0:00:02 > 0:00:04WIND GUSTS
0:00:08 > 0:00:13Stretching out before me is the magnificent Yellowstone.
0:00:17 > 0:00:20This is one of the most dynamic...
0:00:21 > 0:00:23..unpredictable...
0:00:24 > 0:00:26..and exciting environments on Earth.
0:00:27 > 0:00:29HELICOPTER HUMS
0:00:31 > 0:00:33Deep in the Rocky Mountains,
0:00:33 > 0:00:35this vast wilderness is home
0:00:35 > 0:00:39to North America's most iconic wildlife.
0:00:40 > 0:00:41But every year,
0:00:41 > 0:00:45Yellowstone's animals are pushed to their absolute limits.
0:00:47 > 0:00:50Temperatures can swing from minus 40 in winter
0:00:50 > 0:00:53to almost plus 40 during the summer.
0:00:56 > 0:01:00And at the heart of this change is the thaw.
0:01:01 > 0:01:05The melt can last several months from March to July.
0:01:06 > 0:01:10And it's one of the most dramatic, natural events on Earth.
0:01:10 > 0:01:12How do the animals cope
0:01:12 > 0:01:15with such extremes of temperature?
0:01:16 > 0:01:18We're here to find out.
0:01:21 > 0:01:24I'm joined by biologist Patrick Aryee...
0:01:24 > 0:01:26Ooh! That's intense. >
0:01:26 > 0:01:31..and a team of wildlife cameramen and expert scientists.
0:01:35 > 0:01:37Last night, we brought you winter.
0:01:37 > 0:01:41Wildlife struggled in freezing temperatures.
0:01:41 > 0:01:42BISON GROWLS
0:01:43 > 0:01:46Tonight, it's spring.
0:01:46 > 0:01:51Life is about to change, but not always for the better.
0:01:57 > 0:02:00The thaw will melt the snow in the mountains
0:02:00 > 0:02:03and cause over one million tonnes of meltwater
0:02:03 > 0:02:05to crash through Yellowstone.
0:02:09 > 0:02:13Only the strongest animals will survive.
0:02:19 > 0:02:24Welcome to Yellowstone - The Toughest Spring.
0:02:41 > 0:02:45It's the end of April and seven degrees Celsius.
0:02:49 > 0:02:51The thaw is well under way.
0:02:51 > 0:02:54BIRDS CRY
0:02:58 > 0:03:01Spring has come to Yellowstone.
0:03:01 > 0:03:04Mammals, big and small, are emerging from their dens,
0:03:04 > 0:03:06birds are courting and nesting,
0:03:06 > 0:03:09the days are getting longer and warmer.
0:03:09 > 0:03:12Now, you may think that life is going to get easier.
0:03:13 > 0:03:16Yellowstone is bursting into life.
0:03:18 > 0:03:22Hummingbirds return after spending their winter in the warm south.
0:03:24 > 0:03:28Moose feast on the first green shoots.
0:03:29 > 0:03:32BIRD WARBLES
0:03:35 > 0:03:39Bison migrate back to their calving grounds.
0:03:44 > 0:03:47We're in the north-west of the USA,
0:03:47 > 0:03:502,000 metres high up in the Rockies.
0:03:52 > 0:03:55Known as the Greater Yellowstone Ecosystem,
0:03:55 > 0:03:58it's an area the size of Scotland,
0:03:58 > 0:04:00includes two National Parks
0:04:00 > 0:04:04and is bounded on three sides by mountain ranges.
0:04:06 > 0:04:11Biologist, Patrick Aryee, is at the southern end of Greater Yellowstone,
0:04:11 > 0:04:13catching up with one species
0:04:13 > 0:04:16that faces its greatest challenge in spring.
0:04:17 > 0:04:20Behind me are the Teton Mountains
0:04:20 > 0:04:23and this is the Snake River,
0:04:23 > 0:04:26home to North America's largest rodent, the beaver.
0:04:27 > 0:04:30There are 15 of them living on this stretch of river
0:04:30 > 0:04:33and they've built this enormous dam.
0:04:34 > 0:04:37I can't believe that this dam is not a man-made structure
0:04:37 > 0:04:39and that it's been made by beavers.
0:04:39 > 0:04:42I mean, it's incredible.
0:04:42 > 0:04:47Look at the size... of these...branches, and even...
0:04:47 > 0:04:49Look, here we've got these huge rocks as well.
0:04:49 > 0:04:55It acts as a way to slow down the waterways, like speed bumps, almost,
0:04:55 > 0:05:00but I've got to say that they are nature's most incredible engineers.
0:05:01 > 0:05:03But at this time of year,
0:05:03 > 0:05:05their dam is in real danger.
0:05:07 > 0:05:10Soon enough, all that snow is going to melt
0:05:10 > 0:05:13and all that meltwater is going to start flowing down
0:05:13 > 0:05:15into these river systems.
0:05:15 > 0:05:19And as that water starts to flow and cascade along this river,
0:05:19 > 0:05:21it could damage and even destroy,
0:05:21 > 0:05:24not only the dam, but also the lodge.
0:05:24 > 0:05:27And the lodge is where the beavers have been holed up
0:05:27 > 0:05:29for the last five months.
0:05:30 > 0:05:33Now, we've got a camera set up so we can actually see
0:05:33 > 0:05:37exactly what's going on inside there right now.
0:05:38 > 0:05:39If I come over here...
0:05:43 > 0:05:45That is incredible.
0:05:46 > 0:05:49Oh, they're just starting to wake up.
0:05:49 > 0:05:50BEAVERS GROAN
0:05:53 > 0:05:57In this lodge, we've got... There's two yearlings in there,
0:05:57 > 0:06:00two two-year-olds and then one adult.
0:06:00 > 0:06:03There should be another one, but I can't see it.
0:06:04 > 0:06:06This is our beaver family
0:06:06 > 0:06:09and we're going to be getting to know them very well.
0:06:09 > 0:06:11BEAVERS SQUEAK
0:06:11 > 0:06:15And we'll be following them over the coming months to see how they cope
0:06:15 > 0:06:16when the spring floods hit.
0:06:17 > 0:06:19Just seen one dive.
0:06:22 > 0:06:25Beavers are most comfortable in water.
0:06:25 > 0:06:29Their lumbering bodies are quite ungainly on land.
0:06:30 > 0:06:32He's huge!
0:06:32 > 0:06:36This stretch of river is home to 15 beavers.
0:06:36 > 0:06:41In April, the river bank is usually covered in a blanket of snow.
0:06:41 > 0:06:43So the beavers stay snuggled in their den.
0:06:43 > 0:06:48But this year, the snow has already disappeared.
0:06:48 > 0:06:52According to scientists, the thaw is three weeks early.
0:06:52 > 0:06:55That means the beavers can start venturing out
0:06:55 > 0:06:58on the search for new green shoots.
0:07:00 > 0:07:02This is brilliant, I'm just following this beaver...
0:07:02 > 0:07:06and hopefully, I'll get to see him when he comes up onto this next dam.
0:07:08 > 0:07:12This beaver hasn't eaten fresh food for five months.
0:07:17 > 0:07:22Over winter, the beavers' food supply is locked in snow and ice.
0:07:25 > 0:07:30They feed on twigs, leaves and bark of aspen and willow trees.
0:07:30 > 0:07:34But when there's so much snow on the ground, they can't get to it.
0:07:34 > 0:07:36Yet, beavers don't hibernate,
0:07:36 > 0:07:40so they must keep eating throughout the winter to survive.
0:07:41 > 0:07:44Their solution is ingenious.
