0:00:24 > 0:00:27It's a universal dream to fly like a bird.
0:00:29 > 0:00:32To soar on wings into the heavens.
0:00:39 > 0:00:42But it's nothing compared to the reality.
0:00:50 > 0:00:53Experience our planet as never before.
0:00:58 > 0:01:01A bird's-eye view.
0:01:09 > 0:01:12This is a journey that will embrace the world...
0:01:20 > 0:01:23..showing us astonishing natural events...
0:01:30 > 0:01:32..extreme challenges...
0:01:39 > 0:01:40..and hard-won rewards.
0:01:49 > 0:01:51This is the world on the wing.
0:02:14 > 0:02:18It's spring in the Gulf of Mexico, in the far south of North America.
0:02:20 > 0:02:23Families of snow geese are soon to begin an epic journey
0:02:23 > 0:02:25across a continent.
0:02:26 > 0:02:30It's a migration made by millions upon millions of birds.
0:02:36 > 0:02:37Before they leave,
0:02:37 > 0:02:40individual families join thousands of other geese
0:02:40 > 0:02:43preparing for the great trek northwards.
0:02:44 > 0:02:49They become one of a crowd made up of hundreds of other families.
0:03:09 > 0:03:13But this spectacular gathering doesn't go unnoticed.
0:03:13 > 0:03:17Out of the heavens soars America's national bird -
0:03:17 > 0:03:19an aerial predator of cunning and power.
0:03:22 > 0:03:26The parents warn their young, and the message spreads like wildfire.
0:03:43 > 0:03:47The bald eagle flushes the birds, checking for signs of weakness.
0:04:00 > 0:04:02The panic spreads like a Mexican wave,
0:04:02 > 0:04:06but still, among the masses, the families stick together.
0:04:20 > 0:04:22As more and more families take to the air,
0:04:22 > 0:04:26they create a snowstorm of geese that confuses the eagle.
0:04:37 > 0:04:42The massed ranks create a whiteout, impossible to see through.
0:04:50 > 0:04:53She must look for easier options.
0:04:55 > 0:04:58Overwintering coots are a far better bet.
0:05:00 > 0:05:03Unlike geese, they need a long take-off.
0:05:16 > 0:05:18They are also weak fliers.
0:05:18 > 0:05:20It heads back to the water...
0:05:30 > 0:05:33and performs a disappearing act...
0:05:36 > 0:05:38..diving two metres below the surface.
0:05:40 > 0:05:41But the game isn't up yet.
0:05:44 > 0:05:46Another coot dives, but the water's shallow,
0:05:46 > 0:05:50and the eagle's pin-sharp vision can see him.
0:06:03 > 0:06:04It's a fatal mistake.
0:06:09 > 0:06:12The eagle will follow the migrating flocks,
0:06:12 > 0:06:14hoping for yet more opportunities.
0:06:18 > 0:06:20The snow geese have a tight schedule to keep
0:06:20 > 0:06:23if they are to reach the Arctic in time to breed.
0:06:23 > 0:06:25It's 3,000 miles away.
0:06:36 > 0:06:40They become part of a spring migration across the continent,
0:06:40 > 0:06:43one that features millions of other birds.
0:06:57 > 0:07:01Brown pelicans are one of America's most charismatic birds.
0:07:03 > 0:07:06They breed in the warm waters of Baja California,
0:07:06 > 0:07:09but travel north in search of fish.
0:07:17 > 0:07:20The brown juveniles are still learning the ways of the ocean.
0:07:20 > 0:07:25Their more colourful elders have many secrets to pass on.
0:07:28 > 0:07:33The youngsters must learn the habits of every sea creature below.
0:07:42 > 0:07:46He soon discovers that some can guide him to a meal.
0:07:58 > 0:08:01Humpback whales are the easiest to spot -
0:08:01 > 0:08:05they have travelled over 2,500 miles from the Arctic to breed here.
0:08:08 > 0:08:13The males announce their arrival by slapping their five-metre long fins.
0:08:26 > 0:08:30But this "breaching" is a more spectacular way of gaining attention.
0:08:40 > 0:08:4335 tons of blubber make quite a splash!
0:08:50 > 0:08:55The pelican soon discovers that these show-offs have nothing to divulge at this time of year.
