's the first time in months
that these Latvian females have seen an adult male bat.
Most of them are quasi-sedentary,
and live in more southern zones.
Patiently awaiting the day the migrating females
will pass through the area.
The male sings languorously from within his nesting box.
Although hesitant at first,
the females finally respond to the call.
One accepts the invitation.
Then a second.
And even a third.
Migration is one of the only times adult males
and females cross path.
Most migratory animals tend to mate during the spring.
But Nathusius' pipistrelles reproduce at the end
of summer in the middle of migration.
This rendezvous is only one of many to come.
Nathusius' pipistrelles are not
a particularly monogamous species.
The males try to attract as many females
as possible during mating season.
The females however only spend a few days with a male
before they carry on and follow their migratory path
until they reach their next harem.
For the pipistrelles, migration is much more
than a long journey.
It's a key moment in their reproduction.
The male awaits the next female in his nest.
His earlier companions have already taken flight.
This is where the female bats
and their male offspring part ways.
The juvenile males stay with the adult male bats,
while the females continue their journey.
But the next leg of the trip will be perilous.
One of the greatest dangers to migrating bats
are wind turbines, which are only increasing in number.
Christian Voigt know that these rotating steel blades
claim many migrating pipistrelles.
Oh no, we've got a collusion victim here.
It's a Nathusius' pipistrelle that had an accident
while migrating.
These bats are commonly injured by wind turbines
which are located along their migratory route.
The turbines operate at the same height
that the bats are flying at.
We have reliable data for Germany that shows
that each wind turbine injures between 10 to 12 bats a year.
Multiple that by the total number
of wind turbines, and the annual death toll reaches hundreds
of thousands of bats.
Numerous pipistrelles are among the casualties.
This is a real threat to the species.
As you can see,
this animal doesn't seem to have any visible injuries,
which is quite remarkable considering
how fragile their fingers and forearms are.
Then what caused the death of these bats
found beneath the wind turbines?
Dr. Gudrun Wibbelt from the Pathology Department
of the Leibniz Institute for Zoo and Wildlife Research
in Berlin wants to understand what happened.
All the bones are here, everything's intact.
The animal has no exterior injuries
but when we look at the ribcage,
we can see differences in color which seems to indicate
that the ribcage is full of blood.
This is most likely a barotrauma.
Which is obviously not something
that just happens all on its own.
The barotrauma was caused by the violent drop
in air pressure that takes place
behind the wind turbine's blade.
This drop was powerful enough
to make the pulmonary veins of the creature explode,
killing the animal without any contact.
Pipistrelles cannot detect low pressure zones
using echolocation.
However, the carnage could easily be prevented.
The simple solution would be
to stop running the wind turbines
when the bats are in peak migration.
Winds are not strong at that time anyway,
because we know that bats prefer not to fly in strong winds.
When the winds are stronger
than eight or 10 meters per second,
the bats generally stop flying,
so it would be perfectly appropriate.
Since wind turbines are not very efficient
unless wind speeds get up to six or eight meters per second.
We have to come to some sort of consensus.
We can simultaneously find solutions to global warming
while also protecting our planet's biodiversity.
For the pipistrelles who survived
the wind turbines, the journey continues.
The traveling females find many shelters along the way
that are carefully guarded by male bats.
The network of Europe's larger rivers
mostly follows a north to south axis.
A pathway that scientists believe bats follow
as a guide for their migration.
After the Rhin and the Saône,
the pipistrelles start to follow the Rhône,
as they approach the region's frost free areas.
Today, the Rhône is full of channels
and its shores have been tamed.
The wet forests that once ran along the river
have almost completely disappeared.
In Camargue, one of the last remaining green islands
clings to the river's edge.
This two kilometer stretch is called the Tourtoulen Forest.
These old trees are semi-aquatic
and make up a riparian forest,
a rare ecosystem that's very conducive to biodiversity.
Two researchers from the Chiroptera Group of Provence
want to know if this forest could
be a favorable place for bats,
and specifically, for Nathusius' pipistrelles.