0:07:46 > 0:07:49They collect a mound of twigs and vegetation,
0:07:49 > 0:07:52big enough to last for five months
0:07:52 > 0:07:55and store it by the underwater entrance to their home.
0:07:55 > 0:07:58Even when the river freezes over,
0:07:58 > 0:08:01they have a larder just a moment's swim away.
0:08:05 > 0:08:06But by the end of winter,
0:08:06 > 0:08:09the rotten twigs have lost most of their protein.
0:08:09 > 0:08:13This beaver is surviving on his own body fat.
0:08:16 > 0:08:18The early thaw, however, has given him a lifeline.
0:08:20 > 0:08:22This year, he can forage for fresh food
0:08:22 > 0:08:25three weeks earlier than normal.
0:08:29 > 0:08:32It's just what his hungry family needs.
0:08:35 > 0:08:38OK, that adult is now back inside the lodge.
0:08:40 > 0:08:43And it's got a mouth full of vegetation.
0:08:43 > 0:08:46BEAVERS SQUEAK
0:08:46 > 0:08:50These beavers have lost a third of their body weight over winter.
0:08:50 > 0:08:51To gain that weight back,
0:08:51 > 0:08:55they'll need to eat a kilogram of twigs and leaves every single day.
0:08:58 > 0:09:01The younger beavers squeak to demand food
0:09:01 > 0:09:04but when competition is this fierce...
0:09:04 > 0:09:06it's every beaver for itself.
0:09:06 > 0:09:09YOUNG BEAVERS SQUEAK
0:09:16 > 0:09:20In spring, twigs are rich in sugars and proteins,
0:09:20 > 0:09:24just what they need to kick-start their recovery.
0:09:24 > 0:09:28But to reach the fresh food, the beavers must first get to work.
0:09:31 > 0:09:37Not only are beavers phenomenal engineers, building all those dams,
0:09:37 > 0:09:39but they're also professional lumberjacks
0:09:39 > 0:09:42and they can take down whole trees, just like this one,
0:09:42 > 0:09:45and they do that using their super-sharp teeth.
0:09:45 > 0:09:48These bottom teeth are just like chisels
0:09:48 > 0:09:51and what they'll do is get their head in sideways
0:09:51 > 0:09:53and chew away at the bark.
0:09:53 > 0:09:55With smaller trees, they'll go right the way through
0:09:55 > 0:09:57and you'll often see beavers running away
0:09:57 > 0:09:59just before the trees fall down.
0:09:59 > 0:10:01But with bigger ones, like this one,
0:10:01 > 0:10:04they'll only get so far and then they'll move away
0:10:04 > 0:10:06and let the wind do the rest of the job.
0:10:09 > 0:10:13Now this is a really big tree.
0:10:13 > 0:10:16But the part the beavers are really interested in is up here,
0:10:16 > 0:10:17the top third,
0:10:17 > 0:10:21and I can see where they've gnawed away at this end
0:10:21 > 0:10:23and hauled all their branches back to the river
0:10:23 > 0:10:26to construct their lodge and the dams.
0:10:26 > 0:10:29And making sure their dams are reinforced
0:10:29 > 0:10:31before the spring floods hit is crucial.
0:10:34 > 0:10:36There are five dams on this stretch of river.
0:10:37 > 0:10:43This entire waterway, this system has been created by these beavers.
0:10:43 > 0:10:49They've made these...series of dams,
0:10:49 > 0:10:51which kind of create this lock system.
0:10:52 > 0:10:55The beavers have turned, what was a small stream,
0:10:55 > 0:10:57into a series of deep ponds.
0:10:59 > 0:11:02These pools allow them to stay in the water while foraging.
0:11:04 > 0:11:08On land, they're easy prey for predators, like wolves and bears.
0:11:09 > 0:11:14But crucially, this dam system also slows down the floodwater.
0:11:16 > 0:11:22At two metres wide and 40 metres long, the main dam is the strongest.
0:11:22 > 0:11:25But if this one breaches, the others will go too.
0:11:27 > 0:11:31BEAVER MOANS
0:11:31 > 0:11:34Right now, the beavers are safe in their lodge.
0:11:35 > 0:11:37But I'll follow them closely
0:11:37 > 0:11:40and find out how they get on as spring takes hold.
0:11:45 > 0:11:50For many animals, spring is the most important time of year.
0:11:51 > 0:11:53It's now May.
0:11:53 > 0:11:55BIRD WARBLES
0:11:55 > 0:11:58As soon as the snow melts,
0:11:58 > 0:11:59fresh vegetation appears.
0:12:03 > 0:12:06Scientists call it the "spring green-up"
0:12:06 > 0:12:10and it drives all life in Yellowstone.
0:12:10 > 0:12:13One animal relies on it more than almost any other...
0:12:15 > 0:12:17The Rocky Mountain elk.
0:12:18 > 0:12:22There are over 20,000 elk in Yellowstone.
0:12:23 > 0:12:27As the green-up moves from the lowlands to high mountain pastures,
0:12:27 > 0:12:29the elk move with it.
0:12:30 > 0:12:35This magnificent herd of elk that are straddling right across
0:12:35 > 0:12:38the sagebrush here and up the hill,
0:12:38 > 0:12:40are on their way to the calving grounds,
0:12:40 > 0:12:42a little bit further north from here.
0:12:42 > 0:12:44They'll be giving birth over the next few weeks.
0:12:44 > 0:12:48They'll hide their youngsters in the undergrowth
0:12:48 > 0:12:50while they go off to feed.
0:12:51 > 0:12:55This is a crucial move in order to protect them from predation
0:12:55 > 0:12:59by the largest carnivore here... the grizzly bear.
0:13:00 > 0:13:02Now, there are some grizzlies
0:13:02 > 0:13:07that have become particularly adept at finding those youngsters.
0:13:07 > 0:13:10And that source of protein
0:13:10 > 0:13:13is really great for adult bears and their cubs,
0:13:13 > 0:13:17needing desperately to put on weight after a long winter hibernation.
0:13:21 > 0:13:24It's as the snow line retreats from the valleys to the foothills
0:13:24 > 0:13:28that the female grizzlies and their cubs start emerging.
0:13:33 > 0:13:38They usually stay close to their dens until the end of May,
0:13:38 > 0:13:42but this year, the warm weather has enticed the bears away
0:13:42 > 0:13:43earlier than usual.
0:13:46 > 0:13:50But the elk calves won't be born for another month.
0:13:50 > 0:13:55So life for these cubs will now be harder.
0:13:55 > 0:13:59They will have to scratch a living digging up roots and grasses.
0:14:04 > 0:14:07Grizzly expert, Casey Anderson,
0:14:07 > 0:14:11is already seeing signs that there are plenty of bears around.
0:14:12 > 0:14:15They've been using trees as scratching posts.
0:14:16 > 0:14:19Yeah, this thing has been getting hammered, look at all this.
0:14:19 > 0:14:21They're leaving their scent behind.
0:14:22 > 0:14:25Cos there's just all kinds of mud and hair stuck on here, so...
0:14:25 > 0:14:28Yeah, it's going to be cool to see who is back in town.
0:14:30 > 0:14:34Patrick is joining Casey in the Gallatin Mountains
0:14:34 > 0:14:36to find out whether the early bears
0:14:36 > 0:14:38are getting enough to eat.
0:14:40 > 0:14:44Casey's tracked the mother and her two-year-old cubs to a wooded area
0:14:44 > 0:14:46halfway down the mountain.
0:14:47 > 0:14:51They've been feeding all morning and are likely to be somewhere close by.
0:14:52 > 0:14:56From here on, we've got to go slow, even lower our voice.