0:08:55 > 0:08:58They are too preoccupied with each other.
0:09:09 > 0:09:12Dolphins, on the other hand, are a different proposition.
0:09:15 > 0:09:19The youngsters learn that dolphins follow schools of anchovies.
0:09:28 > 0:09:32Huge pods form around the biggest shoals.
0:09:37 > 0:09:41The pelicans take their cue to dive from the dolphins.
0:09:50 > 0:09:53The deeper the fish, the higher the birds dive.
0:10:00 > 0:10:04The young pelican is perfectly adapted for plunge attacks.
0:10:06 > 0:10:07As he hits the surface,
0:10:07 > 0:10:12his legs and wings thrust backwards, forcing his bill around his prey.
0:10:24 > 0:10:28His pouch gapes, sucking in ten litres of water
0:10:28 > 0:10:31as his upper bill closes the trap.
0:11:02 > 0:11:08Brown pelicans leave these pleasant, balmy seas when fishing opportunities arise further north.
0:11:12 > 0:11:17Birds of prey arriving from South America have their own appointments to keep.
0:11:21 > 0:11:24Many red-tail hawks stop for a bite to eat
0:11:24 > 0:11:27at Bracken Cave in southern Texas.
0:11:31 > 0:11:33High on the menu are 20 million free-tailed bats
0:11:33 > 0:11:37that have just arrived from Mexico to breed.
0:11:39 > 0:11:43It's the largest gathering of mammals on the planet.
0:11:49 > 0:11:51As the bats leave to feed each evening,
0:11:51 > 0:11:54they run the gauntlet of the waiting hunters.
0:11:56 > 0:12:00For a rookie hawk that's just arrived, they're quite a challenge.
0:12:09 > 0:12:12She has to learn how to catch them.
0:12:26 > 0:12:31But she's pitched against the fastest and most agile bats in the world.
0:12:36 > 0:12:38They run rings round her.
0:12:44 > 0:12:49The sheer numbers are overwhelming, especially for a beginner.
0:13:11 > 0:13:13She tries again.
0:13:23 > 0:13:25Then again.
0:13:27 > 0:13:31Finally, she spots a bat that's strayed from the mass,
0:13:31 > 0:13:34and flicks her back claw to hook it.
0:13:37 > 0:13:41Impressive stuff, but nothing compared to the real top guns.
0:14:11 > 0:14:16These experienced hunters have learnt the best technique,
0:14:16 > 0:14:20dive-bombing the bats at over 100 mph.
0:14:53 > 0:14:57The best can catch a bat with one talon while still holding another.
0:15:03 > 0:15:07The "best of the best" stay for the entire summer,
0:15:07 > 0:15:12but most migrating hawks simply use the cave as a drive-through restaurant.
0:15:20 > 0:15:24Like hawks, snow geese travel along well-defined routes,
0:15:24 > 0:15:27known as flyways.
0:15:27 > 0:15:29The Mississippi flyway is the most popular,
0:15:29 > 0:15:32as families can drop down any time to rest or feed.
0:15:32 > 0:15:34BOAT HORN BLOWS
0:15:39 > 0:15:42Youngsters stick close to their parents -
0:15:42 > 0:15:46those that are making their first migration will still be learning the way.
0:15:56 > 0:15:58The young will only need to make the journey once
0:15:58 > 0:16:01to remember it for a lifetime.
0:16:02 > 0:16:05But not all routes are quite so easy.
0:16:14 > 0:16:18Birds taking the western route have to cross hazardous deserts,
0:16:18 > 0:16:20such as the Grand Canyon.
0:16:26 > 0:16:31Hawks can use the updraughts rising from the canyon walls
0:16:31 > 0:16:34to glide onwards and upwards to more forgiving lands.
0:16:39 > 0:16:41The master of these travellers is the bald eagle.
0:16:43 > 0:16:47She, too, is exquisitely sensitive to rising air currents.
0:16:56 > 0:16:58She is a slope-soarer,
0:16:58 > 0:17:02adapted to exploit the uplift created by hills and cliffs.
0:17:05 > 0:17:07There is no shortage of uplift here!
0:17:09 > 0:17:11Apart from the wind,
0:17:11 > 0:17:14the canyon appears to offer little for such a water-loving bird.
