0:00:04 > 0:00:08Every year, spectacular seasons transform our planet.
0:00:11 > 0:00:14They're the driving force of all life on Earth...
0:00:16 > 0:00:20..bringing opportunities and huge challenges.
0:00:23 > 0:00:25Spring bursts with new life.
0:00:27 > 0:00:30But it's a race to grab fleeting chances.
0:00:32 > 0:00:35Summer is all about glorious abundance.
0:00:36 > 0:00:39But the heat can push animals to their limits.
0:00:41 > 0:00:43Autumn is nature's great gold rush.
0:00:47 > 0:00:49But competition is fierce.
0:00:52 > 0:00:55Winter creates a frozen wonderland.
0:00:57 > 0:01:00And only the most resourceful will survive.
0:01:08 > 0:01:10In every corner of the planet,
0:01:10 > 0:01:13animals rise to overcome the seemingly impossible...
0:01:15 > 0:01:17..and use extraordinary tricks...
0:01:18 > 0:01:20..to thrive against the odds...
0:01:21 > 0:01:25..as the seasons create the greatest shows on Earth.
0:01:36 > 0:01:37Spring.
0:01:38 > 0:01:42The sun's getting higher, the air's getting warmer...
0:01:45 > 0:01:48..and nature bursts back to life.
0:01:54 > 0:01:56But the clock is ticking.
0:01:56 > 0:01:58The good times are fleeting.
0:02:03 > 0:02:06Life in spring is one big race.
0:02:08 > 0:02:11It's a rush to grow up and find your way in the world.
0:02:13 > 0:02:16But often, the first challenge is simply...
0:02:18 > 0:02:19..getting out of bed.
0:02:25 > 0:02:26In the mountains of Alaska,
0:02:26 > 0:02:30there's one animal that's waking up after six months fast asleep.
0:02:33 > 0:02:39In early spring, all across North America, 60,000 grizzly bears come
0:02:39 > 0:02:41lumbering out of their dens.
0:02:41 > 0:02:45They've slept their way right through the worst of winter.
0:02:50 > 0:02:53This mother gave birth ten weeks ago.
0:02:54 > 0:02:57Her cubs have been fattening up on her milk ever since.
0:03:01 > 0:03:05They're now 20 times bigger than when they were born.
0:03:11 > 0:03:13But there's nothing to eat up here.
0:03:13 > 0:03:16She has to get them down to the warmth of the valleys.
0:03:20 > 0:03:23This is the first time these cubs have seen the outside world.
0:03:26 > 0:03:28But there's no time for playing about.
0:03:28 > 0:03:31Their mother hasn't eaten a thing since autumn.
0:03:34 > 0:03:38Every thousand feet they descend, the air gets one degree warmer.
0:03:40 > 0:03:43Every step takes them closer to spring.
0:03:45 > 0:03:47For some, it's a perilous journey.
0:03:49 > 0:03:53This family chose to make their den 8,000 feet up.
0:03:53 > 0:03:57It kept them safe from attack by other bears through the winter.
0:03:57 > 0:03:59But now they have to get down again.
0:04:06 > 0:04:09These cubs are two years old,
0:04:09 > 0:04:11and they've made this journey before.
0:04:11 > 0:04:14They don't seem too worried by the steep climb down.
0:04:27 > 0:04:32Their mother needs them to keep up, but not everyone's paying attention.
0:04:40 > 0:04:44In just a couple of months, this young bear will leave his mother for good
0:04:44 > 0:04:45and lead his own life...
0:04:47 > 0:04:50..and he's already showing signs of wilful independence.
0:04:54 > 0:04:57But this is probably not the best place to strike out on your own.
0:04:59 > 0:05:01He's in danger of getting lost.
0:05:12 > 0:05:13Bears are good climbers...
0:05:15 > 0:05:20..but this is a risky manoeuvre on a vertical icy slope.
0:05:24 > 0:05:26He got himself into this mess
0:05:26 > 0:05:29and he's going to have to get himself out.
0:05:29 > 0:05:31His mother can't wait for stragglers.
0:05:43 > 0:05:45BEAR PANTS
0:05:47 > 0:05:50This is probably the first time he's ever left her side...
0:05:51 > 0:05:54..and as he's still reliant on her,
0:05:54 > 0:05:55he needs to find her.
0:06:12 > 0:06:16For young grizzly bears, spring is full of life lessons.
0:06:17 > 0:06:21This time next year, they'll be finding their own way down.
0:06:21 > 0:06:24But, for now, Mother knows best.
0:06:27 > 0:06:32Meanwhile, in the lowlands, there's a magical transformation happening.
0:06:39 > 0:06:42In the valleys, the grizzly bears have left the snow behind.
0:06:44 > 0:06:46It's taken them days to get here,
0:06:46 > 0:06:48and they've barely eaten on the way.
0:06:50 > 0:06:52But it's ten degrees warmer down here.
0:06:52 > 0:06:54Spring has landed.