0:14:56 > 0:14:59The wind's really going in the wrong direction.
0:14:59 > 0:15:03The wind's carrying our scent right towards where the grizzly bears are
0:15:03 > 0:15:04and they have a sense of smell
0:15:04 > 0:15:07- that's about seven times stronger than a bloodhound.- All right.
0:15:07 > 0:15:10'Casey thinks the bears will stay in this area for a while
0:15:10 > 0:15:13'to make the most of the early green shoots.
0:15:13 > 0:15:16'So for us, it's a waiting game.'
0:15:17 > 0:15:19It's amazing cos they just come out of nowhere.
0:15:19 > 0:15:21In that deep sagebrush,
0:15:21 > 0:15:23they love to just get down out of this wind
0:15:23 > 0:15:27and lay there, flat to the ground, cuddle up, keep each other warm.
0:15:27 > 0:15:29So we just got to... to keep scanning.
0:15:29 > 0:15:34You'll see like a little ear or they'll stretch their paw up.
0:15:34 > 0:15:36Yeah, look for those little subtle things popping out.
0:15:38 > 0:15:42What makes you think that this is a good spot for bears?
0:15:42 > 0:15:46Look right down here in this... green patch.
0:15:46 > 0:15:49You'll see where the bears have been excavating there
0:15:49 > 0:15:52and that's just classic in the springtime,
0:15:52 > 0:15:55they'll find these first really green spots,
0:15:55 > 0:15:57they've got a lot of moisture, get a lot of sun.
0:15:57 > 0:16:01So there has been a lot of activity down in that meadow there, so...
0:16:01 > 0:16:04there's a really good chance that... whoever did that,
0:16:04 > 0:16:07whatever bears did that are going to come back to that spot
0:16:07 > 0:16:08and do it again.
0:16:10 > 0:16:13But plants contain less than half the protein of meat.
0:16:15 > 0:16:19So they'll have to eat huge amounts to keep up their calorie count.
0:16:27 > 0:16:29- CASEY WHISTLES - Hey, hey...
0:16:29 > 0:16:30Where?
0:16:33 > 0:16:36Right-right... Right down in the trees.
0:16:38 > 0:16:40Oh, yeah! Yeah, yeah, yeah, yeah!
0:16:40 > 0:16:41Three bears.
0:16:41 > 0:16:44HE LAUGHS
0:16:44 > 0:16:46One... Two...
0:16:48 > 0:16:50Three. And there's like...
0:16:50 > 0:16:52Like a... Looks like a big female.
0:16:52 > 0:16:54One light cub, one dark cub.
0:16:55 > 0:16:57- It's pretty awesome.- Yeah.
0:17:00 > 0:17:03Those look like two-year-old cubs.
0:17:03 > 0:17:07Being in this area just draws in females with cubs.
0:17:07 > 0:17:08Spot like that, well, you know,
0:17:08 > 0:17:10we've got dandelions, grasses and sedge.
0:17:10 > 0:17:13Even biscuitroot will grow in that area
0:17:13 > 0:17:14and that's all on the menu
0:17:14 > 0:17:17for a grizzly bear this time of year, so...
0:17:17 > 0:17:19They come right to that spot and they'll keep coming back,
0:17:19 > 0:17:21over and over and over again, till everything's gone.
0:17:21 > 0:17:23So, basically, this area here
0:17:23 > 0:17:26is kind of like an all-you-can-eat veggie buffet?
0:17:26 > 0:17:28Yeah, it's a salad bar.
0:17:29 > 0:17:33A dandelion plant only contains about three calories,
0:17:33 > 0:17:35and grass...even less.
0:17:36 > 0:17:39The early thaw is at least helping.
0:17:39 > 0:17:43But they must find two fields every day with rich sources of food,
0:17:43 > 0:17:46like this one, if they're going to survive.
0:17:54 > 0:17:56It's not just grizzly bears
0:17:56 > 0:17:58that must exploit the best pockets of food.
0:18:01 > 0:18:05The Rocky Mountain elk are still moving north following the green-up.
0:18:10 > 0:18:14And another of Yellowstone's residents is ahead of them...
0:18:14 > 0:18:15the bison.
0:18:16 > 0:18:20This year, the bison and elk are lucky.
0:18:20 > 0:18:22Not only have they had a milder winter,
0:18:22 > 0:18:26but the early thaw means there's plenty of grass around.
0:18:29 > 0:18:32And now in spring, it's full of protein.
0:18:36 > 0:18:39I've come to the Northern Range of Yellowstone.
0:18:40 > 0:18:42Park biologists have reported
0:18:42 > 0:18:44that a large herd of bison
0:18:44 > 0:18:45has come here to feast
0:18:45 > 0:18:47because this year
0:18:47 > 0:18:49the snow has already melted.
0:18:50 > 0:18:53The park is home to about 5,000 bison.
0:18:53 > 0:18:55BISON GROWLS
0:18:55 > 0:18:59This herd has made it through to spring fit and strong,
0:18:59 > 0:19:02which is crucial for the survival of their newborn calves.
0:19:04 > 0:19:08Calving season is just beginning here in Yellowstone National Park.
0:19:09 > 0:19:13This is the very first one I've seen. It's suckling, I think.
0:19:15 > 0:19:19A quarter of the baby bison in Yellowstone
0:19:19 > 0:19:21die in their first two months,
0:19:21 > 0:19:26but this mild weather will give them a much greater chance of survival.
0:19:26 > 0:19:31There's a tiny one, looks like it's probably only a day or two old.
0:19:32 > 0:19:37Beautiful, you can see that lovely chestnutty colour.
0:19:37 > 0:19:39They're extraordinary, bison calves.
0:19:39 > 0:19:42Within half an hour of being born, they're up on their feet
0:19:42 > 0:19:46and within an hour or two, they can run to keep up with their mothers.
0:19:46 > 0:19:49BISON GROWLS
0:19:50 > 0:19:53Sticking close to their mothers is vital.
0:19:58 > 0:20:04Because this spring, Yellowstone's top predator is particularly hungry.
0:20:05 > 0:20:09There are 100 wolves in Yellowstone National Park.
0:20:10 > 0:20:14They rely on bison and elk to survive the winter.
0:20:17 > 0:20:20But whilst the mild winter has been good for the bison,
0:20:20 > 0:20:22it's been bad for the wolves.
0:20:22 > 0:20:24They've struggled to hunt.
0:20:26 > 0:20:29Back in the winter, biologist Doug Smith
0:20:29 > 0:20:31observed the problem first-hand.
0:20:32 > 0:20:36Doug and his team monitor these wolves every single day.
0:20:37 > 0:20:41They've noted that whilst mild winters help the prey,
0:20:41 > 0:20:44they put predators at a disadvantage.
0:20:45 > 0:20:50When you're in ankle-deep snow, or just elbow-deep snow,
0:20:50 > 0:20:55a healthy elk, bison, deer will always outrun a healthy wolf.
0:20:55 > 0:20:59Wolves and elk run at the same speed,
0:20:59 > 0:21:01but elk are built for endurance -
0:21:01 > 0:21:05with sure footing, they can keep going for longer.
0:21:05 > 0:21:09So this winter, the wolves have been going hungry.
0:21:11 > 0:21:15They're having a particularly tough time here in the Northern Range,
0:21:15 > 0:21:20where five packs compete for food and territory.
0:21:22 > 0:21:25Doug has studied these wolves for over 20 years
0:21:25 > 0:21:27and in the last decade,
0:21:27 > 0:21:30their numbers have fallen dramatically.
