0:00:11 > 0:00:13CUCKOO SINGS
0:00:21 > 0:00:26This is perhaps the best-known bird call in Britain.
0:00:29 > 0:00:33A wandering voice, Wordsworth called it.
0:00:33 > 0:00:35The harbinger of Spring.
0:00:35 > 0:00:37An icon of our countryside.
0:00:41 > 0:00:47Yet the owner of this call is a cheat, a thief and a killer.
0:00:53 > 0:00:57Few know what it looks like, and even fewer its unique behaviour.
0:01:01 > 0:01:03The cuckoo never builds a nest.
0:01:03 > 0:01:06Instead, it tricks other species
0:01:06 > 0:01:09into accepting its egg as one of their own.
0:01:14 > 0:01:18It will steal and eat other birds' eggs.
0:01:23 > 0:01:25The new-born cuckoo's first instinct
0:01:25 > 0:01:28is to kill anything else in its nest.
0:01:33 > 0:01:39Finally, and perhaps most remarkably of all, the monstrous cuckoo chick
0:01:39 > 0:01:43manages to fool two tiny foster parents into feeding
0:01:43 > 0:01:45and caring for it, for weeks.
0:01:47 > 0:01:51How does this rule-breaker get away with it?
0:02:08 > 0:02:13100 years of study are only now revealing the cuckoo's secrets.
0:02:25 > 0:02:29Nick Davies is Professor of Behavioural Ecology
0:02:29 > 0:02:31at the University of Cambridge.
0:02:33 > 0:02:38He's one of the country's top scientists and like many ornithologists before him,
0:02:38 > 0:02:43he is intrigued and puzzled by the cuckoo's extraordinary behaviour.
0:02:45 > 0:02:50Nick has spent the last 23 years studying the cuckoo
0:02:50 > 0:02:53and divides his time between college life in Cambridge
0:02:53 > 0:02:55and his study site.
0:03:02 > 0:03:04We're on Wicken Fen in Cambridgeshire,
0:03:04 > 0:03:07right in the heart of the Fenlands and we're here
0:03:07 > 0:03:12because this is a fantastic place for studying cuckoos and what makes it so
0:03:12 > 0:03:15good is that one of the cuckoo's favourite hosts,
0:03:15 > 0:03:19the reed warbler, nests right along this stretch here.
0:03:19 > 0:03:23And the reason the cuckoos love stretches like this is that adjoining
0:03:23 > 0:03:26the lode, this waterway here,
0:03:26 > 0:03:32are these tall trees, from which the cuckoos can watch the hosts.
0:03:38 > 0:03:42It's late April, and the cuckoo's intended host or victim, the reed
0:03:42 > 0:03:46warbler, has made a long journey all the way from Africa.
0:03:50 > 0:03:51The reed warbler
0:03:51 > 0:03:55is just one of 20 species in Europe that the cuckoo takes advantage of.
0:03:55 > 0:03:59In Britain, it has four other favourites - meadow pipits,
0:03:59 > 0:04:02robins, dunnocks and pied wagtails.
0:04:02 > 0:04:06Individual female cuckoos specialise in exploiting
0:04:06 > 0:04:10just one particular species - here it's the reed warbler.
0:04:16 > 0:04:20Nick has spotted the first reed warblers of the season.
0:04:20 > 0:04:24As soon as they arrive, they busily set up territories in reed beds
0:04:24 > 0:04:28along the lode and each male proclaims his territory
0:04:28 > 0:04:30with a striking song.
0:04:35 > 0:04:38Once he's attracted a mate, she works hard building an
0:04:38 > 0:04:42intricate nest, using the old reed heads and spiders' webs.
0:04:48 > 0:04:52But the small warbler's peaceful existence on the fen
0:04:52 > 0:04:54is about to end.
0:04:57 > 0:05:00CUCKOO SINGS
0:05:09 > 0:05:11Cuckoos have arrived from Africa.
0:05:11 > 0:05:14They know exactly when to turn up,
0:05:14 > 0:05:18just as the warblers are building their nests.
0:05:20 > 0:05:24The males arrive first and sing to announce their arrival.
0:05:24 > 0:05:29Male cuckoos set up territories where there are lots of warbler nests.
