BBC Nature_Cuckoo
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00:00This is perhaps the best known bird call in Britain.
00:28A wandering voice, Wordsworth called it.
00:33The harbinger of spring, an icon of our countryside.
00:41Yet the owner of this call is a cheat, a thief and a killer.
00:49Few know what it looks like and even fewer its unique behaviour.
01:01The cuckoo never builds a nest.
01:04Instead it tricks other species into accepting its egg as one of their own.
01:10It will steal and eat other birds' eggs.
01:23The newborn cuckoo's first instinct is to kill anything else in its nest.
01:34Finally, and perhaps most remarkably of all,
01:37the monstrous cuckoo chick manages to fool two tiny foster parents into feeding and caring for it for weeks.
01:47How does this rule breaker get away with it?
02:08A hundred years of study are only now revealing the cuckoo's secrets.
02:25Nick Davis is Professor of Behavioural Ecology at the University of Cambridge.
02:32He's one of the country's top scientists and like many ornithologists before him,
02:38he is intrigued and puzzled by the cuckoo's extraordinary behaviour.
02:45Nick has spent the last 23 years studying the cuckoo
02:49and divides his time between college life in Cambridge and his study site.
03:02We're on Wicken Fen in Cambridgeshire, right in the heart of the Fenlands
03:06and we're here because this is a fantastic place for studying cuckoos.
03:12And what makes it so good is that one of the cuckoo's favourite hosts, the reed warbler,
03:17nests right along this stretch here.
03:20And the reason the cuckoos love stretches like this
03:23is that adjoining the load, this waterway here,
03:27are these tall trees from which the cuckoos can watch the hosts.
03:38It's late April and the cuckoo's intended host or victim, the reed warbler,
03:43has made a long journey all the way from Africa.
03:50The reed warbler is just one of 20 species in Europe that the cuckoo takes advantage of.
03:56In Britain, it has four other favourites.
03:59Meadow pippets, robins, dummocks and pied wagtails.
04:03Individual female cuckoos specialise in exploiting just one particular species.
04:09Here, it's the reed warbler.
04:17Nick has spotted the first reed warblers of the season.
04:21As soon as they arrive, they busily set up territories in reed beds along the load.
04:26And each male proclaims his territory with a striking song.
04:35Once he's attracted a mate, she works hard building an intricate nest
04:39using the old reed heads and spider's webs.
04:44But the small warbler's peaceful existence on the fen is about to end.
05:05Cuckoos have arrived from Africa.
05:09Cuckoos have arrived from Africa.
05:12They know exactly when to turn up, just as the warblers are building their nests.
05:20The males arrive first and sing to announce their arrival.
05:25Male cuckoos set up territories where there are lots of warbler nests.
05:29They dash around at high speed, chasing off rival males.
05:39This will continue for the ten weeks they're in Britain.
05:50Places rich with reed warblers, like Wicken Fen, have several male cuckoos in a small area.
06:02I think it's not completely understood what the cuckoo's call is all about.
06:05It's certainly a territorial call and a keep out for rival males.
06:08So if another male comes, or I come along and mimic another male,
06:13very quickly the resident will approach and get cross.
06:17Here he comes.
06:36The male cuckoos may also be calling for females.
06:41Courtship is a rarely seen aerial display high above the fen.
06:50The female doesn't call like a male bird, but makes a strange, seldom heard, bubbling cry.
06:58Several males often chase a lone female.
07:05It's only once mating has begun that the real cunning begins.
07:13By mid-May, the first reed warbler's nests have been completed and eggs are about to be laid.
07:21Unbeknownst to them, the female cuckoo, with her distinct reddish-brown breast,
07:26is secretly watching and waiting.
07:30Unlike the female reed warbler, she will never build a nest.
07:35She has plans for this warbler nest.
07:40Well, the ancients knew all about cuckoos.
07:43Gilbert White puzzled about this, and he thought, well, maybe God had just done a bad job on cuckoos,
07:48and he called the cuckoos' lack of maternal care a monstrous outrage on maternal affection.
07:55The cuckoos' lack of maternal care was the reason why they were called the cuckoos.