0:21:32 > 0:21:37One pack, known by park biologists as the Lamar Canyon Pack,
0:21:37 > 0:21:39is extremely weak.
0:21:40 > 0:21:42If they don't gain strength quickly,
0:21:42 > 0:21:45they risk losing their territory to other wolves.
0:21:48 > 0:21:52Our wildlife camera team is down in the Lamar Valley following them.
0:21:56 > 0:21:59From a mile off, they spot some unusual behaviour.
0:22:03 > 0:22:06The wolves are taking on a whole herd of bison.
0:22:09 > 0:22:12They're hoping to snatch a newborn calf.
0:22:13 > 0:22:16This is a sign that the pack is desperate.
0:22:18 > 0:22:21Doug is helping us to analyse what's going on.
0:22:23 > 0:22:28These bison are doing classic defence.
0:22:29 > 0:22:31They're grouped together, they're big.
0:22:31 > 0:22:33They're not moving other than to attack the wolves.
0:22:36 > 0:22:40This is an age-old dance between power and agility.
0:22:42 > 0:22:45Wolves may be quick, but bison are strong.
0:22:47 > 0:22:51Getting this close risks a kick to the skull.
0:22:54 > 0:22:57Wolves would like to spread this group apart,
0:22:57 > 0:23:00break it up and then that would free up one of these calves
0:23:00 > 0:23:02that they could grab.
0:23:07 > 0:23:11A female charges, threatening to lash them with her hooves.
0:23:11 > 0:23:17You get caught by a foot, you get caught by a horn - injury, death.
0:23:22 > 0:23:24It's fascinating footage to watch.
0:23:24 > 0:23:28Uh, they're being aggressive, yet very careful.
0:23:28 > 0:23:33Wolves definitely feel safer pursuing a fleeing prey
0:23:33 > 0:23:38cos they can kind of come in from behind, grab and pull down.
0:23:38 > 0:23:42To attack a standing herd means these wolves are desperate.
0:23:44 > 0:23:47As they get hungry and it goes more days without food,
0:23:47 > 0:23:49they're going to take more risks.
0:23:49 > 0:23:51They finally close in.
0:23:51 > 0:23:55This is the break in the herd they've been hoping for.
0:24:03 > 0:24:05ADULT BISON GRUNTS, WOLVES WHINE
0:24:08 > 0:24:11The mother can't do all this by herself, the wolves outnumber her.
0:24:13 > 0:24:17They may get this calf, I can't believe this, you rarely see this.
0:24:17 > 0:24:19ADULT BISON GRUNTS
0:24:27 > 0:24:30But the calf makes it back to the safety of the herd...
0:24:31 > 0:24:34..leaving nine very hungry wolves.
0:24:37 > 0:24:39The Lamar Canyon Pack has failed.
0:24:46 > 0:24:50Doug is concerned that these warmer winters and early springs
0:24:50 > 0:24:52are part of a worrying trend.
0:24:53 > 0:24:56We can see climate change affecting the park now, um...
0:24:56 > 0:25:00Our wolf research has only been going a little over 20 years,
0:25:00 > 0:25:05but you talk to people who were in Yellowstone 40 or 50 years ago,
0:25:05 > 0:25:08and winter severity is just not like it used to be,
0:25:08 > 0:25:12and this is a big factor in how this system works.
0:25:14 > 0:25:18This winter and last winter were two of our...shortest winters we've had.
0:25:18 > 0:25:22This, in a way, in a big picture way, favours the bison.
0:25:22 > 0:25:26So we lost our snow at low elevation very early this year
0:25:26 > 0:25:29and so any time the snow goes away, it gives the bison better footing...
0:25:29 > 0:25:31Uh, they can deal with the wolves,
0:25:31 > 0:25:34and secondly, it exposes the vegetation to sunlight.
0:25:34 > 0:25:37The condition of bison will start to improve earlier, uh,
0:25:37 > 0:25:39and so that hurts the wolves a little bit.
0:25:41 > 0:25:46And so sitting aside and watching life evolve with that going on,
0:25:46 > 0:25:48uh, has got me scared stiff.
0:25:50 > 0:25:54It's the rate of climate change that Doug and his fellow scientists
0:25:54 > 0:25:56find so shocking.
0:25:56 > 0:25:59Temperatures here have been rising fast.
0:26:00 > 0:26:04And the last decade has been the warmest since records began.
0:26:08 > 0:26:11With Yellowstone's winters becoming shorter
0:26:11 > 0:26:14and the spring starting earlier,
0:26:14 > 0:26:17one animal in particular is extremely vulnerable.
0:26:24 > 0:26:26The great grey owl.
0:26:29 > 0:26:33These owls live in some of the coldest forests on Earth,
0:26:33 > 0:26:36from Siberia to Scandinavia.
0:26:36 > 0:26:41Yellowstone is just about cold enough for the owls to survive,
0:26:41 > 0:26:43but if it gets any warmer,
0:26:43 > 0:26:46scientists believe they won't be able to hold on.
0:26:48 > 0:26:50Back in winter, cameraman Jeff Hogan
0:26:50 > 0:26:55followed a pair of great grey owls in the Teton Mountains.
0:27:01 > 0:27:05Great grey owls rely on small rodents like voles and gophers,
0:27:05 > 0:27:07which live beneath the snow.
0:27:07 > 0:27:11But owl biologists believe that the changing climate
0:27:11 > 0:27:13is affecting their hunting.
0:27:13 > 0:27:17These owls have to bust through this crust
0:27:17 > 0:27:20and then maybe another foot of snow to get at their prey.
0:27:22 > 0:27:27But erratic winter weather caused the crust to melt and then refreeze,
0:27:27 > 0:27:29making it impenetrable for the owls.
0:27:34 > 0:27:39And if they can't hunt, great grey owls won't nest or lay eggs.
0:27:45 > 0:27:46Now it's spring,
0:27:46 > 0:27:50Jeff has been searching for owl chicks in the Beartooth Mountains.
0:27:50 > 0:27:53The forest here is 300 years old
0:27:53 > 0:27:56and ideal habitat for great grey owls.
0:28:01 > 0:28:03- There they are, three of 'em. - Oh, wow!
0:28:04 > 0:28:06OWL SCREECHES
0:28:08 > 0:28:10Oh, my goodness.
0:28:13 > 0:28:16The smaller one, she's the runt.
0:28:16 > 0:28:21Despite the difficulties of winter, we found a nest with three chicks.
0:28:21 > 0:28:23This is extremely unusual.
0:28:24 > 0:28:28When food is scarce, the smallest chick can be eaten by its siblings,
0:28:28 > 0:28:30but this runt is still alive.
0:28:31 > 0:28:34How much of a success is it
0:28:34 > 0:28:37that we've got three great grey owl chicks here?
0:28:37 > 0:28:40In these parts here, this neck of the woods,
0:28:40 > 0:28:44- it's about a 20% success rate for the third chick.- Mm.
0:28:44 > 0:28:46So most, you know,
0:28:46 > 0:28:49four out of five perish by this... by this time.
0:28:51 > 0:28:53So we've got some great parents here?
0:28:53 > 0:28:57Yeah, the reason is our winters have been quite mild,
0:28:57 > 0:28:59so the rodents, the prey, has really benefited
0:28:59 > 0:29:01and their populations have really grown,
0:29:01 > 0:29:04which of course then benefits the owls.
0:29:04 > 0:29:06And that could be making the difference
0:29:06 > 0:29:08on whether that third chick survives or not.
0:29:09 > 0:29:13The early thaw this year has turned the owls' fortunes around.
0:29:13 > 0:29:17The milder weather has led to a boom in rodent numbers.