0:17:21 > 0:17:24But the gorge was carved out by water.
0:17:30 > 0:17:33And here, the mighty Colorado still flows.
0:17:41 > 0:17:45It has sustained life in these deserts for the last 17 million years.
0:17:45 > 0:17:48It still feeds the eagles that are travelling through.
0:18:01 > 0:18:06Just 85 miles further east lies Monument Valley.
0:18:06 > 0:18:09It has no obvious attractions for migrating birds at all.
0:18:09 > 0:18:13Even so, many snow geese taking the western route
0:18:13 > 0:18:15pass over it as they travel north.
0:18:15 > 0:18:17Like their Mississippi counterparts,
0:18:17 > 0:18:21the adults recognise the landscape features,
0:18:21 > 0:18:23and guide their young through.
0:18:36 > 0:18:40But geese needing food or water face more of a challenge out here.
0:18:44 > 0:18:50Unlike the Mississippi, the desert offers no food or water at all.
0:18:55 > 0:18:59From the goose's perspective, this is a hard land to cross.
0:19:15 > 0:19:18Geese must flap hard and fast to stay aloft
0:19:18 > 0:19:23and push onwards to the fertile lands that lie further north.
0:19:23 > 0:19:25But like the walls of the Grand Canyon,
0:19:25 > 0:19:29the huge sandstone buttresses deflect air upwards.
0:19:29 > 0:19:32It's just the help a family needs.
0:19:33 > 0:19:36Migrating geese appreciate these free rides,
0:19:36 > 0:19:39using them to join other flocks flying overhead.
0:19:46 > 0:19:49On the West Coast, pelicans are also on the move.
0:19:49 > 0:19:51They must keep to a strict timetable
0:19:51 > 0:19:54if they are not to miss a great feeding event.
0:19:57 > 0:20:00They say, "A wonderful bird is the pelican,
0:20:00 > 0:20:03"his bill can hold more than his belly can".
0:20:03 > 0:20:08While it's true he could swallow five kilos of fish in one gulp,
0:20:08 > 0:20:10these rays are way out of his league.
0:20:17 > 0:20:21They trawl for tiny shrimps using their mouths as a net.
0:20:22 > 0:20:24They are also known as Devil rays,
0:20:24 > 0:20:28and at times they appear to display supernatural powers.
0:20:35 > 0:20:37It seems they too can fly!
0:21:02 > 0:21:07Why they indulge in such weird aerobatics is anyone's guess.
0:21:08 > 0:21:12But the pelican's aerial perspective
0:21:12 > 0:21:16reveals that the jumps happen around the edge of the shoal.
0:21:16 > 0:21:19Perhaps they panic the shrimps into the mouths of the other rays.
0:21:40 > 0:21:42Or maybe they're just showing off!
0:21:51 > 0:21:55But the rays' aerobatics won't delay the pelicans.
0:21:55 > 0:22:01They are heading for one of the most bizarre natural spectacles in North America.
0:22:02 > 0:22:05The youngsters follow the adults as they navigate
0:22:05 > 0:22:08to a very special beach in the Sea of Cortez.
0:22:14 > 0:22:16They are right on time.
0:22:16 > 0:22:19The tide has reached its highest point.
0:22:23 > 0:22:26The sea now reveals its prize.
0:22:33 > 0:22:36An entire shoreline, carpeted in gulf grunion.
0:22:38 > 0:22:42The adult pelicans know exactly when the grunion will arrive,
0:22:42 > 0:22:46passing on this knowledge to the youngsters travelling with them.
0:22:48 > 0:22:51The grunion surf ashore, and the females burrow backwards,
0:22:51 > 0:22:54laying their eggs in the exposed sand.
0:22:55 > 0:22:58The males then coil around to fertilise them.
0:23:05 > 0:23:09They are laying their eggs out of the reach of aquatic predators,
0:23:09 > 0:23:11above the high tide mark.
0:23:13 > 0:23:17Their young will hatch on the next spring tide and return to the sea.
0:23:19 > 0:23:23These fish out of water should be a doddle for a pelican to catch.
0:23:23 > 0:23:26But they're missing the right tools for the job.