07:59The cuckoos were called the cuckoos because they were the only ones who could produce young cuckoos.
08:05The cuckoos were called the cuckoos because they were the only ones who could produce young cuckoos.
08:10A monstrous outrage on maternal affection.
08:13These quaint ideas, of course, seem ridiculous now, but before Darwin came up with the idea of evolution,
08:19the cuckoos' habits really just were very odd. They made no sense at all.
08:26Darwin correctly suggested that the cuckoo's strange instinct to lay eggs in another bird's nest
08:33evolved from ancestors that had built nests.
08:40By becoming parasitic, cuckoos were freed from nest building and parental duties,
08:45so they could lay many more eggs.
08:49So successful was this cheating that it was passed on through their young.
09:01But how does the cuckoo deceive another species into raising its young?
09:10Early bird watchers were uncertain whether the cuckoo sneaked an egg or even a hatchling into the host's nest.
09:17It's a question that has puzzled naturalists since Aristotle.
09:21Remarkably, no one knew for sure until as late as the 1920s.
09:30Back then, one Englishman discovered more about cuckoo behaviour than anyone before him.
09:37Finally proving just how it manages to lay its egg in another bird's nest.
09:47Edgar Chance was a businessman, but his passion was oology, egg collecting, which is illegal today.
09:57As a wealthy man, he spent much of his time travelling the country, finding eggs to add to his collection,
10:04and he would go to any lengths to get them.
10:10By 1915, his focus had changed from being a simple egg collector to becoming a brilliant naturalist,
10:18obsessed with trying to understand the mysterious habits of the cuckoo.
10:27This is one, I think, of the greatest bits of bird watching ever done, was by Edgar Chance, who's one of my great heroes.
10:34And in the 1920s, he did some brilliant observations on a common in Worcestershire in central England.
10:41And he was the one who, for the very first time, showed how the cuckoo lays her egg.
10:47Pound Green Common
10:54Pound Green Common was close to Edgar Chance's home.
10:58It had a good population of cuckoos and one of their other favourite hosts, the meadow pipit.
11:08Chance paid local children to scour the area to find nests.
11:13He paid them well, because nestling amongst the pipit eggs were highly prized cuckoo eggs.
11:31He carefully examined the cuckoo eggs and discovered that most shared the same colour and spots.
11:37He realised that they must have all been laid by the same cuckoo.
11:43Chance named her Cuckoo A and began following her.
11:48He found all the nests she was using and collected her eggs.
12:02The egg of any individual cuckoo is unique to her.
12:08It has an avian fingerprint on the surface of its shell.
12:19By identifying individual eggs, he was able to determine which nests she had visited and when.
12:26Almost everything Chance learnt about the cuckoo came from just studying her eggs.
12:37Many, said Chance, have remained unaware how much of the bird's life story is written upon the empty shells.
12:48Chance made meticulous notes on each and every cuckoo he observed
12:53and in doing so made some remarkable discoveries about their behaviour.
12:59One of the things he learnt was the cuckoo lays every other day.
13:03He also learnt that the cuckoo lays her egg in the afternoon.
13:06That was a big shock. Most birds lay their eggs early in the morning.
13:10It really took many weeks of getting up at dawn and before to realise that the cuckoo must have laid the previous evening.
13:18Once he then knew the timing of the egg laying, then he could watch the cuckoo's behaviour in more detail.
13:25And he got so good at predicting which nest the cuckoo would choose next that he was able to set up a hide
13:33and for the very first time actually film the egg laying of the cuckoo.
13:40This remarkable footage, shot for Edgar Chance in 1921 by cameraman Oliver Pike, is one of the earliest wildlife films ever made.
13:56Once Chance had decided which pipit nest the cuckoo was going to target,
14:01he placed his hide and camera close by, hoping to solve the mystery of how the cuckoo deposits its egg.
14:11This is what he filmed.
14:13The female cuckoo glides in from a distant tree to a pipit nest she has been observing carefully for several days,
14:21concealed in a tuft of heather.
14:26Next, the cuckoo hops on the ground and lays its egg directly into the nest while the adult pipits try to attack her.