0:29:20 > 0:29:23To reveal how this has been helping the owls,
0:29:23 > 0:29:27Jeff has been searching for their pellets.
0:29:28 > 0:29:31Yeah, I've got a pellet here I collected this morning.
0:29:31 > 0:29:33And, look, you'll see the size of this...
0:29:33 > 0:29:35- It's kind of... - PATRICK LAUGHS
0:29:35 > 0:29:36It's kind of disgusting.
0:29:36 > 0:29:39OK, well, if I just break this apart...
0:29:39 > 0:29:42Well, you can see, it is literally just...
0:29:43 > 0:29:46..hair and bone.
0:29:46 > 0:29:47Yeah.
0:29:48 > 0:29:52- GASPING:- Look at that!
0:29:52 > 0:29:55That is an entire skull.
0:29:55 > 0:29:57- Look at that, you can even see the teeth!- Mm-hm.
0:29:57 > 0:29:59Oh! Here we go, look, look, look, look!
0:29:59 > 0:30:02- There you go.- Oh, that is brilliant.
0:30:02 > 0:30:03Yeah.
0:30:03 > 0:30:05A second skull,
0:30:05 > 0:30:08which means they would have had two meals in quick succession.
0:30:09 > 0:30:11So far, the owls are doing well,
0:30:11 > 0:30:15but spring in Yellowstone is unpredictable.
0:30:17 > 0:30:21After I leave, the weather takes a turn for the worse.
0:30:21 > 0:30:23A storm blows in.
0:30:23 > 0:30:25It could mean death for the chicks.
0:30:25 > 0:30:29This rain makes...makes it a little more difficult for me...
0:30:29 > 0:30:31to work in, but...
0:30:32 > 0:30:36..it makes it even more difficult for the adult owls to hunt,
0:30:36 > 0:30:38being that it's just so noisy,
0:30:38 > 0:30:42and that masks any sounds that the rodents may be making.
0:30:42 > 0:30:45The runt is at the bottom of the pecking order.
0:30:45 > 0:30:49As soon as food becomes scarce, she'll be the first to lose out.
0:30:49 > 0:30:54If the weather doesn't change, the runt may starve.
0:30:54 > 0:30:56THUNDER RUMBLES
0:31:04 > 0:31:08May is always the wettest month of the year in Yellowstone.
0:31:08 > 0:31:12But this year, it's far wetter than average.
0:31:15 > 0:31:18The reason can be found in the Pacific.
0:31:18 > 0:31:22The warming ocean creates much more moisture in the air.
0:31:23 > 0:31:27This is channelled directly up to Yellowstone.
0:31:27 > 0:31:29And when it hits the wall of mountains,
0:31:29 > 0:31:32the fury of the monsoon is unleashed.
0:31:32 > 0:31:34THUNDER RUMBLES
0:31:38 > 0:31:42We experience the most violent storm that's been seen here
0:31:42 > 0:31:43for many years.
0:31:44 > 0:31:47Rain falling over three mountain ranges
0:31:47 > 0:31:51drains straight into the Yellowstone basin.
0:31:51 > 0:31:54Overnight, water levels rise by a foot.
0:31:57 > 0:31:58For two weeks,
0:31:58 > 0:32:03rain pours down and thunderstorms reverberate around the mountains.
0:32:04 > 0:32:07Then, at last, in early June...
0:32:09 > 0:32:11..the weather breaks.
0:32:13 > 0:32:15But there's water everywhere.
0:32:17 > 0:32:19Melting snow, as well as rainwater,
0:32:19 > 0:32:22is now flowing into the river systems.
0:32:26 > 0:32:30Scientists have told me to come to Yellowstone Falls
0:32:30 > 0:32:32to see the full effect.
0:32:53 > 0:32:56Back in March, Yellowstone Falls was just a mere trickle,
0:32:56 > 0:33:00and now it's a dramatic torrent of tumbling water.
0:33:05 > 0:33:07Every year the thaw is different
0:33:07 > 0:33:10and it's the rivers that hold the clues
0:33:10 > 0:33:14to tell us how this year's thaw is progressing.
0:33:27 > 0:33:30Thousands of animals rely on this river to survive.
0:33:32 > 0:33:36Greater Yellowstone sits on the Continental Divide.
0:33:36 > 0:33:41This water flows west to the Pacific Ocean and east to the Atlantic.
0:33:42 > 0:33:45Without this huge reservoir of water,
0:33:45 > 0:33:50agriculture and industry, as well as wildlife in the American West,
0:33:50 > 0:33:52would die.
0:33:52 > 0:33:56So understanding how MUCH water is draining into the river systems
0:33:56 > 0:33:58is vital.
0:33:58 > 0:33:59Teams of scientists
0:33:59 > 0:34:02have monitored the streamflow in Yellowstone's rivers
0:34:02 > 0:34:05for the last 86 years.
0:34:05 > 0:34:08They've noticed that recently the pattern of the spring runoff
0:34:08 > 0:34:10has been changing.
0:34:11 > 0:34:14This piece of equipment measures streamflow.
0:34:17 > 0:34:21Hydrologist Cheryl Miller is keeping a close eye on the rising water
0:34:21 > 0:34:23in the Yellowstone River.
0:34:23 > 0:34:28The data she gathers will reveal how big the spring runoff will be.
0:34:30 > 0:34:32It's all done with this little orange boat
0:34:32 > 0:34:37that uses sonar to measure the depth and speed of the water.
0:34:37 > 0:34:41Combine those and you get the flow, and that's the key figure.
0:34:42 > 0:34:46The rain has already raised the water levels,
0:34:46 > 0:34:50but it's the thousands of tonnes of snow still in the mountains
0:34:50 > 0:34:53that will make the real difference.
0:34:53 > 0:34:55- OK, I've got it.- All right.
0:34:56 > 0:35:00So, if you do get a very sharp rise in temperature,
0:35:00 > 0:35:02if that snow pattern melts very quickly...
0:35:02 > 0:35:04And, I mean, you know, I've been in that snow pattern,
0:35:04 > 0:35:06the snow was above my head.
0:35:06 > 0:35:09Presumably, if that DOES melt very quickly,
0:35:09 > 0:35:14the results downstream must be potentially quite dramatic,
0:35:14 > 0:35:16quite-quite destructive.
0:35:16 > 0:35:20Indeed. High energy floods can make great changes along the river.
0:35:20 > 0:35:23What's happening now is the days are warm, but that more importantly,
0:35:23 > 0:35:25the nights are warmer,
0:35:25 > 0:35:27and so that snow pack never refreezes at night.
0:35:27 > 0:35:30It continues to slowly melt during the night
0:35:30 > 0:35:32and so that's what allows the snowmelt and the runoff
0:35:32 > 0:35:34- to accelerate during this time of year.- Yeah.
0:35:36 > 0:35:37Just looking at the river,
0:35:37 > 0:35:41does it appear that the flow is faster now than it was
0:35:41 > 0:35:42when you last collected data?
0:35:42 > 0:35:45It is, it has come up in the last three or four days that it's...
0:35:45 > 0:35:48It may not be a lot faster, but the volume is definitely bigger,
0:35:48 > 0:35:51there's more streamflow than there was just a few days ago.
0:35:54 > 0:35:57Cheryl's data reveals this year the melting snow
0:35:57 > 0:36:01is coming off the mountains more quickly than usual.
0:36:03 > 0:36:08The faster it runs off, the less water is absorbed into the ground.
0:36:10 > 0:36:15These rapid spring melts are more likely to cause flash floods.