To see if bats are present, Lorenza Buono and Lionel Bruhat,
installed automated sensors in some
of the oldest local trees and let them run
for an entire year.
The riparian forest makes up less than 2%
of southern territory here,
but we need this tree diversity in order to have areas
that are welcoming for the bats.
These trees have lots of little cavities
that make great hiding places for bats
while also protecting them from the elements
like the cold, the wind, and the rain.
Here's one, two.
When night falls, the bats come out of hiding.
Among them are Nathusius' pipistrelles.
This study has helped better understand at what time
of the year the pipistrelles are present here in Camargue.
Our study clearly indicates
that from May to August,
only males are present here in Camargue.
Afterwards we see that the females arrive in September.
It seems that the only reason the females come here
is to mate, so the Provence region is really only
for female mating season or hibernating.
Tonight, we had two contacts with a Nathusius' pipistrelle.
This one emitted the sounds typical made by males in autumn,
when they're trying to attract females for mating purposes.
For the females, one of the biggest risks
of migrating, is that they may not find shelter before dawn.
The males scattered along their routes
have carefully chosen shelters
and are essential to the female's survival.
Located on the edge of France's largest wetland,
the Tourtoulon Forest seems like an ideal place
to spend winter.
In the Camargue marshes, the temperature rarely drops
below freezing and insects are abundant
ten months out of twelve.
Yet despite these seemingly ideal conditions,
we've recently learned that some pipistrelles
continue their journey even further south.
A pipistrelle that was ringed in Latvia,
was recently found south of the Pyrenees.
For a record breaking 2,225 kilometers of migration.
In 2010, A Catalan scientist discovered
that the females migrate as far
as the Ebro Delta in southern Catalonia.
This means the area where the species can be found
extends 600 kilometers further south than previously known.
Like the Rhône river,
the mouth of the Ebro has a large fertile delta.
While some protected areas remain,
80% of the delta's surface is used
for intensive rice farming.
Old trees which are natural pipistrelle nests,
have rarefied.
At the far end of the delta on Buddha Island,
a few proud trunks still stand tall.
It's now early November,
and the bats will seek shelter in the nest boxes
hung in these century old eucalyptus trees.
For migrating bats, the south of Catalonia
marks the end of their journey.
They will still hunt for a few more weeks,
mate with the local males,
and then finally begin their hibernation.
Here lies the incredible paradox of this species.
The females travel thousands of kilometers to escape winter.
But then they hibernate upon arrival at their destination.
They cross all of Europe to sleep.
Yet, they don't actually choose the nesting spots laid
by the local scientists.
They go elsewhere to an unknown location.
A team of scientists led by Carles Flaquer,
the man who discovered their presence in the delta,
would give anything to find out where the bats go.
They've come to Buddha Island to catch a few bats
for an upcoming new experiment.
(speaking in foreign language)
To unravel the mystery,
the team will attempt to follow three Pipistrelles
equipped with radio transmitters.
Today we want to know where these guys are going
during winter days.
We have to use nowadays,
the only thing that we have is the radio transmitters
and follow them.
When they are handled,
the pipistrelles sometimes resort to a technique
they also employ to fool their predators.
He's pretending that he's dead,
so from something like cat, is doing nothing,
because it is strange, they are not moving,
and they love it.
It's a strategy that bats usually do.
Two females and a male were equipped
with a transmitter.
It's 11 PM.
The three pipistrelles are released.
The tracking begins.
(Dramatic music)
It's three in the morning and the male's transmitter
indicates he's returned to the island.
Meanwhile, the two females have disappeared from the radar.
The next morning, the male is finally found.
He returned to his normal hiding place,
a eucalyptus on Buddha Island.
It seems that they are here, maybe?
Yeah, here's one, yeah.
Let me check.
Bats are here.
The females have vanished without a trace.
They must have left the island.
The scientists scan the entire delta in vain.
In a late afternoon,
Carles and Maria detect a brief weak signal
near the edge of the delta, but it quickly fades.