0:23:34 > 0:23:38Their bills are too unwieldy to scoop the grunion from the sand.
0:23:44 > 0:23:48The pelicans are relegated to fishing in the surf zone
0:23:48 > 0:23:50as the orgy goes on behind.
0:23:54 > 0:23:58Not the easiest place to fish, especially on a rough day.
0:24:22 > 0:24:26But as the fish make their getaway, the pelicans seize their chance
0:24:26 > 0:24:31and go back to what they know best -
0:24:31 > 0:24:33plunge diving on to their prey.
0:24:50 > 0:24:53The marshlands of South Carolina are the setting
0:24:53 > 0:24:56for another strange fishing story.
0:24:58 > 0:25:02One that involves a very talented pod of dolphins.
0:25:10 > 0:25:15They are closely watched by a flock of great egrets,
0:25:15 > 0:25:18who have become experts on the dolphins' behaviour.
0:25:44 > 0:25:47As the dolphins manoeuvre, the egrets shadow them,
0:25:47 > 0:25:51leap-frogging from one spot to the next.
0:25:54 > 0:25:58Their cue is the moment a dolphin pops to the surface,
0:25:58 > 0:26:02checking the position of the nearest mud bank.
0:26:02 > 0:26:06The egrets are poised and ready for action.
0:26:06 > 0:26:07Then it happens.
0:26:10 > 0:26:13The dolphins drive the fish shoal ashore.
0:26:18 > 0:26:21As the fish flounder, the dolphins scoop them up.
0:26:27 > 0:26:30But the egrets also pile in.
0:26:38 > 0:26:43This is the only place in the world where dolphins "strand feed" like this,
0:26:43 > 0:26:46and the local birds have learnt to make the most of it.
0:26:59 > 0:27:02As the tide drops further,
0:27:02 > 0:27:04the dolphins search for more fish shoals.
0:27:24 > 0:27:29Strangely, the dolphins always use their right sides to push ashore.
0:27:34 > 0:27:38Their bellies flush pink with the excitement of it all.
0:27:54 > 0:27:57The young dolphins pick up this technique from their parents,
0:27:57 > 0:27:59as do the egrets that follow them.
0:28:02 > 0:28:04Many no longer fish for themselves,
0:28:04 > 0:28:08and totally rely upon the dolphins' cast-offs.
0:28:12 > 0:28:16The egrets are living proof of birds' extraordinary ability
0:28:16 > 0:28:18to adapt to the opportunities on offer.
0:28:25 > 0:28:29Snow geese taking the Mississippi route pass over Nebraska.
0:28:29 > 0:28:32Here, over the last 150 years,
0:28:32 > 0:28:37natural grasslands have been transformed into America's grain belt.
0:28:37 > 0:28:42From the goose's perspective, the changes are a major improvement,
0:28:42 > 0:28:46and now the snow geese population is booming.
0:28:48 > 0:28:51It is increasing at an incredible 5% per year
0:28:51 > 0:28:54as farmers grow crops that the geese can eat on their migration.
0:29:40 > 0:29:42Many of the birds that touch down here
0:29:42 > 0:29:44will have been flying continuously for three days.
0:29:47 > 0:29:51They would have travelled 800 miles, burnt over 3,000 calories
0:29:51 > 0:29:55and lost a third of their body weight to reach here.
0:29:55 > 0:29:58The first priority is to top up their lost reserves.
0:30:03 > 0:30:06But even best-laid plans are at the mercy of the weather.
0:30:19 > 0:30:21An overnight snowfall can blanket everything.
0:30:32 > 0:30:35They have no option but to move, even on empty stomachs.
0:30:39 > 0:30:41But flying burns calories,
0:30:41 > 0:30:45and many of the migrating geese are pretty much running on empty.
0:30:59 > 0:31:03Any young now have to rely on their parents' knowledge of the area -
0:31:03 > 0:31:06without food, they simply won't make it.
0:31:14 > 0:31:16In San Francisco Bay,
0:31:16 > 0:31:19the young pelican reaches the end of his journey.
0:31:20 > 0:31:27The Golden Gate Bridge spans waters teeming with some of the most diverse marine life on the planet.
0:31:29 > 0:31:31Pelicans only make short migrations,
0:31:31 > 0:31:34and for many, it's the end of their journey.