0:36:15 > 0:36:19Patrick's finding out how that's affecting the beavers.
0:36:25 > 0:36:28Here in the Snake River Valley,
0:36:28 > 0:36:31all the meltwater from the Tetons is running down the mountains
0:36:31 > 0:36:34and it's headed directly for our beaver dam.
0:36:35 > 0:36:39'I'm meeting up with cameraman Charlie Hamilton James.'
0:36:40 > 0:36:41- Hey, Charlie.- Hey, Patrick.
0:36:41 > 0:36:44- How's it going, man?- Yeah, it's good. Lovely weather, isn't it?
0:36:44 > 0:36:46'He's spent the last week
0:36:46 > 0:36:48'watching the beavers struggle with the rising river.'
0:36:53 > 0:36:56The water level is so much higher
0:36:56 > 0:36:59compared to the last time I was here.
0:36:59 > 0:37:01Yeah, actually, really only in the last few days.
0:37:01 > 0:37:05- There was a lot of rain.- All of that water has resulted in this?
0:37:05 > 0:37:06Yeah, and what it's done,
0:37:06 > 0:37:09- it's actually starting to flood the dam.- Mm.
0:37:09 > 0:37:13In some places, the dam has already breached.
0:37:14 > 0:37:18The beavers are working nonstop to make sure all the leaks are plugged.
0:37:21 > 0:37:26They built this dam five years ago and every year they fortify it.
0:37:28 > 0:37:31This one's been strengthened with a particularly high number of stones.
0:37:34 > 0:37:37So what makes this spot prime habitat?
0:37:37 > 0:37:39Well, actually, weirdly, beavers have made this spot.
0:37:39 > 0:37:42This actually started life as a stream
0:37:42 > 0:37:45and it's the beavers that came along and stuck these dams in it
0:37:45 > 0:37:50and blocked its path and created these long, slow ponds,
0:37:50 > 0:37:52and when you create a pond, you create another pond,
0:37:52 > 0:37:55you create another pond... You create a wetland ecosystem.
0:37:55 > 0:37:58They're basically engineers, but they're also gardeners.
0:37:58 > 0:37:59I like that, gardeners.
0:37:59 > 0:38:02So they're creating this whole environment that suits THEM.
0:38:02 > 0:38:04But it's not just them that benefit from this, right?
0:38:04 > 0:38:07- You've got amphibians, other mammals...- Oh, absolutely.
0:38:07 > 0:38:08..birds that rely on this.
0:38:08 > 0:38:11I mean, just sitting here now, there's birds everywhere.
0:38:11 > 0:38:12You know, swallows flying round,
0:38:12 > 0:38:15drinking and catching flies off the surface.
0:38:15 > 0:38:16There's kingfishers eating fish.
0:38:16 > 0:38:18There's lots of minnows here,
0:38:18 > 0:38:19lots of small fish perfect for kingfishers.
0:38:19 > 0:38:22Otters come through here, ospreys, eagles...
0:38:22 > 0:38:26So, you know, they're ecosystem builders, is what they are, beavers
0:38:26 > 0:38:28and more so, really, than any other animal.
0:38:31 > 0:38:33If the main dam is knocked out,
0:38:33 > 0:38:36it's not just the beavers that will lose their home.
0:38:39 > 0:38:41The top is just about holding.
0:38:41 > 0:38:44But Charlie and I are heading underwater
0:38:44 > 0:38:46to find out how solid the base is.
0:38:55 > 0:38:57Just where my feet are now, it actually starts here.
0:38:57 > 0:39:00These... The foundations have been laid here.
0:39:00 > 0:39:02So, you know, it's like ten foot wide.
0:39:02 > 0:39:05You can't see it all cos, you know, most of it's underwater.
0:39:05 > 0:39:07We've got these drysuits on.
0:39:07 > 0:39:10Yeah, and really, Patrick, in the spirit of full immersion...
0:39:12 > 0:39:14..we should go in and actually have a look.
0:39:14 > 0:39:16- Is this where I find out I've got the dud suit?- Yeah.
0:39:20 > 0:39:23Actually, this is so much better than I thought it would be.
0:39:23 > 0:39:24This is awesome!
0:39:24 > 0:39:27It's much clearer than I thought, you can really see it.
0:39:28 > 0:39:32It's only when you get underwater that you see the skill
0:39:32 > 0:39:34that's gone into building this dam.
0:39:37 > 0:39:40The beavers choose logs up to two metres long...
0:39:42 > 0:39:46..and place them at an angle of 30 degrees to hold back the water.
0:39:46 > 0:39:49This is precision engineering.
0:39:54 > 0:39:55This is amazing, isn't it?
0:39:55 > 0:39:56It's so impressive.
0:39:56 > 0:39:59This is, you know, it's-it's kind of a work of art.
0:40:02 > 0:40:05But the beavers' hard work may not be enough.
0:40:05 > 0:40:10If the river rises any further, the water could break through.
0:40:10 > 0:40:13And this watery world might just disappear.
0:40:16 > 0:40:18It's getting cold now, innit?
0:40:18 > 0:40:21THEY CHATTER
0:40:24 > 0:40:29It's now mid-June and temperatures reach 28 degrees...
0:40:31 > 0:40:34Seven degrees above average
0:40:34 > 0:40:35for this time of year.
0:40:43 > 0:40:46The snow recedes halfway up the mountains...
0:40:46 > 0:40:51revealing the highest feeding grounds in Greater Yellowstone.
0:40:51 > 0:40:53And close behind are all the animals
0:40:53 > 0:40:58taking advantage of the final stage of this year's spring green-up.
0:41:06 > 0:41:08At 2,500 metres,
0:41:08 > 0:41:13this vast wilderness provides food for thousands of animals.
0:41:15 > 0:41:17Elk that overwintered down in the valleys,
0:41:17 > 0:41:21finally arrive back in these high pastures,
0:41:21 > 0:41:22where they'll raise their calves.
0:41:27 > 0:41:32These meadows are a feasting ground, full to the brim with wildlife...
0:41:32 > 0:41:35the hunters as well as the hunted.
0:41:44 > 0:41:48Opportunistic black bears take advantage of a low bird's nest
0:41:48 > 0:41:51and defenceless chicks.
0:41:51 > 0:41:55At this time of year, any food is fair game.
0:41:57 > 0:42:01The grizzly bears are also joining the feeding frenzy.
0:42:01 > 0:42:03I'm catching up with Casey,
0:42:03 > 0:42:08who's following the fortunes of one young bear family.
0:42:08 > 0:42:11These newborn cubs are only four months old.
0:42:12 > 0:42:14With nowhere to hide from predators,
0:42:14 > 0:42:17this meadow is a dangerous place for baby bears.
0:42:18 > 0:42:20But their mother has no choice.
0:42:22 > 0:42:27She's following the melting snow to make sure that both she and her cubs
0:42:27 > 0:42:30get enough protein from the new growth.
0:42:33 > 0:42:35- CASEY CHUCKLES - Aw, they're so cute!
0:42:36 > 0:42:38So small.
0:42:41 > 0:42:42So, you know this bear?
0:42:42 > 0:42:44Yeah, you can count on her like clockwork.
0:42:44 > 0:42:50Everything that a little cub needs to learn to survive its lifetime,
0:42:50 > 0:42:52they're learning in these first few years with its mother.
0:42:52 > 0:42:55Like where to go, what time of the year, how to dig the roots.
0:42:55 > 0:42:57They're learning all of that right now.
0:42:57 > 0:43:00This one here, you know, the year after year that I've watched her,
0:43:00 > 0:43:02she usually keeps almost all of her cubs.