0:06:58 > 0:07:00And this is what they've come for -
0:07:00 > 0:07:02the fresh greens.
0:07:04 > 0:07:08At this time of year, it's all new growth, rich in sugar,
0:07:08 > 0:07:12and that's just what they need after their long winter fast.
0:07:15 > 0:07:18They've got through spring's first challenge -
0:07:18 > 0:07:21to wake up and find a feast.
0:07:25 > 0:07:29While thousands of grizzly bears are emerging across North America,
0:07:29 > 0:07:32further south, in Mexico,
0:07:32 > 0:07:37there's a tiny creature whose spring awakening is on a truly epic scale.
0:07:39 > 0:07:42In the forests of the Sierra Madre Mountains,
0:07:42 > 0:07:46the trees are festooned with monarch butterflies.
0:07:48 > 0:07:51They've spent the winter asleep, huddled together for warmth.
0:07:55 > 0:07:56There are millions of them.
0:07:58 > 0:08:00These trees have been the perfect roost.
0:08:01 > 0:08:04The branches shelter them like a blanket,
0:08:04 > 0:08:06keeping out the freezing nights.
0:08:10 > 0:08:15On a sunny spring morning, the butterflies begin to stir.
0:08:36 > 0:08:39When the sun warms their bodies above 13 degrees Centigrade...
0:08:41 > 0:08:42..they start to fly.
0:08:56 > 0:08:59More than 100 million take to the air.
0:08:59 > 0:09:03It's the biggest gathering of butterflies in the world.
0:09:07 > 0:09:10They're getting ready for a long and arduous journey.
0:09:10 > 0:09:14They stream from the colony and make their way to water to drink.
0:09:18 > 0:09:20Then they bask in the spring sun
0:09:20 > 0:09:22to get their muscles warmed up for flight.
0:09:28 > 0:09:29Soon they'll be off.
0:09:31 > 0:09:34They're all going in search of one very special plant...
0:09:38 > 0:09:40..and it only flowers in spring...
0:09:43 > 0:09:44..milkweed.
0:09:45 > 0:09:48The butterflies won't lay their eggs on any other plant,
0:09:48 > 0:09:51so they must time their wake-up perfectly
0:09:51 > 0:09:53for when the milkweed blooms.
0:09:55 > 0:09:59Generations of butterflies fly 3,000 miles to Canada...
0:10:01 > 0:10:04..following the flowering of this one particular plant.
0:10:05 > 0:10:08It's the greatest insect migration on Earth,
0:10:08 > 0:10:12and it only starts when the sun wakes them up in spring.
0:10:17 > 0:10:20Life in spring is all about timing.
0:10:20 > 0:10:22As soon as the light returns,
0:10:22 > 0:10:25animals have to be ready to seize the moment.
0:10:25 > 0:10:29And, for some, that means attracting a mate.
0:10:38 > 0:10:42For a few weeks, birds suddenly become sensitive to the longer days.
0:10:44 > 0:10:47The extra sunlight flicks a switch deep inside their brains...
0:10:48 > 0:10:51..and this kick-starts their sex hormones.
0:10:53 > 0:10:58All around the world, song birds sing at dawn to keep out rivals
0:10:58 > 0:10:59and attract mates.
0:11:03 > 0:11:06But it's not just singing that'll get you noticed.
0:11:06 > 0:11:11As males rush to impress females, it leads to some outrageous flirting.
0:11:13 > 0:11:17In the North American grasslands, a sage grouse woos the females by
0:11:17 > 0:11:20inflating his magnificent chest sacs.
0:11:26 > 0:11:31An Asian Temminck's tragopan starts by hiding from the female.
0:11:32 > 0:11:34But then he plays peekaboo,
0:11:34 > 0:11:37trying to dazzle her with his exotic good looks.
0:11:43 > 0:11:47And in India, a peacock shakes his flamboyant tail,
0:11:47 > 0:11:51hoping to impress a passing hen by shimmying his feathers.
0:11:52 > 0:11:55Success is never guaranteed.
0:11:59 > 0:12:02On the lakes of North America,
0:12:02 > 0:12:05one bird's courtship display is unlike any other.
0:12:10 > 0:12:11Clark's grebes.
0:12:13 > 0:12:15They've just flown in from the Pacific coast,
0:12:15 > 0:12:17and here in Oregon is where they'll breed.
0:12:20 > 0:12:24At this time of year, these sheltered freshwater lakes are
0:12:24 > 0:12:27the perfect place to raise a family.
0:12:30 > 0:12:32These two lovebirds paired up last season.
0:12:33 > 0:12:37But they've spent the winter apart so they need to get reacquainted.
0:12:40 > 0:12:42If they stick together as parents,
0:12:42 > 0:12:45they'll give their chicks the best start in life.
0:12:48 > 0:12:51They need to prove their mutual devotion...
0:12:53 > 0:12:54..so they dance.
0:13:03 > 0:13:06They start by copying each other's movements
0:13:06 > 0:13:08as they preen their feathers.