0:31:42 > 0:31:45In fact, the prison island of Alcatraz
0:31:45 > 0:31:49is named after the old Spanish word for pelican.
0:31:54 > 0:31:58Throughout their travels, pelicans have learnt to find food
0:31:58 > 0:32:01by watching other animals or using their ingenuity.
0:32:02 > 0:32:05But here, the going is easy.
0:32:05 > 0:32:08He can either catch fish in the bountiful waters of the bay
0:32:08 > 0:32:11or scavenge on waste discarded by fishing boats.
0:32:14 > 0:32:17He can even partner with Californian sea lions.
0:32:18 > 0:32:21Like dolphins, they help find and corral the shoals.
0:32:23 > 0:32:25With plenty of food to be had,
0:32:25 > 0:32:29more than 1,000 sea lions laze around without a care in the world.
0:32:41 > 0:32:44It's a carefree end to the pelicans' journey,
0:32:44 > 0:32:46but it is the beginning of another.
0:32:49 > 0:32:51These are California gulls,
0:32:51 > 0:32:55a bird that lives on his wits around the harbours of the Californian coast.
0:32:57 > 0:32:59But despite their love of the seaside,
0:32:59 > 0:33:02each spring they hear the call of the wild
0:33:02 > 0:33:06and all 65,000 of them up sticks and leave.
0:33:10 > 0:33:16They head 200 miles inland to a barren salty lagoon known as Mono Lake.
0:33:19 > 0:33:24Here, towers of calcium carbonate rise from the lake surface,
0:33:24 > 0:33:26creating a surreal landscape.
0:33:31 > 0:33:35It makes a stark contrast to the comforts of the gulls' usual home
0:33:35 > 0:33:37along the San Francisco seafront.
0:33:41 > 0:33:42Even the water is poisonous -
0:33:42 > 0:33:46a caustic chemical brew of alkaline salts.
0:33:46 > 0:33:51Despite its lack of amenities, the gulls flock here in their thousands.
0:33:53 > 0:33:58Each morning, they wait for an event that happens as the day warms up.
0:33:59 > 0:34:04Suddenly, the shoreline fills with vast clouds of brine flies,
0:34:04 > 0:34:06billions of them.
0:34:12 > 0:34:15For the gulls, it's a feast like no other.
0:34:17 > 0:34:21As the flies gather to breed and feed, they turn the sand black.
0:34:31 > 0:34:34His technique is anything but sophisticated -
0:34:34 > 0:34:36he just opens his mouth and runs.
0:34:53 > 0:34:56It may look inefficient,
0:34:56 > 0:34:59but each fly contains seven calories.
0:34:59 > 0:35:00He just has to catch 60 a day.
0:35:41 > 0:35:44Here, the living is easy
0:35:44 > 0:35:48so the gulls come here each summer to raise their families.
0:35:56 > 0:36:00Food is also top priority for snow goose families.
0:36:02 > 0:36:05When snow covers everything, there is none to be found.
0:36:15 > 0:36:17YOUNG GEESE SQUAWK
0:36:17 > 0:36:20But mother geese know exactly where to go.
0:36:23 > 0:36:27In fact, thousands of snow goose families have exactly the same idea.
0:36:32 > 0:36:35The marshes of the Platte River Basin
0:36:35 > 0:36:37offer plenty of natural food,
0:36:37 > 0:36:38even in bad weather.
0:36:41 > 0:36:44Two million geese take refuge here
0:36:44 > 0:36:47as they wait for the weather to change for the better.
0:37:04 > 0:37:05But where geese gather,
0:37:05 > 0:37:07so do the eagles.
0:37:18 > 0:37:21The youngsters keep close to their parents,
0:37:21 > 0:37:22hiding among the masses.
0:37:24 > 0:37:27As before, the eagle's tactic is to divide and conquer,
0:37:27 > 0:37:31harassing and fragmenting the flocks to single out the weaklings.
0:37:32 > 0:37:35But healthy snow geese are ambitious targets
0:37:35 > 0:37:38and there are thousands of other waterfowl on offer.
0:37:46 > 0:37:50Mallards are certainly worth a try.
0:37:50 > 0:37:53The eagle encourages others to join the hunt.