0:43:02 > 0:43:03She's a great mother.
0:43:03 > 0:43:07In the ten years that Casey's been following this female,
0:43:07 > 0:43:11he's also seen her teach her offspring a more gruesome skill.
0:43:12 > 0:43:14She's a known predator.
0:43:14 > 0:43:15During this time of year,
0:43:15 > 0:43:19she'll actually go around looking for elk calves that were just born,
0:43:19 > 0:43:21that are stashed out here in the sagebrush
0:43:21 > 0:43:23and these little cubs will be in tow, and they'll be watching.
0:43:23 > 0:43:25So it's a very learned behaviour
0:43:25 > 0:43:28and some bears, right in the same area, don't hunt elk calves at all,
0:43:28 > 0:43:30cos their mother didn't teach 'em.
0:43:30 > 0:43:32They didn't get the chance to learn this.
0:43:32 > 0:43:34These little guys are going to learn that
0:43:34 > 0:43:35and I'll tell you, at this time of year,
0:43:35 > 0:43:38- that protein for these little bear cubs is essential...- Mm-hm.
0:43:38 > 0:43:40..and that may just be the thing
0:43:40 > 0:43:43that gives them the advantage to be strong bears.
0:43:43 > 0:43:46But of all the survival skills they'll need to learn,
0:43:46 > 0:43:49the most important is how to spot danger.
0:43:51 > 0:43:55Now, we've got Mum in the middle of this sagebrush
0:43:55 > 0:43:58and these little cubs bobbing their head up and down.
0:43:58 > 0:44:03Surely, you know, this is, this is a dangerous time for them as well.
0:44:03 > 0:44:08How difficult is it, you know, to be a newborn grizzly bear cub?
0:44:08 > 0:44:13Very difficult. Stats say about 50% of bear cubs in their first year,
0:44:13 > 0:44:14they don't make it.
0:44:14 > 0:44:18And there are so many things out there that are trying to kill them.
0:44:19 > 0:44:20A male grizzly's one of them.
0:44:20 > 0:44:22You know, a male grizzly knows that
0:44:22 > 0:44:24she won't mate as long as she has cubs.
0:44:24 > 0:44:26If he comes in there and he can eliminate those cubs,
0:44:26 > 0:44:29then she will come into oestrus and she will mate.
0:44:29 > 0:44:33So... Big male bears - big danger for these guys.
0:44:35 > 0:44:39Male grizzlies are known to roam this area,
0:44:39 > 0:44:41so their mother must stay on high alert.
0:44:44 > 0:44:46And what's that scar on her head?
0:44:46 > 0:44:48It looks like she's got some sort of scar.
0:44:48 > 0:44:50Yeah, it's new for her,
0:44:50 > 0:44:53so I'm assuming now that she's got these little cubs,
0:44:53 > 0:44:56that scar is probably from protecting them against a big male.
0:44:56 > 0:45:01And when it comes to that defence, these females, they don't care.
0:45:01 > 0:45:04They're going to go against the biggest male grizzly
0:45:04 > 0:45:06and it looks like she probably protected her cubs,
0:45:06 > 0:45:08but came away with a pretty big scar.
0:45:10 > 0:45:12Oh, she's nursing right now.
0:45:12 > 0:45:14She's laying on her back.
0:45:14 > 0:45:16- Oh, yeah...- Yeah, they're just going to get in there,
0:45:16 > 0:45:17they're going to relax.
0:45:17 > 0:45:20Nothing better for a little cub than that right there,
0:45:20 > 0:45:22but Mum... she still has to pay attention.
0:45:22 > 0:45:25And you can see, she'll just keep picking her head up and watching,
0:45:25 > 0:45:26looking for danger.
0:45:26 > 0:45:29There's a good chance these two little dudes will grow up to be adult bears.
0:45:30 > 0:45:33And she's... She's a rock star mum, for sure.
0:45:34 > 0:45:36Bears spend the heat of the day
0:45:36 > 0:45:38bedded down in the shade of the forest.
0:45:38 > 0:45:41But before this mother takes her cubs back to the woods,
0:45:41 > 0:45:44she has one more trick to teach them -
0:45:44 > 0:45:46how to keep cool in the hot sun.
0:45:48 > 0:45:49These little patches of snow
0:45:49 > 0:45:51that are left from the snowdrifts in the winter
0:45:51 > 0:45:53are everything for these bears,
0:45:53 > 0:45:56cos it's really their AC unit, cos the hotter it gets...
0:45:56 > 0:45:58They really seek out these snowfields
0:45:58 > 0:46:01just to go and lay their bellies on and cool down.
0:46:09 > 0:46:11CASEY LAUGHS
0:46:11 > 0:46:14That's the best thing, they're so playful, so full of energy, just...
0:46:15 > 0:46:17They'll roll around like that for hours.
0:46:20 > 0:46:24You know, they're using that snow like a slip and slide,
0:46:24 > 0:46:27just like kids playing on toboggans and sleds.
0:46:27 > 0:46:30They're starting to learn how to use their claws and their paws
0:46:30 > 0:46:31and get their balance.
0:46:31 > 0:46:34If you look around out here, I mean, this is a pretty rugged terrain,
0:46:34 > 0:46:38so they'd better get their feet underneath them pretty quick.
0:46:38 > 0:46:42I really believe that they like to have fun, just like any kid.
0:46:44 > 0:46:48But these snowy playgrounds will soon disappear
0:46:48 > 0:46:51as temperatures are now climbing every day.
0:46:54 > 0:46:57In late June, we hit 30 degrees.
0:46:59 > 0:47:01The snowpack is collapsing.
0:47:07 > 0:47:11The great Yellowstone thaw reaches its peak.
0:47:24 > 0:47:29Thousands of tonnes of meltwater cascade through the canyons...
0:47:30 > 0:47:33..bulldozing everything in their path
0:47:33 > 0:47:36and crashes on into the valleys.
0:47:41 > 0:47:44Over 250 cubic metres of water
0:47:44 > 0:47:47is passing through the river per second.
0:47:54 > 0:47:59This meltwater is vital for the regeneration of the land.
0:48:04 > 0:48:06But it comes at a cost.
0:48:09 > 0:48:10Despite the floodwater,
0:48:10 > 0:48:13Yellowstone's residents have to continue
0:48:13 > 0:48:15their daily quest to find food.
0:48:16 > 0:48:19Biologists have just told us that a herd of bison
0:48:19 > 0:48:24is attempting a river crossing not far from Yellowstone Falls.
0:48:24 > 0:48:26BISON GROWLS
0:48:26 > 0:48:28Our camera team arrives
0:48:28 > 0:48:32to find a two-month-old bison calf already struggling.
0:48:43 > 0:48:45It's been swept away from its mother.
0:48:47 > 0:48:49BISON BLEATS
0:48:51 > 0:48:53Without her body to shield it,
0:48:53 > 0:48:57the calf is pounded by the full force of the current.
0:49:04 > 0:49:07The other females have their own young to protect.
0:49:07 > 0:49:10This one is on its own.
0:49:12 > 0:49:14And it's losing energy fast.
0:49:14 > 0:49:16BISON GRUNTS
0:49:33 > 0:49:36But the exhausted calf makes it safely
0:49:36 > 0:49:39to the new pastures on the other side.
0:49:41 > 0:49:43This one is lucky.
0:49:48 > 0:49:52And further south, the Snake River is also running high.
0:49:52 > 0:49:57There's so much floodwater that one of the beavers' dams has burst.
0:49:58 > 0:50:00Yeah, this is up, isn't it?
0:50:00 > 0:50:03Look at that. I mean, it's just... It's just blown out, hasn't it?