14:41A very determined Edwardian naturalist had finally solved the age-old riddle.
14:47The cuckoo lays directly into the host nest.
14:56But though Edgar Chance had evidence of how the cuckoo lays her egg,
15:00the question of how it actually fools the host into accepting it wouldn't be solved for another 50 years.
15:15The reason that I got interested in the cuckoo is that, of course, it's one of nature's most famous cheats.
15:20And, in theory, this cheating should provoke an evolutionary arms race between the hosts and the cuckoo.
15:28Now, once hosts start evolving defences, that should then provoke improved trickery by the cuckoo.
15:34And that, in turn, would provoke even better host defences and so on.
15:38So the two sides in the arms race should improve their adaptations and counter-adaptations over evolutionary time.
15:46And I wanted to try and test by experiment whether this was going on.
15:50I mean, Edgar Chance had shown very beautifully what the cuckoo does.
15:54I wanted to try and understand why does the cuckoo behave in this particular way
15:58and have the hosts evolve counter-tricks to try and defend themselves against the cuckoo.
16:04Nick's approach isn't just to observe, but also to scientifically test the reasons for the cuckoo's cheating ways.
16:12The cuckoo is a threatened species.
16:14There are fewer than half the number of cuckoos in the UK today than there were in Chance's day.
16:20And it's detailed knowledge like Nick's that might help to save them.
16:24On Wycombe Fen, the cuckoos lay in reed warbler nests.
16:28So their eggs have to look exactly like the eggs of the reed warbler.
16:32They have to be the same colour, pattern and size.
16:36Nick can test how important the mimicry is by placing a rail of leaves.
16:42On Wycombe Fen, the cuckoos lay in reed warbler nests.
16:45They have to be the same colour, pattern and size.
16:55Nick can test how important the mimicry is by placing a rail of leaves.
16:59Nick can test how important the mimicry is by placing a wrongly coloured egg into the
17:07reed warbler's nest and seeing how the reed warbler reacts.
17:27The reed warbler returns and checks her nest.
17:30She settles down seemingly unaware of the rogue egg, but then she senses something isn't
17:36quite right.
17:50She starts to peck the egg.
18:01Each time she returns to the nest, she repeatedly targets the new egg until eventually she punctures
18:08it.
18:10Next, she drinks some of the contents until it's safe to move it without spilling something
18:17over the other eggs.
18:22To destroy an egg that might hatch out into your own chick would be a calamity.
18:27There is enough variation in their own eggs that they could make a mistake, but it's worth
18:32the risk.
18:33They have to ensure the survival of their own offspring.
18:42If you give reed warblers a blue egg or a white egg or a brown egg, very different from
18:47their own green eggs, they throw them out.
18:50But if you give them a green egg matching their own eggs, in other words mimicking what
18:54the cuckoo actually does, the reed warblers tend to accept that.
18:59If you give them a giant egg, the reed warblers find it very difficult to sit on and will
19:03often desert.
19:05So the cuckoo's egg not only has to match the reed warbler's eggs in colour, but it
19:09also has to match reasonably in size too, if the cuckoo's got to get its egg accepted.
19:14So those very simple experiments show that this egg mimicry by the cuckoo is a crucial
19:18part of their trickery.
19:21The common cuckoo species is divided into several races, each with a distinctive egg
19:27that matches the colour of its particular host.
19:35Blue cuckoo eggs to copy red start eggs, speckled green cuckoo eggs to copy warbler eggs and
19:42so on.
19:43They are the only species that can do this, but each individual female cuckoo can only
19:49lay one egg type.
19:52So the reed warblers do have ways of protecting their nests, wrong colour, size or shape and
19:59the egg is out.
20:02Only if the cuckoo's egg is a good match will she outsmart her host.
20:08A well-matched egg though doesn't mean the cuckoo's work is done.
20:12She now has to decide exactly when to lay her egg.
20:19The reed warbler lays a single egg every day for four or five days.
20:25The female cuckoo must keep watch on the reed warblers to make sure she lays on the same
20:29days they do.
20:35The female cuckoo glides in.