0:37:53 > 0:37:55EAGLE CRIES
0:38:01 > 0:38:05They pick out any weakened by the ordeals of the journey.
0:38:10 > 0:38:12MALLARD QUACKS
0:38:35 > 0:38:39With so much prey around, the eagles can relax and socialise.
0:38:39 > 0:38:42They even start to play with their food.
0:38:44 > 0:38:45Missed!
0:38:53 > 0:38:55The youngster sees if he can do better.
0:39:03 > 0:39:05Anyone can have a go.
0:39:11 > 0:39:14Locking talons is a popular tactic.
0:39:20 > 0:39:21Tackling is allowed
0:39:21 > 0:39:23and soon everyone piles in.
0:39:32 > 0:39:35Even scraps are used for passing practice.
0:39:40 > 0:39:43These games are all about establishing relationships
0:39:43 > 0:39:45and perfecting combat skills.
0:39:58 > 0:40:01Bald eagles are the most social of all eagles.
0:40:01 > 0:40:02By playing games,
0:40:02 > 0:40:06the young birds learn survival skills from the adults.
0:40:15 > 0:40:17The sport passes the time
0:40:17 > 0:40:20as all the birds wait for the weather to change.
0:40:24 > 0:40:26As the days go by,
0:40:26 > 0:40:28all along Nebraska's Platte River,
0:40:28 > 0:40:30there is an air of excitement.
0:40:30 > 0:40:34The weather has changed in their favour.
0:40:34 > 0:40:35It's time to check out.
0:40:44 > 0:40:48100,000 snow geese launch themselves into the air.
0:40:53 > 0:40:55And that's just a start.
0:40:55 > 0:40:58Two million birds are soon on the move again.
0:41:11 > 0:41:15Many families head on north over the Badlands of Dakota.
0:41:26 > 0:41:30The barren landscape soon gives way to natural grassland.
0:41:34 > 0:41:37The prairies stretch for hundreds of miles
0:41:37 > 0:41:40and provide grazing for the largest land mammal in America.
0:41:47 > 0:41:51The bison attract a very different flying traveller,
0:41:51 > 0:41:52cowbirds.
0:42:02 > 0:42:04As the bison migrate,
0:42:04 > 0:42:06the cowbirds travel with them.
0:42:06 > 0:42:11Like cuckoos, they even lay their eggs in other birds' nests,
0:42:11 > 0:42:13so they never have to stop moving.
0:42:18 > 0:42:21They rely on the bison to stir up insects.
0:42:21 > 0:42:24They know their every move...
0:42:26 > 0:42:29..and when it's wisest to keep out of the way.
0:42:32 > 0:42:35A dust-bathing bison is a force to be reckoned with
0:42:35 > 0:42:39but the cowbirds risk their lives to grab fleeing insects.
0:42:52 > 0:42:53BISON SNORTS
0:42:58 > 0:43:02Males fighting over females are a frequent occupational hazard.
0:43:02 > 0:43:05But the birds are experts on bison behaviour
0:43:05 > 0:43:09and know just how to dodge the battling beasts.
0:43:12 > 0:43:16The bisons' collisions are like a car crashing at 30 mph.
0:43:21 > 0:43:25Wisely, the birds stay out of the impact zone.
0:44:15 > 0:44:18These one-ton beasts and their feathered friends
0:44:18 > 0:44:19once numbered millions.
0:44:24 > 0:44:28But the same landscape changes that have helped the snow geese prosper
0:44:28 > 0:44:32have given the bison and cowbirds little space to roam.
0:44:39 > 0:44:41On the East Coast, in Delaware Bay,
0:44:41 > 0:44:44another animal migration takes place
0:44:44 > 0:44:46just as it has done for aeons of time.
0:44:50 > 0:44:53Over one million shore birds arrive from South America
0:44:53 > 0:44:56to take part in this extraordinary event.
0:45:00 > 0:45:04They time their visit to the highest spring tides.
0:45:05 > 0:45:09It's now that thousands of bizarre, prehistoric creatures
0:45:09 > 0:45:11emerge from the ocean.
0:45:14 > 0:45:16Horseshoe crabs.