0:05:29 > 0:05:33They dash around at high speed, chasing off rival males.
0:05:39 > 0:05:43This will continue for the ten weeks they are in Britain.
0:05:50 > 0:05:52Places rich with reed warblers,
0:05:52 > 0:05:56like Wicken Fen, have several male cuckoos in a small area.
0:05:59 > 0:06:01HE MIMICS THE CUCKOO SONG
0:06:01 > 0:06:05It's not completely understood what the cuckoo's call is all about.
0:06:05 > 0:06:08It's certainly a territorial call and a "Keep Out" for rival males.
0:06:08 > 0:06:13So if another male comes, or I come along and mimic another male,
0:06:13 > 0:06:16very quickly the resident will approach and get cross.
0:06:16 > 0:06:18Here he comes.
0:06:38 > 0:06:42The male cuckoos may also be calling for females.
0:06:42 > 0:06:47Courtship is a rarely seen aerial display high above the fen.
0:06:51 > 0:06:57The female doesn't call like a male bird but makes a strange,
0:06:57 > 0:06:59seldom heard, bubbling cry.
0:07:03 > 0:07:08Several males often chase a lone female.
0:07:10 > 0:07:15It's only once mating has begun that the real cunning begins.
0:07:18 > 0:07:22By mid-May, the first reed warblers' nests have been completed
0:07:22 > 0:07:25and eggs are about to be laid.
0:07:28 > 0:07:31Unbeknownst to them, the female cuckoo,
0:07:31 > 0:07:34with her distinct reddish brown breast,
0:07:34 > 0:07:37is secretly watching and waiting.
0:07:37 > 0:07:42Unlike the female reed warbler, she will never build a nest.
0:07:42 > 0:07:45She has plans for this warbler nest.
0:07:47 > 0:07:50Well, the Ancients knew all about cuckoos.
0:07:50 > 0:07:54They knew they were parasites and they just couldn't work out why
0:07:54 > 0:07:58a bird would bother producing young and not look after them.
0:08:01 > 0:08:04Gilbert White puzzled about this and thought maybe God
0:08:04 > 0:08:06had just done a bad job on cuckoos,
0:08:06 > 0:08:10and he called the cuckoo's lack of maternal care a monstrous
0:08:10 > 0:08:13outrage on maternal affection.
0:08:13 > 0:08:17These quaint ideas seem ridiculous now but before Darwin came up with
0:08:17 > 0:08:19the idea of evolution, the cuckoo's
0:08:19 > 0:08:24habits really just were very odd and they made no sense at all.
0:08:25 > 0:08:30Darwin correctly suggested that the cuckoo's strange instinct to lay
0:08:30 > 0:08:36eggs in another bird's nest evolved from ancestors that had built nests.
0:08:39 > 0:08:41By becoming parasitic,
0:08:41 > 0:08:45cuckoos were freed from nest-building and parental duties,
0:08:45 > 0:08:48so they could lay many more eggs.
0:08:48 > 0:08:50So successful was this cheating,
0:08:50 > 0:08:53that it was passed on through their young.
0:09:00 > 0:09:04But how does the cuckoo deceive another species
0:09:04 > 0:09:06into raising its young?
0:09:10 > 0:09:12Early birdwatchers were uncertain
0:09:12 > 0:09:16whether the cuckoo sneaked an egg or even a hatchling into the host nest.
0:09:16 > 0:09:20It's a question that has puzzled naturalists since Aristotle.
0:09:20 > 0:09:25Remarkably, no-one knew for sure until as late as the 1920s.
0:09:29 > 0:09:32Back then, one Englishman
0:09:32 > 0:09:36discovered more about cuckoo behaviour than anyone before him...
0:09:37 > 0:09:41..finally proving just how it manages to lay its egg
0:09:41 > 0:09:42in another bird's nest.
0:09:46 > 0:09:52Edgar Chance was a businessman, but his passion was oology,
0:09:52 > 0:09:55egg collecting, which is illegal today.
0:10:00 > 0:10:02As a wealthy man, he spent much of his time
0:10:02 > 0:10:06travelling the country, finding eggs to add to his collection
0:10:06 > 0:10:09and he would go to any lengths to get them.