0:50:03 > 0:50:06Wow. Yeah, that is completely busted.
0:50:06 > 0:50:08- Yeah.- You know, that water's flow...
0:50:08 > 0:50:09And it's a lot faster as well.
0:50:09 > 0:50:11Do you know what? You're not going to stop this.
0:50:11 > 0:50:13The river's got to come down here, hasn't it?
0:50:13 > 0:50:17It'll either bust through the dam or it'll go over the top of the dam, or it'll go around the dam.
0:50:17 > 0:50:21- Yeah.- And this dam just obviously wasn't quite strong enough, it's just bust through it.
0:50:21 > 0:50:24And, you know, when this dam busts, it can take out the next dam.
0:50:24 > 0:50:26- You know? It's a kind of domino effect.- Right.
0:50:26 > 0:50:28Oh, yeah. I mean, it's just...
0:50:28 > 0:50:31That is strong. So there's no way this can...
0:50:31 > 0:50:33you know, a pile of sticks is going to stop this.
0:50:33 > 0:50:36Are they even going to bother to fix that bit?
0:50:36 > 0:50:39No, I don't think they will because...
0:50:39 > 0:50:41Well, actually, you know, it'd be impossible to fix it.
0:50:41 > 0:50:45- Yeah.- They're going to... I reckon they'll wait and in a month maybe
0:50:45 > 0:50:49or more, until the water goes right back down to pre-flood level,
0:50:49 > 0:50:51then they'll build it back up.
0:50:51 > 0:50:54You know, it doesn't look like much when you look upstream,
0:50:54 > 0:50:56but right here,
0:50:56 > 0:50:59all this water is pushing up against my body and there's so much force,
0:50:59 > 0:51:02and that's what the dam is having to contend with,
0:51:02 > 0:51:04that's what it's trying to hold back.
0:51:05 > 0:51:07If this dam has bust...
0:51:08 > 0:51:10..what's happened to the beavers?
0:51:27 > 0:51:30The beavers are busier than ever.
0:51:30 > 0:51:33The water is flowing right over the top of their dam.
0:51:33 > 0:51:36But at the moment, it's still holding.
0:51:36 > 0:51:40They won't be able to fix it until the water levels fall.
0:51:51 > 0:51:56Inside the lodge, all the old bedding is pushed into the water.
0:51:58 > 0:52:00They leave it to soak for a while...
0:52:00 > 0:52:04and when it's clean, they drag it back in...
0:52:04 > 0:52:07giving them fresh new bedding for the summer.
0:52:13 > 0:52:16With temperatures now reaching 30 degrees,
0:52:16 > 0:52:18the owl chicks are struggling.
0:52:19 > 0:52:21One has already left the nest.
0:52:22 > 0:52:26These are birds that thrive in colder temperatures,
0:52:26 > 0:52:30but their nest is now exposed to the full force of the sun.
0:52:33 > 0:52:36Great grey owls struggle to lose heat,
0:52:36 > 0:52:39so the remaining two chicks must leave the nest
0:52:39 > 0:52:40as quickly as they can.
0:52:44 > 0:52:47The second chick is getting up on the lip,
0:52:47 > 0:52:51on the highest point of the nest and just flaps like crazy.
0:52:51 > 0:52:53He's looking over the edge
0:52:53 > 0:52:56and this is what they do before they jump.
0:52:57 > 0:52:58But the chicks are vulnerable.
0:52:59 > 0:53:02Their wings aren't strong enough to fly.
0:53:02 > 0:53:08When they jump, they'll plummet ten metres to the forest floor.
0:53:09 > 0:53:12Oh, he's going to go, he's going to go! Jump, jump!
0:53:14 > 0:53:16Go, go, go, go! Oh!
0:53:16 > 0:53:20He's leaning way over on the other edge.
0:53:20 > 0:53:21He could go any second.
0:53:24 > 0:53:28It's the riskiest moment of this owl's life.
0:53:36 > 0:53:39It's not the most graceful maiden flight,
0:53:39 > 0:53:41but it's success nonetheless.
0:53:41 > 0:53:44He's hanging upside down!
0:53:45 > 0:53:46Got it all messed up.
0:53:46 > 0:53:48Oh, he is struggling.
0:53:49 > 0:53:51He's hanging by one talon.
0:53:53 > 0:53:55There he goes!
0:53:59 > 0:54:03It's one thing to launch, it's another thing to land.
0:54:05 > 0:54:07The chick lands safely,
0:54:07 > 0:54:10but it must head straight back up into the tree tops,
0:54:10 > 0:54:13away from predators on the forest floor.
0:54:16 > 0:54:19With Yellowstone's climate getting warmer each year,
0:54:19 > 0:54:23these owls may not be able to live here for much longer.
0:54:28 > 0:54:31Summer temperatures are becoming too hot.
0:54:36 > 0:54:39She doesn't know what to do or where to go.
0:54:40 > 0:54:43The runt is now in danger.
0:54:44 > 0:54:48She's being struck directly by the intense heat of the sun.
0:54:50 > 0:54:51In these conditions,
0:54:51 > 0:54:54great grey owls struggle to regulate their body temperature.
0:55:04 > 0:55:06Panting helps her to cool down.
0:55:07 > 0:55:11But her downy feathers, which are so efficient in cold weather,
0:55:11 > 0:55:13may prove fatal.
0:55:18 > 0:55:19CHICK CHIRPS
0:55:20 > 0:55:22Our little girl's up again.
0:55:23 > 0:55:25Finally, but...
0:55:26 > 0:55:30She's hot too. She... She wants out of there. I can tell.
0:55:30 > 0:55:34This is a leap of faith she has GOT to make.
0:55:39 > 0:55:40She's back on the launch pad.
0:55:40 > 0:55:42This is where I think she's going to go.
0:55:46 > 0:55:47She's...
0:55:59 > 0:56:02She launched! She launched!
0:56:04 > 0:56:06CHICK CHIRPS
0:56:09 > 0:56:14So this forest is now home to three more great grey owls.
0:56:15 > 0:56:18I'm impressed. I'm proud of this little girl.
0:56:18 > 0:56:22You know, maybe one day she's going to just come back here
0:56:22 > 0:56:26and raise a great grey owl family of her own.
0:56:40 > 0:56:45As the longest day of the year approaches, spring becomes summer.
0:56:46 > 0:56:48The animals we've followed
0:56:48 > 0:56:51have made it through Yellowstone's most challenging season.
0:56:53 > 0:56:57This year, the thaw started three weeks earlier than usual.
0:57:00 > 0:57:05An area of 30,000 square miles has been transformed...
0:57:08 > 0:57:12..and the mountains have released over a million tonnes of water.
0:57:14 > 0:57:20And finally, from top to bottom, Yellowstone is free of snow.
0:57:21 > 0:57:25Now our animal families face the new challenge of summer,
0:57:25 > 0:57:28when temperatures can soar and vegetation wither.
0:57:29 > 0:57:31And that leads to a new threat -
0:57:31 > 0:57:33wildfires.
0:57:39 > 0:57:41Tomorrow night, we bring you summer,
0:57:41 > 0:57:45when soaring temperatures force the animals of Yellowstone
0:57:45 > 0:57:47to change their behaviour.
0:57:48 > 0:57:52Bears enter cowboy country as they search for food.
0:57:53 > 0:57:57The wolves display unusual hunting strategies
0:57:57 > 0:57:59to keep their 11 pups alive.
0:58:02 > 0:58:04And the beavers flee from their lodge
0:58:04 > 0:58:08in a dramatic twist to life in this changing environment.