0:45:29 > 0:45:31Like the grunion in Mexico,
0:45:31 > 0:45:37they too aim to lay their eggs out of the reach of predatory fish.
0:45:37 > 0:45:41It's a plan that worked millions of years ago, before birds evolved
0:45:41 > 0:45:43but now the secret's out
0:45:43 > 0:45:45and the crabs suffer the consequences.
0:45:48 > 0:45:52Sanderlings and sandpipers are first to gather on the strand line.
0:45:59 > 0:46:02They are like kids playing dare with the waves
0:46:02 > 0:46:06as they pluck the freshly-laid eggs from the sand.
0:46:21 > 0:46:23They gorge themselves silly.
0:46:23 > 0:46:27They have a journey of 2,000 miles ahead of them.
0:46:42 > 0:46:45If the sanderlings' onslaught wasn't bad enough for the crabs,
0:46:45 > 0:46:48then the ruddy turnstones arrive!
0:46:49 > 0:46:51Their rather unfortunate name
0:46:51 > 0:46:54actually refers to their reddish plumage.
0:47:02 > 0:47:04But turnstone is a good description.
0:47:04 > 0:47:07They really do leave no stone unturned
0:47:07 > 0:47:10as they search for eggs missed by the manic sanderlings.
0:47:15 > 0:47:17Dunlin have longer bills
0:47:17 > 0:47:19and can afford to take their time.
0:47:19 > 0:47:23They probe for eggs that were successfully buried by the crabs.
0:47:38 > 0:47:40In this battle for the beaches
0:47:40 > 0:47:42the crabs suffer huge losses.
0:47:42 > 0:47:46Hundreds upturned by the waves find it impossible to right themselves.
0:47:55 > 0:47:57But for all this carnage,
0:47:57 > 0:47:59the horseshoe crabs are great survivors.
0:48:02 > 0:48:04Only a few eggs need to escape
0:48:04 > 0:48:08to allow enough young to return to the beach in future years.
0:48:10 > 0:48:12The birds now head northwards,
0:48:12 > 0:48:15satisfied by an egg breakfast that will last them the journey.
0:48:19 > 0:48:22Snow geese travelling up the East Coast of America
0:48:22 > 0:48:24pass one of the greatest cities in the world.
0:48:34 > 0:48:37They've covered two-thirds of their journey
0:48:37 > 0:48:39in a little more than two weeks.
0:48:50 > 0:48:53New York is a busy crossroads for travelling birds.
0:49:03 > 0:49:07Over 250 migrating species still pass through each year
0:49:07 > 0:49:11even though their marshlands have been replaced by real estate.
0:49:13 > 0:49:16Although things ain't what they used to be,
0:49:16 > 0:49:20the Big Apple does give some help to the exhausted snow geese.
0:49:22 > 0:49:26The buildings act like the buttresses of Monument Valley,
0:49:26 > 0:49:27channelling wind upwards
0:49:27 > 0:49:31and giving geese a much-needed boost as they move inland.
0:49:33 > 0:49:35CAR HORNS, SIRENS
0:49:35 > 0:49:38Now they will continue into Canada
0:49:38 > 0:49:41and onwards to their breeding grounds.
0:49:42 > 0:49:44EAGLE CRIES
0:49:46 > 0:49:48Bald eagles, also heading north,
0:49:48 > 0:49:51spread out across the vast forests,
0:49:51 > 0:49:53never straying far from water.
0:50:07 > 0:50:10EAGLE CRIES
0:50:13 > 0:50:17They head for melt-water streams and estuaries,
0:50:17 > 0:50:19as this is their best chance for a meal.
0:50:29 > 0:50:32One particular event draws eagles like no other.
0:50:34 > 0:50:36It also attracts grizzly bears.
0:50:42 > 0:50:44BEARS GRUNT
0:50:46 > 0:50:49The soaring eagle gets the first sight of what's to come -
0:50:49 > 0:50:51thousands of sockeye salmon
0:50:51 > 0:50:54on the starting blocks for a race up the river to breed.
0:51:08 > 0:51:10The eagle bides his time
0:51:10 > 0:51:13as the bears lay claim to the best fishing spots.
0:51:19 > 0:51:24Younger eagles watch from the sidelines, anxious to learn.