0:10:12 > 0:10:17By 1915, his focus had changed from being a simple egg collector
0:10:17 > 0:10:20to becoming a brilliant naturalist,
0:10:20 > 0:10:25obsessed with trying to understand the mysterious habits of the cuckoo.
0:10:26 > 0:10:30This is one, I think, of the greatest bits of bird watching ever done,
0:10:30 > 0:10:33was by Edgar Chance, who is one of my great heroes.
0:10:33 > 0:10:38And in the 1920s, he did some brilliant observations on a common
0:10:38 > 0:10:40in Worcestershire in central England
0:10:40 > 0:10:43and he was the one who, for the very first time,
0:10:43 > 0:10:45showed how the cuckoo lays her egg.
0:10:54 > 0:10:58Pound Green Common was close to Edgar Chance's home.
0:10:58 > 0:11:00It had a good population of cuckoos
0:11:00 > 0:11:04and one of their other favourite hosts - the meadow pipit.
0:11:08 > 0:11:12Chance paid local children to scour the area to find nests.
0:11:12 > 0:11:14He paid them well, because
0:11:14 > 0:11:19nestling amongst the pipit eggs were highly-prized cuckoo eggs.
0:11:19 > 0:11:21Look, I've found it!
0:11:23 > 0:11:25- Found one!- Well done, children!
0:11:28 > 0:11:31- There we go.- Thank you very much.
0:11:31 > 0:11:35He carefully examined the cuckoo eggs and discovered that most
0:11:35 > 0:11:37shared the same colour and spots.
0:11:37 > 0:11:42He realised that they must have all been laid by the same cuckoo.
0:11:43 > 0:11:47Chance named her Cuckoo A and began following her.
0:11:47 > 0:11:52He found all the nests she was using and collected her eggs.
0:12:02 > 0:12:06The egg of any individual cuckoo is unique to her.
0:12:09 > 0:12:13It has an avian fingerprint on the surface of its shell.
0:12:20 > 0:12:22By identifying individual eggs,
0:12:22 > 0:12:27he was able to determine which nests she had visited and when.
0:12:27 > 0:12:30Almost everything Chance learnt about the cuckoo
0:12:30 > 0:12:32came from just studying her eggs.
0:12:37 > 0:12:40"Many", said Chance, "have remained unaware how
0:12:40 > 0:12:45"much of the bird's life-story is written upon the empty shells".
0:12:48 > 0:12:52Chance made meticulous notes on each and every cuckoo he observed
0:12:52 > 0:12:58and in doing so made some remarkable discoveries about their behaviour.
0:12:59 > 0:13:03One of the things he learnt was the cuckoo lays every other day.
0:13:03 > 0:13:06He also learnt the cuckoo lays her egg in the afternoon and
0:13:06 > 0:13:09that was a big shock - most birds lay their eggs early in the morning.
0:13:09 > 0:13:15And it really took many weeks of getting up at dawn and before,
0:13:15 > 0:13:18to realise that the cuckoo must have laid the previous evening.
0:13:18 > 0:13:21And once he then knew the timing of the egg-laying,
0:13:21 > 0:13:25then he could watch the cuckoo's behaviour in more detail
0:13:25 > 0:13:28and he got so good at predicting which nests
0:13:28 > 0:13:30the cuckoo would choose next
0:13:30 > 0:13:33that he was able to set up a hide and for the very first time
0:13:33 > 0:13:37actually film the egg-laying of the cuckoo.
0:13:40 > 0:13:45This remarkable footage, shot for Edgar Chance in 1921 by
0:13:45 > 0:13:46cameraman Oliver Pike,
0:13:46 > 0:13:49is one of the earliest wildlife films ever made.
0:13:57 > 0:14:01Once Chance had decided which pipit nest the cuckoo was going to target,
0:14:01 > 0:14:04he placed his hide and camera close by,
0:14:04 > 0:14:09hoping to solve the mystery of how the cuckoo deposits its egg.
0:14:11 > 0:14:13This is what he filmed.
0:14:13 > 0:14:18The female cuckoo glides in from a distant tree to a pipit nest
0:14:18 > 0:14:22she has been observing carefully for several days,
0:14:22 > 0:14:24concealed in a tuft of heather.