0:13:19 > 0:13:23Then, to show he really means it, the male will find a fish...
0:13:26 > 0:13:28..and offer it to her as a gift.
0:13:52 > 0:13:54With eyes only for each other,
0:13:54 > 0:13:58they reach the climax of their romantic display.
0:14:15 > 0:14:21Only grebes can do this. No other bird this heavy can run on water.
0:14:22 > 0:14:27But this display might help them get their breeding cycles in sync.
0:14:41 > 0:14:42At this time of year,
0:14:42 > 0:14:46the pressure on males to impress a female is relentless.
0:14:48 > 0:14:52One ploy is to provide her with the perfect nest to lay her eggs.
0:14:56 > 0:15:00But if you live in Antarctica, the most desolate place on Earth...
0:15:01 > 0:15:04..nest-building materials are hard to come by.
0:15:13 > 0:15:16At Cape Crozier,
0:15:16 > 0:15:18Adelie penguins are in a hurry.
0:15:22 > 0:15:25They've spent the whole winter at sea but,
0:15:25 > 0:15:30as the days get longer, more than 150,000 come rushing back to land...
0:15:32 > 0:15:33..to breed.
0:15:36 > 0:15:38The season is so short here,
0:15:38 > 0:15:40they have to be in the right place to build
0:15:40 > 0:15:43a nest as soon as the ice thaws.
0:15:47 > 0:15:49The males are first to arrive,
0:15:49 > 0:15:52racing each other to stake a claim to last year's patch.
0:15:54 > 0:15:57But with over 60 miles of ice to cross,
0:15:57 > 0:16:00just getting to the nest site is a challenge in itself.
0:16:18 > 0:16:20They're heading for this rocky slope,
0:16:20 > 0:16:23the first to feel the warmth of the spring sunshine.
0:16:30 > 0:16:33When the females arrive in a few days' time,
0:16:33 > 0:16:35the males must impress them with a good-looking nest.
0:16:37 > 0:16:40But with temperatures rarely above freezing,
0:16:40 > 0:16:42no trees or shrubs can grow here.
0:16:45 > 0:16:47One thing there's plenty of...
0:16:48 > 0:16:49..is pebbles.
0:16:56 > 0:17:00A neat little pebble nest will stop the eggs rolling around on the ice.
0:17:07 > 0:17:12But finding just the right stones is a time-consuming business.
0:17:16 > 0:17:19To get ahead, some are prepared to cheat.
0:18:02 > 0:18:04The victim senses something's wrong...
0:18:06 > 0:18:10..but can't quite put his finger on what.
0:18:16 > 0:18:19The thief's nest is coming along nicely.
0:18:23 > 0:18:26But even a villain can become a victim.
0:18:26 > 0:18:27PENGUIN SQUAWKS
0:18:30 > 0:18:33Although he's a lot quicker to spot when he's being robbed.
0:18:33 > 0:18:34PENGUIN SQUAWKS
0:18:34 > 0:18:39Well, he would, because it takes one to know one.
0:18:42 > 0:18:43PENGUINS SQUAWK
0:18:43 > 0:18:45Finally, they're finished,
0:18:45 > 0:18:47and just in the nick of time -
0:18:47 > 0:18:49the females are back.
0:18:52 > 0:18:55The first to arrive will grab the best-looking nests.
0:18:58 > 0:19:02But what also impresses them is a male with a good voice.
0:19:04 > 0:19:05PENGUINS SQUAWK AND CHATTER
0:19:05 > 0:19:09A well-held note is the sign of a well-fed male.
0:19:11 > 0:19:14And that's just what the female wants.
0:19:14 > 0:19:18It shows he has the resources to stay and incubate the egg while
0:19:18 > 0:19:20she's off feeding out at sea.
0:19:23 > 0:19:27Choices made and thousands of couples start breeding.
0:19:33 > 0:19:38Within six weeks, the slopes are crowded with spring chicks.
0:19:40 > 0:19:43CHICK CHEEPS
0:19:57 > 0:20:01This is the season when many animals race to pair up and breed.
0:20:01 > 0:20:06All this activity leads to one thing - an abundance of babies.
0:20:14 > 0:20:17With more hours of daylight and plenty of food,
0:20:17 > 0:20:21spring is the perfect time for young animals to fatten up and grow.
0:20:24 > 0:20:26But the season moves fast.
0:20:26 > 0:20:28And it won't be long before
0:20:28 > 0:20:30they have to start fending for themselves.
0:20:30 > 0:20:33So it's also the time to learn the basics of survival.
0:20:39 > 0:20:41This Alaskan grizzly bear mother
0:20:41 > 0:20:43might look like she's just playing...
0:20:45 > 0:20:47..but this rough and tumble
0:20:47 > 0:20:50is teaching her four-month-old cub a vital lesson...
0:20:53 > 0:20:54..how to hold his own in fights
0:20:54 > 0:20:57with other males when he's a grown-up bear.