20:47The egg laying is completed in seconds.
20:58If the cuckoo leaves it too late and the warblers have laid all their eggs, then the cuckoo
21:03chick might not hatch out in time.
21:07But if the cuckoo lays too early, there's a problem there too.
21:10If we put our model eggs in before the hosts have begun to lay, those model eggs always
21:16get thrown out.
21:18So very sensibly the female reed warbler knows that if she hasn't started to lay eggs yet,
21:22that egg in the nest can't possibly be mine.
21:28You get a completely different perspective down here at the water level.
21:36I'm normally up on the bank looking for reed warblers' nests and down here you really enter
21:41the reed warbler's world and you can see the habitat from their perspective.
21:46Every 20 metres or so there's a new territory.
21:48I've just seen one of the birds hopping around in amongst the reeds there.
21:52They don't seem to mind us at all as we go.
21:55And all the while, cuckoos up in those trees behind, birdwatching, you must imagine what
22:02it's like to be a reed warbler in the reeds being observed up there.
22:15The female cuckoo's job, although not as laborious, is every bit as time-consuming as that of
22:20the reed warblers.
22:22A cuckoo can lay 10 eggs or more in one season, but she only lays one egg per nest.
22:28This means she has to stake out dozens of reed warbler nests within her territory to
22:34ensure she can lay each precious egg in the best nest at the best time.
22:39She may wait days or weeks for the timing to be perfect.
22:48When we ourselves have adopted the strategy of the cuckoo and have tried to do these model
22:52egg experiments, by the end of the day we've convinced ourselves that it's actually a crazy
22:57thing to do.
22:58It's such hard work looking for host nests and I think if I was a bird I'd just be an
23:03honest worker and raise my own young.
23:11Just how many eggs a cuckoo can lay in any one season fascinated Edgar Chance, but not
23:17only for scientific reasons.
23:26There's no doubt that one of his motivations for discovering the cuckoo's laying procedure
23:32was so that he could collect the most number of eggs that a cuckoo had laid in a season.
23:42Edgar Chance was determined to beat a rival collector in Germany who claimed the world
23:46record for the number of eggs laid by a cuckoo in one season.
24:00Edgar Chance did get his world record.
24:02He managed to get 25 eggs in one season from his beloved cuckoo egg.
24:12He collected every pippet and cuckoo egg that he could and his vast collection is now held
24:18at the Natural History Museum at Tring.
24:26We now know that a typical cuckoo will lay about 10 eggs in any one season, but Chance
24:33intervened to make sure his female could lay more.
24:39Actually this world record was achieved with Edgar Chance's help.
24:44What Chance did is he used to farm the medipippet's nests in the sense that if incubation was
24:50already underway and the cuckoo had missed that nest, Edgar Chance would collect all
24:54the eggs and that would force those pippets to start a replacement nest and by farming
25:00nests in this way and making more new nests available for the cuckoo at a suitable stage,
25:05he managed to get the world record.
25:11Egg collecting was key to many of Chance's discoveries about cuckoos.
25:18It wasn't illegal as it is now, but even back then some naturalists disapproved.
25:31Chance was actually drummed out of the British Ornithologist Union because of his egg collecting
25:35habits.
25:36So even back in the 1920s, many people regarded egg collecting as something which you simply
25:42shouldn't do.
25:45There's a final twist in this amazing story.
25:49Chance's trick of removing eggs to encourage the host bird to lay another clutch is actually
25:55just what a female cuckoo would do.
26:05The cuckoo is the only British bird to do this.
26:08Their behaviour is unique.
26:21This cuckoo will eat whole clutches of eggs.
26:25Just like Chance, she wants to encourage the reed warbler to lay more clutches.
26:37Regardless of his methods, Chance's record stood and no one thought that any single cuckoo
26:43would ever lay as many eggs in one season.
27:00Sixty-five years after Edgar Chance, another amateur ornithologist appeared on the scene,
27:07but he never took a single egg.
27:16I first became interested in cuckoos in June 1983.
27:20I was doing a water rail survey, and to alleviate the boredom of sitting there, I watched some
27:25reed warblers.