0:14:28 > 0:14:30Next, the cuckoo hops on the ground
0:14:30 > 0:14:35and lays its egg directly into the nest, while the adult pipits
0:14:35 > 0:14:36try to attack her.
0:14:42 > 0:14:45A very determined Edwardian naturalist
0:14:45 > 0:14:48had finally solved the age-old riddle.
0:14:48 > 0:14:52The cuckoo lays directly into the host nest.
0:14:58 > 0:15:02But though Edgar Chance had evidence of how the cuckoo lays her egg, the
0:15:02 > 0:15:05question of how it actually fools the host into accepting it
0:15:05 > 0:15:09wouldn't be solved for another 50 years.
0:15:16 > 0:15:20The reason that I got interested in the cuckoo is that, of course, it's
0:15:20 > 0:15:23one of nature's most famous cheats.
0:15:23 > 0:15:25And in theory this cheating should
0:15:25 > 0:15:29provoke an evolutionary arms race between the hosts and the cuckoo.
0:15:31 > 0:15:36Once hosts start evolving defences, that should then provoke
0:15:36 > 0:15:37improved trickery by the cuckoo,
0:15:37 > 0:15:41and that in turn would provoke even better host defences and so on.
0:15:41 > 0:15:45So the two sides in the arms race should improve their adaptations and
0:15:45 > 0:15:50counter adaptations over evolutionary time and I wanted to try and test by
0:15:50 > 0:15:52experiment whether this was going on.
0:15:52 > 0:15:56I mean, Edgar Chance had shown very beautifully what the cuckoo does,
0:15:56 > 0:15:58I wanted to try and understand
0:15:58 > 0:16:01why does the cuckoo behave in this particular way
0:16:01 > 0:16:03and have the hosts evolved counter tricks
0:16:03 > 0:16:06to try and defend themselves against the cuckoo?
0:16:12 > 0:16:15Nick's approach isn't just to observe, but also to
0:16:15 > 0:16:20scientifically test the reasons for the cuckoo's cheating ways.
0:16:20 > 0:16:22The cuckoo is a threatened species.
0:16:22 > 0:16:26There are fewer than half the number of cuckoos in the UK today
0:16:26 > 0:16:30than there were in Chance's day, and it's detailed knowledge like
0:16:30 > 0:16:32Nick's that might help to save them.
0:16:40 > 0:16:44On Wicken Fen, the cuckoos lay in reed warbler nests,
0:16:44 > 0:16:48so their eggs have to look exactly like the eggs of the reed warbler.
0:16:48 > 0:16:51They have to be the same colour, pattern and size.
0:17:01 > 0:17:04Nick can test how important the mimicry is
0:17:04 > 0:17:08by placing a wrongly-coloured egg into the reed warbler's nest
0:17:08 > 0:17:11and seeing how the reed warbler reacts.
0:17:27 > 0:17:30The reed warbler returns and checks her nest.
0:17:30 > 0:17:33She settles down, seemingly unaware of the rogue egg,
0:17:33 > 0:17:37but then she senses something isn't quite right.
0:17:50 > 0:17:52She starts to peck the egg.
0:18:00 > 0:18:03Each time she returns to the nest
0:18:03 > 0:18:08she repeatedly targets the new egg, until eventually, she punctures it.
0:18:12 > 0:18:16Next, she drinks some of the contents until it's safe to move it
0:18:16 > 0:18:20without spilling something over the other eggs.
0:18:22 > 0:18:26To destroy an egg that might hatch out into your own chick
0:18:26 > 0:18:27would be a calamity.
0:18:27 > 0:18:31There is enough variation in their own eggs that they could make
0:18:31 > 0:18:33a mistake, but it's worth the risk.
0:18:33 > 0:18:36They have to ensure the survival of their own offspring.
0:18:42 > 0:18:46If you give reed warblers a blue egg or a white egg or a brown egg,
0:18:46 > 0:18:50very different to their own green eggs, they throw them out,
0:18:50 > 0:18:53but if you give them a green egg matching their own eggs,
0:18:53 > 0:18:55in other words mimicking what the cuckoo actually does,
0:18:55 > 0:18:57the reed warblers tend to accept that.
0:18:59 > 0:19:00If you give them a giant egg,
0:19:00 > 0:19:03the reed warblers find it very difficult to sit on
0:19:03 > 0:19:04and will often desert.