0:21:01 > 0:21:06This otter mum in Yellowstone is taking her ten-week-old cubs for
0:21:06 > 0:21:07a fishing lesson.
0:21:08 > 0:21:13They're already expert swimmers, but catching a cut-throat trout is
0:21:13 > 0:21:14beyond their skills.
0:21:19 > 0:21:21So it's a case of watch...
0:21:21 > 0:21:23OTTER SQUEALS ..and learn.
0:21:30 > 0:21:32And, for now,
0:21:32 > 0:21:34they'll have to make do with caviar.
0:21:45 > 0:21:50The Pantanal in Brazil, the largest wetlands in the world.
0:21:52 > 0:21:53It's the end of the dry season.
0:21:55 > 0:21:58For a few weeks, this is their spring.
0:21:58 > 0:22:03Light showers of rain bring lush grasses and swarms of insects
0:22:03 > 0:22:06before the Pantanal is flooded by summer storms.
0:22:07 > 0:22:09This is when animals have their young...
0:22:11 > 0:22:14..born into a world where there's plenty to eat.
0:22:21 > 0:22:26In a nearby forest, the trees are full of ring-tailed coatis.
0:22:31 > 0:22:36They're related to raccoons and they stick together in big family groups.
0:22:37 > 0:22:39While their pups are young,
0:22:39 > 0:22:42they're kept out of danger in the safety of the woods.
0:22:45 > 0:22:47But now they're ten weeks old...
0:22:48 > 0:22:52..and it's time to make their first trip out into the spring grasslands
0:22:52 > 0:22:54to start looking for their own food.
0:22:59 > 0:23:00COATI SQUEAKS
0:23:06 > 0:23:11There may be 30 in the family band so there should be plenty of adults
0:23:11 > 0:23:12keeping watch.
0:23:15 > 0:23:18In this wide-open space, it's easy to get lost.
0:23:20 > 0:23:25So for every young coati, the first rule to remember is never lose sight
0:23:25 > 0:23:26of your mother's tail.
0:23:38 > 0:23:43They're surrounded by unfamiliar faces, but who's friendly...
0:23:44 > 0:23:45..and who should you avoid?
0:23:48 > 0:23:51Southern lapwings are good to have around.
0:23:52 > 0:23:56They follow the coatis, picking up little bugs the family leave behind.
0:23:58 > 0:24:01In return, they keep an eye out for predators.
0:24:06 > 0:24:08Armadillos only eat insects...
0:24:12 > 0:24:13..and so do giant anteaters.
0:24:16 > 0:24:17Nothing to worry about here.
0:24:22 > 0:24:27But this savanna hawk is watching with a less than friendly interest.
0:24:27 > 0:24:28LAPWINGS TRILL
0:24:42 > 0:24:45The mother makes a dash for the forest to get her pups to safety.
0:24:51 > 0:24:55But with so many babies, it's hard to check if everyone's made it home.
0:24:59 > 0:25:01COATI SQUEAKS
0:25:02 > 0:25:05Sure enough, this pup's got left behind.
0:25:07 > 0:25:08COATI SQUEAKS
0:25:10 > 0:25:12Mistakes like this can be fatal.
0:25:18 > 0:25:20COATI SQUEAKS
0:25:34 > 0:25:37He knows he must somehow get back to the safety of the forest.
0:25:57 > 0:25:59Spring is full of perils.
0:26:02 > 0:26:05And this coati pup just got a little wiser.
0:26:06 > 0:26:09Round here, it pays to stick with your family.
0:26:23 > 0:26:26As the spring sun warms the air,
0:26:26 > 0:26:29the urge to breed isn't just confined to animals.
0:26:30 > 0:26:33Plants are doing it, too,
0:26:33 > 0:26:35and they have some clever tactics.
0:26:43 > 0:26:47In Japan, the mass blossoming of cherry trees is one of the planet's
0:26:47 > 0:26:50most spectacular spring events.
0:26:50 > 0:26:54The trees need to attract insects to spread their pollen,
0:26:54 > 0:26:57but they're only fertile for a few days,
0:26:57 > 0:27:02so they flower all at the same time in one glorious show.
0:27:07 > 0:27:10Their extra-sweet nectar keeps the insects coming back for more.
0:27:16 > 0:27:21North American water lilies spend all winter asleep under the ice but,
0:27:21 > 0:27:26in spring, it takes them just four days to rise through the water
0:27:26 > 0:27:27and bloom.
0:27:32 > 0:27:37As the sun thaws the frozen North, the Arctic poppy flowers.
0:27:40 > 0:27:43Not many plants can survive above the Arctic Circle,
0:27:43 > 0:27:46but this one has a special trick.
0:27:48 > 0:27:50As the sun passes overhead,
0:27:50 > 0:27:54it follows its progress, capturing extra warmth...
0:27:55 > 0:27:58..and pollinating insects linger
0:27:58 > 0:28:00a little longer inside its cosy petals.
0:28:04 > 0:28:07A British woodland in early spring.