27:26And when I found the nest, I was surprised to see a cuckoo egg in it.
27:30And I thought, well, this is far more interesting than watching water rails that aren't there,
27:34so I'll see if there's any more reed warblers in the area.
27:37And I found another three pairs, all of which contain cuckoo eggs of the same female.
27:44Now you might say I've been bitten by the bug.
27:49Mike Bayliss proved himself to be every bit as skilful as Edgar Chance, even though he
27:55had a full-time job and could only search for reed warbler nests in his time off.
28:09It wasn't as if I was really trying to break his record, or even equal it.
28:13It was only when I passed the 20 mark that I realised it was attainable.
28:18We did hear one singing across here, didn't we?
28:22Let's just cruise along here.
28:25Mike observed reed warblers in the reed beds along the Thames.
28:31And like Chance, he got lucky.
28:34One female he called Cuckoo X returned to the same site year after year.
28:41I'd say the best year was obviously when my Cuckoo X in 1988 achieved a world record
28:49under natural conditions when she laid 25 eggs.
28:52This had previously been done by Edgar Chance under experimental conditions in 1922.
29:00Mike had shown just how productive one cuckoo could be under natural conditions.
29:07Cuckoo X returned to Oxford for eight seasons and laid a total of 113 eggs.
29:15Again, like Chance, Mike used his detailed knowledge to get amazing footage of a cuckoo
29:24laying her eggs.
29:32This is Cuckoo X laying her egg in a reed warbler's nest in 1989.
29:38First, the female cuckoo removes a warbler egg.
29:43Holding it carefully in her bill, she then slips forward to lay her own egg.
29:48Now.
29:52It only takes a few seconds.
29:56So, why does the cuckoo have to be so quick?
30:02You can test this by experiment, and what we've done is we've put a stuffed cuckoo
30:09next to a reed warbler nest to simulate, if you like, a female who's very slow at laying.
30:26Not surprisingly, if the reed warblers see this cuckoo, they mob it like mad.
30:33They've got this harsh, scolding noise like this.
30:44What is surprising, though, is that not all reed warblers react in the same way.
30:49Naive birds tend to treat a cuckoo like a dangerous bird of prey.
30:54It looks rather similar, and so they won't get too close.
31:05Experienced parent reed warblers will close right in.
31:09They know they aren't in any danger.
31:16This shows Nick that whilst reed warblers instinctively know to reject certain eggs,
31:22they actually have to learn to recognise the cuckoo.
31:26The interesting result is that when you take the cuckoo away,
31:30the reed warblers seem much more alert to any change in their nest contents.
31:35And our experiments show that they now are very likely to reject
31:38even a good matching model egg from their nest.
31:45The arms race is very much alive at this stage.
31:49The warblers can fight back, and their defences can work.
31:55For the cuckoo's trickery to be successful, there is a lot she has to do.
32:07She must first remove one or more warbler egg to make room for her egg.
32:20She has to lay a similar looking egg, so it's not recognised and thrown out.
32:26And she has to do this quickly, so she doesn't alert the warblers.
32:34If the cuckoo gets all this right, the trap is set.
32:50Warblers who have been tricked can have no idea of what is to come.
32:59Those whose nests have remained safe from the cuckoo
33:03are ready for a brood of their own hungry chicks to emerge.
33:11The long days of summer with endless supplies of insects
33:15are bountiful for the warblers of Wicken Fen.
33:26In a good year, a pair of warblers can raise two broods each of up to four or five chicks.
33:34Those that have been tricked by the cuckoo will have no time for a second brood.
33:39It takes as much effort to raise one cuckoo as ten of their own chicks.
33:53The cuckoo chick has just hatched, and now the reed warbler is ready.
33:59The cuckoo chick has just hatched, and now the reed warblers have lost everything.
34:09Their lives will be totally dominated by this imposter, and there is nothing they can do about it.
34:21Just 24 hours old and still naked and blind,
34:25the cuckoo chick instinctively pushes out any other eggs in the nest.
34:31So why is it left to the newborn hatchling to take on this Herculean task?