The team knows they won't find
the pipistrelles again this year.
This was their third attempt, and the mystery remains.
The closest wetlands are some 200 kilometers to the south.
But no Nathusius' pipistrelles
have ever been observed there.
So where are they?
Are they hiding in some of the many abandoned buildings
found along the edges of the river?
Carles Flaquer believes it's the most likely explanation.
Wherever they have found refuge,
the pipistrelles will adopt the only strategy
that allows them to survive both the cold
and the lack of food.
Hibernation.
Soon, the insects in the delta will all but disappear.
The bat's only option is to slow down its metabolism
and enter a state of extreme lethargy.
All of the creature's biological functions will slow down
as the pipistrelles hibernate for almost four months.
In early April, the delta awakes.
The swollen Ebro river floods the shores again.
The marshes are full of water once more
and the midges and mosquitoes are metamorphosing.
Millions of larvae hatch at the same time.
This is the pipistrelles cue to wake up.
It's time to emerge from their torpor.
Dusk becomes the backdrop of an aerial procession,
where thousands of starving bats feast on aquatic insects.
It's time for Carles Flaquer and Maria Mas,
to see if their female bats have returned to the island.
A female.
Although the gestation period of pipistrelles
only lasts two months, none of the females
have given birth yet.
They can preserve the sperma and decide herself
when they can speculate and begin gestation.
They are able to adjust their cycle
that their phenology to the weather.
Capable of stocking spermatozoa
for up to eight months, the females decide
when is the best time for gestation to begin.
That moment is now, when food sources are abundant.
But the females will not give birth
to their young in Catalonia.
They will do so once they've returned to Northern Europe.
The countdown has begun.
The bats took three months to get
to their current destination,
but they'll need to be much faster on the way back.
In early May, after only a month and a half of traveling,
the pipistrelles from the church of Apšu have returned.
A few weeks later, they finally give birth to their young.
When they are born, the baby bats are completely blind,
bald, and incapable of flying.
They spend their days clinging to their mother,
absorbing her body heat and drinking her milk.
For Gunārs Pētersons, the return of the migrating bats
is his chance to check on the colony's overall health.
Usually they have twins, and during migration
as they mate with several other males,
so it's quite normal that one baby has one father
and another one, another father.
It's good that there is some exchange of genes
during migration as they meet the partners
which are not related to them coming from other populations.
Twice a year,
this minuscule bat discreetly flies
over the European continent.
For Nathusius' pipistrelles,
migration isn't just a way to escape the winter cold.
It's also a crucial period
in the creature's biological cycle.
Only the strongest bats will survive the perilous journey.
The mortality rate among the juveniles
seems rather high this year.
But the rest of the colony appears to be in good health.
And that's not the only positive news for Gunārs.
He's just received a letter from France.
Oh, great.
Madam Labente Daneo,
I can't read in French address properly.
So, it's a ringed bat with number LATVIA RIGA SA7925.
From this I know that it's ringed by us
and wearing only during migration in Pape.
And bat is killed by my cat, please find this ring,
found on a bat killed by my cat.
Ya, it's one more case when bat is killed by cat.
If I walk, then I can choose more direct way,
but probably not so direct as bats can,
and then I have to cover 1,955 kilometers.
For the question of how long it would take for one bat,
then the will be comparable flight from Pape.
This bat flies this distance in 29 nights.
This record-breaking bat
achieved a significant feat.
It flew from Latvia and traveled some 66 kilometers
per night for almost a month.
Proof of this species' incredible endurance.
In the tower of the church of Apšu,
the baby bats are now covered in thick fur.
Still feeding on motherly milk,
these juveniles form social bonds through play,
like all young mammals do.
In a few weeks time, the bravest ones
will jump out over the edge to take their first flight.
Clumsy at first, they won't venture far from the bell tower,
but they will quickly gain confidence
and eventually become fully autonomous.
That's when they'll accompany their mothers
on their great journey across Europe.
(dramatic music)