0:51:28 > 0:51:30BEAR GROWLS
0:51:37 > 0:51:41Around 15,000 fish create the critical mass
0:51:41 > 0:51:44that fires the starting pistol.
0:51:44 > 0:51:46Once one goes, they all do.
0:51:55 > 0:51:58As the river fills with salmon, the bears go wild.
0:52:04 > 0:52:06But the eagles hold back.
0:52:06 > 0:52:09They're happy to let the bears do all the hard work.
0:52:17 > 0:52:22The cubs are torn between stuffing themselves or catching more.
0:52:40 > 0:52:42An eagle seizes the moment.
0:52:49 > 0:52:51But she must eat fast.
0:52:51 > 0:52:55Mother bear doesn't like others stealing her breakfast.
0:53:08 > 0:53:10But the bears don't just compete with eagles.
0:53:10 > 0:53:14Arguments over feeding rights keep tempers running high.
0:53:22 > 0:53:25BEARS ROAR
0:53:28 > 0:53:30The eagle spots another opportunity.
0:53:34 > 0:53:37As she eats, she uses her wings to hide the fish
0:53:37 > 0:53:40but she has just seconds to spare.
0:53:49 > 0:53:51BEAR CUB YOWLS
0:54:04 > 0:54:06Finally, the bears are stuffed,
0:54:06 > 0:54:10eating just the fatty skin and leaving the rest,
0:54:10 > 0:54:11much to the birds' delight.
0:54:16 > 0:54:19And the eagle finally gets to eat in peace.
0:54:28 > 0:54:31It's time for the eagles to move on.
0:54:31 > 0:54:33They've got one more appointment to make.
0:54:37 > 0:54:42All the snow goose families are nearing the end of their journeys.
0:54:45 > 0:54:50Some pass over Niagara Falls on the border between the USA and Canada.
0:55:01 > 0:55:03Further west, others cross the border
0:55:03 > 0:55:06and head right for the heart of the tundra.
0:55:18 > 0:55:21As the geese catch up with the retreating snow,
0:55:21 > 0:55:23they stop one last time.
0:55:26 > 0:55:29But hungry bald eagles have gathered here too,
0:55:29 > 0:55:33anticipating the snow goose's arrival.
0:55:33 > 0:55:37Maybe now, they will finally get to feed on their favourite prey,
0:55:37 > 0:55:42especially as the exhausted geese are at their most vulnerable.
0:55:44 > 0:55:47The young eagle buzzes the geese.
0:55:53 > 0:55:56This time he's found a sitting duck...
0:55:58 > 0:56:01..a youngster weakened by her long flight.
0:56:05 > 0:56:07GEESE HONK
0:56:21 > 0:56:24The snow goose fights back.
0:56:24 > 0:56:25She even grabs his tail.
0:56:35 > 0:56:39But the eagle puts his training into practice.
0:56:44 > 0:56:46GOOSE SQUAWKS
0:57:12 > 0:57:14A remarkable escape.
0:57:17 > 0:57:18Or is it?
0:57:24 > 0:57:28The snow goose survives against the odds.
0:57:28 > 0:57:31And the young eagle returns to the classroom.
0:57:34 > 0:57:37Like all birds, he will never stop learning.
0:57:42 > 0:57:46As the weather clears, the geese make the final short hop
0:57:46 > 0:57:49onwards to their Arctic breeding grounds.
0:57:49 > 0:57:52They have travelled 3,000 miles to reach here.
0:57:54 > 0:57:58It's only now that the young leave their parents
0:57:58 > 0:57:59to raise chicks of their own.
0:58:01 > 0:58:05The bond with their mother that guided them across North America
0:58:05 > 0:58:08is now formed with their own offspring.
0:58:09 > 0:58:13They will learn from her the many secrets known only to birds.
0:58:13 > 0:58:14CHICKS SQUEAK
0:58:18 > 0:58:20In the next journey across a continent,
0:58:20 > 0:58:22we will fly with vultures,
0:58:22 > 0:58:24flamingos and fish eagles
0:58:24 > 0:58:28as we experience a bird's-eye view of Africa.
0:58:51 > 0:58:54Subtitles by Red Bee Media Ltd
0:58:54 > 0:58:57E-mail subtitling@bbc.co.uk