34:41You might think that one of the things the female cuckoo could do
34:44is simply remove all the host eggs and leave her egg instead.
34:48Well, the host will always desert a single egg, so she can't do that.
34:52And that explains very nicely why it's the young cuckoo that has to take on this task of rejecting the host eggs.
34:59Because although the hosts always desert a single egg, they never desert a single chick.
35:06The cuckoo chick is astonishingly strong and has a distinctive hollow back
35:11that helps balance the host egg or chick before throwing it out of the nest.
35:23Nothing the little ogre does alarms the foster parents,
35:27even when their own eggs are being forced out of the nest from right beneath them.
35:39The simple fact is that a warbler nest won't be big enough
35:43to hold both reed warbler chicks and the growing cuckoo chick.
35:47The imposter will need all the food that its adopted parents can bring.
35:52Sometimes the reed warbler's eggs are more advanced,
36:19and both warbler and cuckoo chicks hatch together.
36:23Again, it falls to the blind cuckoo chick to deal with the situation.
36:53The cuckoo chick will have to deal with the warbler and the warbler will have to deal with the warbler.
37:03The warbler will have to deal with the warbler and the warbler will have to deal with the warbler.
37:13You might think the cuckoo's cruel, and of course in a way it is,
37:40but it's no more cruel than the reed warbler.
37:43If I was a fanatic of damselflies and dragonflies,
37:47I might get very upset when I see a reed warbler murder a damselfly and feed it to its chicks,
37:53just as I would when I see a cuckoo chick eject reed warbler eggs or reed warbler young.
37:58In nature, individuals are cruel.
38:01They're all out for what they can get, exploiting others as food or hosts or whatever.
38:11Those reed warbler pairs who managed to escape the attention of the cuckoo,
38:15or spotted the egg and ejected it, are now busy looking after their own brood.
38:27These reed warbler chicks are nine days old and demand to be fed whenever there's daylight.
38:41In the nest nearby, the cuckoo chick is about eight days old.
38:46Having cleared the nest of all competition, there's now just one hungry mouth,
38:51devouring all the food the warblers can bring to the nest.
39:09Just two days later, and the cuckoo chick has grown massively.
39:17It's about halfway through its time in the nest now and can barely still fit inside.
39:27When I was a young student, I saw in the fens here, actually,
39:31a little reed warbler feeding an enormous cuckoo chick.
39:35And this just amazed me.
39:37I think this is one of the most astonishing things you can see in the whole of nature.
39:44Its foster parents are lavishing as much care and attention upon it as they would for their own brood,
39:50instinctively caring for whatever hatches from an egg they assume is their own.
40:06The cuckoo chick is well-fed and it's huge in comparison to reed warbler chicks of the same age.
40:20It's a very strange-looking youngster and bears no resemblance at all to a reed warbler chick.
40:27It has fooled a pair of adult warblers into working as hard as they possibly can 16 hours a day,
40:34an extraordinary feat of manipulation.
41:00Reed warblers are programmed to treat any chick in their nest as one of their own,
41:05but why do they feed it so well?
41:09It's a question that has puzzled bird watchers and academics since it was first observed.
41:21The cuckoo chick does have a problem and the problem is how on earth does it stimulate the reed warblers
41:26to bring as much food as they would to a whole brood of their own hungry young?
41:37Nick believes he's discovered the answer.
41:40It's a trick the cuckoo chick uses from the very moment it hatches to make sure it gets as much food as it needs.
41:52If you listen to the cuckoo's begging call it is absolutely remarkable.
41:56Most chicks when they're hungry just go cheep cheep,
41:59but the cuckoo's got the most incredibly rapid call.
42:02It goes cheep cheep cheep cheep cheep cheep very fast.
42:06And when it's a week of age actually it sounds like a whole brood of hungry reed warbler chicks
42:11and by two or three weeks of age it sounds like two broods of hungry reed warbler chicks.
42:17So we thought maybe it's this very rapid begging which simulates lots of hungry young
42:22which spurs the reed warbler foster parents onto extra work.
42:29Nick and his colleague Becky Kilner tested this idea with an ingenious experiment.