0:28:07 > 0:28:10The trees are still bare.
0:28:10 > 0:28:15But down on the floor, there's a whole host of plants just waiting to
0:28:15 > 0:28:16grab their moment in the sun.
0:28:21 > 0:28:23Early flowers race to bloom
0:28:23 > 0:28:27before the trees grow their leaves and steal the daylight.
0:28:40 > 0:28:43With tough competition, you need a trick or two to get ahead.
0:28:48 > 0:28:51Yellow and white flowers reflect more light,
0:28:51 > 0:28:54making them stand out to insects down here in the shade.
0:29:09 > 0:29:14In this race for the light, one plant puts on a show on a scale seen
0:29:14 > 0:29:16nowhere else on Earth.
0:29:20 > 0:29:21With their rich soils,
0:29:21 > 0:29:25Britain's ancient woodlands are home to half the world's population...
0:29:26 > 0:29:27..of bluebells.
0:29:33 > 0:29:37These vibrant carpets of flowers give bees a vital boost of nectar at
0:29:37 > 0:29:39this early time of year.
0:29:52 > 0:29:55But spring flowers need more than just sunshine.
0:29:56 > 0:29:58They need rain.
0:30:01 > 0:30:02In South Africa,
0:30:02 > 0:30:06one of the driest places on Earth is about to be transformed.
0:30:06 > 0:30:08THUNDER RUMBLES
0:30:13 > 0:30:15The Namaqua Desert,
0:30:15 > 0:30:19a 600-mile strip of land along South Africa's west coast.
0:30:21 > 0:30:24For most of the year, this is an arid place,
0:30:24 > 0:30:27just sand and wind-blasted rock.
0:30:30 > 0:30:33But for a few weeks after the winter rains,
0:30:33 > 0:30:36spring puts on a dazzling display.
0:30:59 > 0:31:04These Namaqua daisies don't waste precious energy producing scent.
0:31:04 > 0:31:07They want to attract one special pollinating insect...
0:31:09 > 0:31:12..and that insect is only drawn to colour.
0:31:13 > 0:31:14And here he is...
0:31:16 > 0:31:17..a monkey beetle.
0:31:19 > 0:31:22He's spent the winter underground, but he's come out
0:31:22 > 0:31:24just in time for the flower show.
0:31:27 > 0:31:29As he hops from flower to flower...
0:31:31 > 0:31:32..he's spreading the daises' pollen...
0:31:35 > 0:31:38..but he's only interested in finding a female.
0:31:42 > 0:31:43There's one.
0:31:44 > 0:31:46She's a golden beauty.
0:31:49 > 0:31:50But he's got competition.
0:32:01 > 0:32:03The female is so busy feeding,
0:32:03 > 0:32:06she doesn't even notice the two males brawling over her.
0:32:12 > 0:32:15But the battling beetles are good news for the flower.
0:32:15 > 0:32:17They're now covered in pollen,
0:32:17 > 0:32:20which they'll spread to the next flower they visit.
0:32:24 > 0:32:26The female has lost interest.
0:32:28 > 0:32:30But the males fight on.
0:32:54 > 0:32:55He's won the brawl...
0:32:56 > 0:32:57..but the female's gone.
0:33:03 > 0:33:06With so many beetles drawn to these flowers,
0:33:06 > 0:33:07he shouldn't be single for long.
0:33:15 > 0:33:18A result - the beetle finds a mate...
0:33:21 > 0:33:23..and the flowers are pollinated.
0:33:27 > 0:33:31Now next year's spring display will be just as spectacular.
0:33:37 > 0:33:38When winter releases its hold,
0:33:38 > 0:33:43spring can bring great opportunities for those prepared to take advantage of them.
0:33:45 > 0:33:49In the search for food or a safe place to breed,
0:33:49 > 0:33:52many animals will make extraordinary journeys.
0:33:59 > 0:34:02In the Arctic, the ocean fills with fish.
0:34:04 > 0:34:06Little auks fly nearly 2,000 miles
0:34:06 > 0:34:09to Svalbard just to raise their chicks on
0:34:09 > 0:34:11the cliffs around these teeming waters.
0:34:15 > 0:34:17Off the coast of Alaska,
0:34:17 > 0:34:19millions of Pacific herring
0:34:19 > 0:34:23swim up from the depths to spawn in the warming shallows.
0:34:28 > 0:34:31As the Arctic tundra unlocks in Greenland,
0:34:31 > 0:34:35snow geese fly all the way from Mexico to raise their young
0:34:35 > 0:34:37on the fresh new grass.
0:34:43 > 0:34:47These long migrations can be gruelling tests of endurance.
0:34:48 > 0:34:55In Africa, one small bird is about to set off on a 3,000-mile journey
0:34:55 > 0:34:58across one of the most inhospitable places on Earth.
0:34:58 > 0:35:00SWALLOW CHEEPS
0:35:02 > 0:35:05Barn swallows have spent the winter in Nigeria...
