0:00:04 > 0:00:07Each year in the heart of southern Sweden,
0:00:07 > 0:00:10a special lake hosts a natural spectacle.
0:00:10 > 0:00:16It's a stop off point for thousands of cranes migrating north.
0:00:17 > 0:00:21A sight in itself, but when the moment is just right,
0:00:21 > 0:00:26the cranes stage a remarkable performance which once seen is never forgotten.
0:00:26 > 0:00:29The dance of the cranes.
0:00:39 > 0:00:41It's the end of March in Sweden,
0:00:41 > 0:00:45and at Lake Hornborga, spring should have arrived.
0:00:45 > 0:00:49But this winter has been one of the worst in recent memory.
0:00:49 > 0:00:56Half a metre of ice has covered the shallow lake for months, and snow still lies on the ground.
0:00:58 > 0:01:02But regardless of this, the cranes have already started to arrive.
0:01:05 > 0:01:11They have spent the winter in sunny Spain, but their breeding grounds are in northern Scandinavia.
0:01:16 > 0:01:20Gliding in gracefully, they take their place on the stage.
0:01:33 > 0:01:38When it's warmer and all of the cranes have arrived, the show will begin.
0:01:46 > 0:01:50The dance they perform will be between couples,
0:01:50 > 0:01:54and will strengthen relationships prior to breeding.
0:01:56 > 0:02:01But for now, food is the first priority.
0:02:01 > 0:02:05And food is the main reason the cranes first chose to come here.
0:02:05 > 0:02:12These fields were once full of potatoes, the perfect fuel for migrating cranes.
0:02:13 > 0:02:16When the farmers here stopped growing potatoes,
0:02:16 > 0:02:21they were so attached to these visitors that they fed them grain instead.
0:02:21 > 0:02:28And today, the cranes continue to be fed, ensuring that they return to the lake each year.
0:02:33 > 0:02:37As April arrives, more and more cranes show up at the lake.
0:02:37 > 0:02:41With them, they have finally brought the spring.
0:02:52 > 0:02:55Their performance is just days away.
0:03:06 > 0:03:12The ice on the lake has melted, and in the open water other visitors have gathered.
0:03:19 > 0:03:22Whooper swans also stop off on their journey north.
0:03:28 > 0:03:34And great crested grebes have arrived, already showing signs of courtship.
0:03:43 > 0:03:50As each day draws to an end, the cranes head off to their roosting ground at the north end of the lake.
0:03:55 > 0:04:00But come the morning, they'll be back for more grain.
0:04:11 > 0:04:15Lake Hornborga is a big success story in Sweden.
0:04:15 > 0:04:19Over many years, farming practices had made water levels drop.
0:04:19 > 0:04:25There was little left of the lake apart from boggy, scrub-filled fields.
0:04:25 > 0:04:30But in recent years, scrub has been cleared and water levels restored.
0:04:32 > 0:04:35The cranes are well looked after.
0:04:40 > 0:04:46It's mid-April and the number of cranes has peaked at around 10,000.
0:04:48 > 0:04:52Nobody knows exactly what triggers the dance, but for whatever reason,
0:04:52 > 0:04:56the cranes still don't quite seem ready for their curtain call.
0:04:58 > 0:05:01With so many birds in such a small area,
0:05:01 > 0:05:05it's easy to understand why they have to be fed.
0:05:05 > 0:05:08And these are big birds, taller than most men.
0:05:12 > 0:05:16They have an appetite to match their considerable size.
0:05:17 > 0:05:21The cranes soon pick out all the grain from the grassy banks.
0:05:21 > 0:05:25It will give them the energy they need to continue the journey north.
0:05:38 > 0:05:43The numbers of whooper swans on the lake have also reached a peak.
0:05:43 > 0:05:45The lake is getting crowded.
0:05:47 > 0:05:51With less space, males become more aggressive.
0:05:51 > 0:05:53They too are preparing to breed.
0:05:57 > 0:06:00But the swans are the supporting act.
0:06:08 > 0:06:13The stars are now ready for the show to begin.
0:06:13 > 0:06:17It is the climax of the gathering at Lake Hornborga.
0:06:17 > 0:06:19The dance of the cranes.
0:06:37 > 0:06:41Cranes mate for life, so some will have joined their partners
0:06:41 > 0:06:44in this display for as many as 20 years.
0:06:51 > 0:06:55Like us, some are better dancers than others.
0:07:03 > 0:07:05But whether clumsy or graceful,
0:07:05 > 0:07:10the dance affirms bonds between couples like a renewal of vows.
0:07:15 > 0:07:20The whole performance ensures that that when they reach the breeding grounds,
0:07:20 > 0:07:24they will be ready to get straight to the business of raising a family.
0:07:50 > 0:07:54With the dance completed, the time has come for the cranes to move on.
0:07:54 > 0:07:58Their breeding site is much further north.
0:07:58 > 0:08:00When conditions are right,
0:08:00 > 0:08:06they take off and fly up to great heights, using thermals for lift.
0:08:06 > 0:08:10Gradually as more and more leave, the numbers at the lake dwindle.
0:08:10 > 0:08:14By the end of April, the cranes will have gone.
0:08:14 > 0:08:16But the show must go on.
0:08:16 > 0:08:20In autumn they will stop off at the lake on their return journey south,
0:08:20 > 0:08:25with some new troop members in training for next season's performance.
0:08:32 > 0:08:36And with a little help from man to ensure this site provides food
0:08:36 > 0:08:41and shelter, this will continue to be the lake of the dancing cranes.