42:37First they carefully borrowed a blackbird chick the same size as a cuckoo chick
42:42and temporarily swapped them around.
42:45So now a blackbird chick is in the nest that was occupied by a similar size cuckoo chick.
42:51Next to this nest they placed a tiny loudspeaker connected to a tape player.
42:58They measured how much food the warbler's brought to the blackbird chick on its own
43:03and compared this with how much food the warbler's brought in
43:06when they played cuckoo chick begging calls through the speaker.
43:10Their results were astonishing.
43:13The reed warblers are coming now, I can see the reeds moving.
43:16Here we go, here we go.
43:18Right, blackbird begging now.
43:20Okay.
43:22Still begging.
43:25Okay, feeding now.
43:27The size of the chick is not important.
43:30A big chick alone doesn't encourage the reed warbler parents to bring more food.
43:34A cuckoo's begging call is vital.
43:39And now she's gone, stop begging.
43:43The warblers will feed any chick in their nest,
43:46but only with the cuckoo's begging calls will they bring enough food to satisfy a chick this big.
43:54The female cuckoo uses the visual trickery to get her egg accepted
43:59and the cuckoo chick uses vocal trickery to get enough food.
44:10It's now the beginning of July,
44:12just ten weeks since the adult cuckoos arrived in Britain
44:16and they're already leaving for home.
44:18With no more new reed warbler nests being built
44:21and no new opportunities for the cuckoo, they set off.
44:27Cuckoo.
44:30Cuckoo.
44:32Cuckoo.
44:34Cuckoo.
44:36Cuckoo.
44:38They have the shortest breeding season of any British migrant
44:42and the birds can be back under African skies
44:45before the last of their offspring has even left the nest.
44:49Cuckoo.
44:51Cuckoo.
44:57Cuckoo.
44:59The giant cuckoo chick is 20 days old.
45:04The nest can hardly hold it any longer.
45:08Soon it will have to leave the nest,
45:11but still the reed warblers will continue to feed it for another two weeks
45:15until it becomes independent.
45:19This reed warbler's season has been wasted
45:23raising another species' offspring.
45:27In a way, it's surprising there are not more cheats to exploit honest workers.
45:32The big question is why.
45:35There's only one parasitic bird in Britain, that's the cuckoo.
45:39In fact, only about 100 birds out of the 10,000 species in the world
45:43are professional cheaters.
45:45And I think the reason is simply that cheating seems a wonderful thing to do
45:49until those who are duped begin to fight back.
45:52And I think it's the fact that the hosts fight back
45:55which really limits the cuckoo's options
45:58and is the reason why cheating doesn't really prosper so well in the very long term.
46:09It's hard to believe
46:12It's hard to believe that in three to four weeks
46:16this clumsy chick will begin its very first journey
46:21a 3,000 mile flight to Africa.
46:26If it survives the long and testing flight
46:29it will return to the fence next year
46:32ready and able to trick the reed warblers yet again.
46:42I think people often like the idea of individuals
46:46who make a living in a rather unusual way, don't follow the crowd.
46:50I think they admire cuckoos because they wonder how on earth they can get away with it.
46:55They equate the cuckoo's behaviour with tremendous cunning.
47:00They don't follow the crowd.
47:02I think they admire cuckoos because they wonder how on earth they can get away with it.
47:07They equate the cuckoo's behaviour with tremendous cunning and wit, if you like.
47:13There's an old saying, I think by Edward Topsel
47:16who says the grand creator has given the cuckoo extra wit
47:20to make up for the fact that it lacks maternal affection.
47:28The cuckoo will need all the wit it can find
47:31for its future is uncertain.
47:35Nick's research will be vital for saving it.
47:39For not only is the cuckoo in an arms race with all the host species
47:44but the cuckoo has also had to cope with huge changes in our countryside.
47:50We should treasure the brief summer visit of the cuckoo
47:54and listen out for that delightful call.
47:58I for one hope that it continues to announce spring for years to come.
48:19Tom Courage next on BBC Two
48:21wetting your appetite with brunches and roasts for the weekend
48:25among his best dishes ever.