0:35:07 > 0:35:09..but the rising temperatures are their cue to leave.
0:35:12 > 0:35:15Two million birds are on the move.
0:35:18 > 0:35:20There's plenty of food here,
0:35:20 > 0:35:22but it's not the best place to bring up chicks.
0:35:24 > 0:35:25There's too much competition...
0:35:28 > 0:35:30..so they're heading for spring in Europe.
0:35:38 > 0:35:40These birds are tiny.
0:35:40 > 0:35:44They weigh not much more than a couple of pound coins,
0:35:44 > 0:35:47but they're about to undertake one of nature's toughest migrations.
0:35:50 > 0:35:52Ahead of them, there's a deathtrap.
0:35:58 > 0:36:00The Sahara Desert.
0:36:03 > 0:36:05It's much too big to fly around.
0:36:07 > 0:36:10The swallows have no choice but to fly across it.
0:36:14 > 0:36:171,000 miles of almost nothing but sand.
0:36:27 > 0:36:30It will take them four days to get across.
0:36:30 > 0:36:34Somehow, they will have to find water on the way
0:36:34 > 0:36:36otherwise they will die of thirst.
0:36:45 > 0:36:46As they cross Libya...
0:36:47 > 0:36:52..in an ocean of sand, there's a tiny speck of blue.
0:37:00 > 0:37:02Umm al-Maa,
0:37:02 > 0:37:07a lake of ancient ground water seeping to the surface.
0:37:07 > 0:37:10But this oasis is not what it seems.
0:37:15 > 0:37:19Over thousands of years, the lake has been evaporating.
0:37:19 > 0:37:21It's nearly five times saltier than the sea.
0:37:23 > 0:37:26If the swallows tried to drink this, it would kill them.
0:37:28 > 0:37:29FLIES BUZZ
0:37:29 > 0:37:32But there's an unexpected solution.
0:37:32 > 0:37:36At this time of year, thousands of brine flies emerge from the lake.
0:37:38 > 0:37:39Their bodies filter out the salt...
0:37:42 > 0:37:46..so, effectively, they're tiny packages of fresh water.
0:37:48 > 0:37:52The swallows gorge on them and get all the water they need.
0:37:55 > 0:37:58And they're not the only ones.
0:37:58 > 0:38:01Wagtails also break their northern journey for the fly bonanza.
0:38:10 > 0:38:13This is the swallows' only stopover,
0:38:13 > 0:38:15but it's enough to get them across the desert.
0:38:21 > 0:38:23By the time they reach Europe,
0:38:23 > 0:38:25spring has arrived in the Northern Hemisphere.
0:38:29 > 0:38:33They now have four months to feed up and raise their family before they
0:38:33 > 0:38:36fly all the way back to Africa.
0:38:46 > 0:38:48For other animals on the move,
0:38:48 > 0:38:52the challenge is not so much about distance, but navigation.
0:38:57 > 0:38:59In the Arctic, spring unlocks
0:38:59 > 0:39:02the land, but the vast areas of frozen sea
0:39:02 > 0:39:03take longer to crack.
0:39:05 > 0:39:07So animals here must find their way
0:39:07 > 0:39:10through a constantly shifting landscape.
0:39:13 > 0:39:18As the sun gets more intense and the sea ice melts, pools begin to form.
0:39:20 > 0:39:25The darker water absorbs warmth and the pace of the ice melt increases.
0:39:34 > 0:39:37The great sheet of ice starts to split.
0:39:39 > 0:39:43These huge cracks are known as leads,
0:39:43 > 0:39:46and they're just what one animal is looking for.
0:39:53 > 0:39:54Narwhal.
0:39:57 > 0:40:03These strange whales are some of the most secretive animals in the ocean.
0:40:04 > 0:40:07Over winter, they've been feeding at
0:40:07 > 0:40:09the edge of the sea ice but, in spring,
0:40:09 > 0:40:14as the ice begins to shrink, predators start to move in.
0:40:20 > 0:40:21Killer whales.
0:40:22 > 0:40:24They have a taste for narwhal.
0:40:28 > 0:40:32To avoid them, the narwhal begin a 600-mile journey north to find
0:40:32 > 0:40:34safer waters close to land.
0:40:37 > 0:40:40And these giant leads are the only pathway through.
0:40:57 > 0:40:59It's a hazardous journey.
0:40:59 > 0:41:01These cracks could close over at any time.
0:41:03 > 0:41:04The narwhal will have to navigate
0:41:04 > 0:41:07carefully through a constantly shifting maze of ice.
0:41:21 > 0:41:24They've barely got going and they've hit a dead end.
0:41:26 > 0:41:29They use their heads and tusks to try to batter their way through.
0:41:40 > 0:41:41The ice is too thick.
0:41:44 > 0:41:48Rather than go back, their only option is to look for another lead.
0:41:51 > 0:41:53But that means going under.
0:41:57 > 0:42:00They need to surface every 15 minutes to breathe...
0:42:01 > 0:42:04..or they'll drown,
0:42:04 > 0:42:06so it's a constant search for the next airhole.
0:42:28 > 0:42:30They've found another lead.
0:42:36 > 0:42:38But the gap has suddenly narrowed.
0:42:40 > 0:42:42There's barely room for one-way traffic.
0:42:47 > 0:42:50And they hit another snag.
0:42:50 > 0:42:53Another pod of narwhal is trying to find their way through,
0:42:53 > 0:42:56and they're coming from the opposite direction.
0:42:58 > 0:42:59It's a stand-off.
0:43:10 > 0:43:12There's nothing to be gained from a fight.
0:43:12 > 0:43:16The newcomers give way and everyone continues in the same direction.
0:43:24 > 0:43:25These tussles will soon be over.
0:43:34 > 0:43:35The Arctic sea ice will melt...
0:43:37 > 0:43:41..and the narwhal will spend their summer breeding in the bays.
0:43:57 > 0:43:59Some animals spend their whole life travelling.
0:44:02 > 0:44:04In the North American tundra,
0:44:04 > 0:44:08caribou make the longest migration of any land mammal on Earth.
0:44:12 > 0:44:15When calves are born in the spring, they have to be ready to run.
0:44:18 > 0:44:21This far north, by the end of spring,
0:44:21 > 0:44:25the sun is warming the tundra almost 24 hours a day.
0:44:25 > 0:44:27The grass is fresh and nutritious,
0:44:27 > 0:44:30and the caribou barely stop eating.
0:44:42 > 0:44:44More than two million animals
0:44:44 > 0:44:47move across the tundra on a constant search
0:44:47 > 0:44:48for the best grazing.
0:44:56 > 0:45:00They might travel over 20 miles a day without even stopping to sleep.
0:45:05 > 0:45:08But while they're benefiting from spring's good times,
0:45:08 > 0:45:10it's about to bring out their worst enemy.
0:45:14 > 0:45:15At this time of year,
0:45:15 > 0:45:18the sun is strong enough to melt the permafrost,
0:45:18 > 0:45:21the frozen soil beneath the grass.
0:45:21 > 0:45:27The tundra is covered in pools of water, and they're the perfect place
0:45:27 > 0:45:29for mosquitoes to breed.
0:45:35 > 0:45:39Great clouds of them emerge and head straight for the caribou.
0:45:49 > 0:45:52The mosquitoes are voracious.
0:45:52 > 0:45:55They can drain half a pint of blood in a day.
0:45:56 > 0:45:59They drive the caribou crazy.
0:46:03 > 0:46:06The caribou do anything they can to avoid being bitten.
0:46:08 > 0:46:11The ones on the outside of the herd are the most exposed...
0:46:14 > 0:46:18..so everyone tries to shuffle into the middle for maximum protection.
0:46:27 > 0:46:31When they can't stand it any more, they bolt to higher ground.
0:46:48 > 0:46:52Up here, there are still patches of snow, and it's too cold for
0:46:52 > 0:46:54the mosquitoes to follow.
0:46:56 > 0:46:58The caribou finally get some relief.
0:47:08 > 0:47:11The constant search for fresh grass
0:47:11 > 0:47:14drives the caribou's nomadic way of life.
0:47:14 > 0:47:19Every year, they cover distances of 3,000 miles.
0:47:29 > 0:47:34Round the world, spring is one big race to grab the good times.
0:47:35 > 0:47:38The increasing power of the sun makes plants grow...
0:47:39 > 0:47:41..and brings out the insects...
0:47:42 > 0:47:45..and that provides food for everything else.
0:47:50 > 0:47:55As the sun's strength increases, in forests, a feast is about to appear.
0:47:57 > 0:48:02After 1,000 hours of winter chilling, buds start to burst.
0:48:04 > 0:48:07Inside each one, the leaves are neatly pleated,
0:48:07 > 0:48:10spring-loaded to expand as quickly
0:48:10 > 0:48:13as possible and start capturing the sun.
0:48:23 > 0:48:26But from the moment they unfurl, they're under attack.
0:48:32 > 0:48:35Caterpillars are insatiable,
0:48:35 > 0:48:39but they're a vital link in the spring food chain.
0:48:44 > 0:48:49Bluetits feed each of their chicks 15,000 caterpillars by the time
0:48:49 > 0:48:50they leave the nest.
0:48:55 > 0:48:58Even in rivers, sunlight brings out the insects.
0:49:00 > 0:49:01In southern France,
0:49:01 > 0:49:05a damselfly is just beginning her new life as an adult.
0:49:11 > 0:49:14But when there's a hungry marsh frog in the water...
0:49:17 > 0:49:18..it's rather brief.
0:49:23 > 0:49:27In some places, the emergence of insects is truly sensational.
0:49:29 > 0:49:32Beneath the surface of an English river,
0:49:32 > 0:49:35mayflies are beginning to stir.
0:49:37 > 0:49:41They've spent the last two years living underwater as larvae.
0:49:43 > 0:49:46But on one day in spring,
0:49:46 > 0:49:48as the sun gets more intense,
0:49:48 > 0:49:53they emerge as adults in spectacular numbers.
0:49:56 > 0:49:59Their only mission is to find a mate and lay their eggs.
0:50:02 > 0:50:06By hatching all together, they increase their chances of success.
0:50:14 > 0:50:18They may only live for 24 hours, so they have to be quick.
0:50:29 > 0:50:33They appear like this on rivers across Europe every spring.
0:50:37 > 0:50:41It's a brief free-for-all, and everyone joins in.
0:51:04 > 0:51:08But millions escape and live just long enough to lay their eggs.
0:51:11 > 0:51:14The show's over until next spring.
0:51:18 > 0:51:20But it's in the open ocean
0:51:20 > 0:51:24that spring puts on the most spectacular feast of all.
0:51:27 > 0:51:29It all starts very small.
0:51:29 > 0:51:32In seas with plenty of nutrients,
0:51:32 > 0:51:37the increasing sunlight sparks the growth of microscopic plankton.
0:51:40 > 0:51:44This assorted collection of tiny life forms drifts around with
0:51:44 > 0:51:48the currents but, in spring,
0:51:48 > 0:51:49their population explodes.
0:51:53 > 0:51:57They multiply 1,000 times in just a few days and create great
0:51:57 > 0:52:02oceanic blooms that stretch for hundreds of miles.
0:52:06 > 0:52:09They're so big, they can be seen from Space.
0:52:19 > 0:52:23These explosions of plankton are so rich,
0:52:23 > 0:52:27they can support some of the biggest animals on Earth.
0:52:27 > 0:52:29WHALE WHISTLES
0:52:34 > 0:52:39Humpback whales - a mother and her two-week-old calf.
0:52:47 > 0:52:50They've just left Hawaii, where her baby was born.
0:52:52 > 0:52:56They're heading north to the plankton-rich seas off Alaska.
0:53:04 > 0:53:08The warm waters of Hawaii were the perfect nursery,
0:53:08 > 0:53:10but there wasn't enough food to sustain them.
0:53:10 > 0:53:13She's had nothing to eat for six months,
0:53:13 > 0:53:15and she's still providing her calf
0:53:15 > 0:53:18with more than 100 litres of milk a day.
0:53:23 > 0:53:26They have a 3,000-mile journey ahead of them.
0:53:29 > 0:53:34Together, they have to cross the North Pacific, the world's biggest ocean.
0:53:39 > 0:53:41It's a tough journey for an adult.
0:53:41 > 0:53:44For a tiny calf, it's a marathon.
0:53:46 > 0:53:50His mother slows her pace to let him keep up and supports him from below
0:53:50 > 0:53:51when he gets tired.
0:53:58 > 0:54:00But if she times it right,
0:54:00 > 0:54:04they'll arrive to a feast of such epic proportions,
0:54:04 > 0:54:06it will keep them fed for months.
0:54:10 > 0:54:14In the seas off Alaska, the water is filling with life.
0:54:17 > 0:54:21Vast numbers of animals are turning up to graze on the plankton.
0:54:26 > 0:54:31Billions of krill arrive - tiny shrimp-like animals.
0:54:35 > 0:54:38A single swarm can contain two million tonnes of them.
0:54:41 > 0:54:44They're one of the most abundant animals on the planet.
0:54:48 > 0:54:52And that, in turn, feeds a lot of fish.
0:55:00 > 0:55:04Huge shoals of herring come up from the depths to feed on the krill.
0:55:15 > 0:55:18And they are the feast that everyone has been waiting for.
0:55:23 > 0:55:27Sea birds have travelled thousands of miles to make the most of it.
0:55:39 > 0:55:41And the herring bonanza draws in
0:55:41 > 0:55:43one of the mightiest predators of them all.
0:56:09 > 0:56:13For a humpback whale and her calf, this is what their journey has all been for.
0:56:16 > 0:56:19She might eat up to a tonne of herring a day.
0:56:22 > 0:56:25It's a feast that will carry on all through summer.
0:56:34 > 0:56:38All across the world, spring brings unique opportunities.
0:56:40 > 0:56:42But you've got to be ready to make the most of them.
0:56:44 > 0:56:47Those who time it right will reap the rewards.
0:56:50 > 0:56:54In Britain, barn swallows will spend the long summer days feeding up
0:56:54 > 0:56:56before they head back to Africa.
0:56:58 > 0:57:00In Alaska, young grizzly bears
0:57:00 > 0:57:03will spend a few more months with their mothers,
0:57:03 > 0:57:06learning all they need to know for a life on their own.
0:57:12 > 0:57:17In Antarctica, Adelie penguin parents will fatten up their chicks
0:57:17 > 0:57:18with the bounty from the sea.
0:57:23 > 0:57:25Spring may have brought challenges
0:57:25 > 0:57:31but, for many animals, it's given them the perfect start in life.