0:19:04 > 0:19:08So the cuckoo's egg not only has to match the reed warbler's eggs in colour,
0:19:08 > 0:19:11but it also has to match reasonably in size too,
0:19:11 > 0:19:14if the cuckoo's got to get it's egg accepted.
0:19:14 > 0:19:18So this very simple experiment shows that this egg mimicry by the cuckoo
0:19:18 > 0:19:20is a crucial part of their trickery.
0:19:20 > 0:19:25The common cuckoo species is divided into several races, each
0:19:25 > 0:19:30with a distinctive egg that matches the colour of its particular host.
0:19:34 > 0:19:38Blue cuckoo eggs to copy redstart eggs,
0:19:38 > 0:19:42speckled green cuckoo eggs to copy warbler eggs and so on.
0:19:42 > 0:19:46They are the only species that can do this.
0:19:46 > 0:19:51But each individual female cuckoo can only lay one egg type.
0:19:52 > 0:19:56So the reed warblers do have ways of protecting their nests.
0:19:56 > 0:20:00Wrong colour, size or shape and the egg is out.
0:20:01 > 0:20:07Only if the cuckoo's egg is a good match will she outsmart her host.
0:20:07 > 0:20:11A well-matched egg, though, doesn't mean the cuckoo's work is done.
0:20:11 > 0:20:15She now has to decide exactly when to lay her egg.
0:20:19 > 0:20:24The reed warbler lays a single egg every day for four or five days.
0:20:24 > 0:20:29The female cuckoo must keep watch on the reed warblers to make sure
0:20:29 > 0:20:31she lays on the same days they do.
0:20:39 > 0:20:42The female cuckoo glides in.
0:20:47 > 0:20:50The egg-laying is completed in seconds.
0:20:58 > 0:21:00If the cuckoo leaves it too late
0:21:00 > 0:21:02and the warblers have laid all their eggs,
0:21:02 > 0:21:06then the cuckoo chick might not hatch out in time.
0:21:06 > 0:21:10But if the cuckoo lays too early, there's a problem there, too.
0:21:10 > 0:21:15If we put our model eggs in before the hosts have begun to lay,
0:21:15 > 0:21:17those model eggs always get thrown out,
0:21:17 > 0:21:19so very sensibly the female reed
0:21:19 > 0:21:22warbler knows that if she hasn't started to lay eggs yet,
0:21:22 > 0:21:24that egg in the nest can't possibly be mine.
0:21:31 > 0:21:36You get a completely different perspective down here at the water level.
0:21:36 > 0:21:39I'm normally up on the bank looking for reed warbler's nests
0:21:39 > 0:21:43and down here you really enter the reed warbler world and you can see
0:21:43 > 0:21:45the habitat from their perspective
0:21:45 > 0:21:48and every 20 metres or so there's a new territory.
0:21:48 > 0:21:52I've just seen one of the birds hopping around in amongst the reeds there.
0:21:52 > 0:21:55They don't seem to mind us at all as we go.
0:21:55 > 0:22:00And all the while, cuckoos up in those trees behind, birdwatching.
0:22:00 > 0:22:04You can almost imagine what it must be like to be a reed warbler
0:22:04 > 0:22:07in the reeds being observed, up there.
0:22:15 > 0:22:18The female cuckoo's job, although not as laborious,
0:22:18 > 0:22:21is every bit as time consuming as that of the reed warbler's.
0:22:21 > 0:22:25A cuckoo can lay 10 eggs or more in one season,
0:22:25 > 0:22:28but she only lays one egg per nest.
0:22:28 > 0:22:32This means she has to stake out dozens of reed warbler nests within
0:22:32 > 0:22:35her territory, to ensure she can lay each precious egg
0:22:35 > 0:22:38in the best nest at the best time.
0:22:38 > 0:22:42She may wait days or weeks for the timing to be perfect.
0:22:48 > 0:22:51When we ourselves have adopted the strategy of the cuckoo
0:22:51 > 0:22:54and have tried to do these model egg experiments,
0:22:54 > 0:22:58by the end of the day we've convinced ourselves that it's a crazy thing to do,
0:22:58 > 0:23:01it's such hard work looking for host nests and I think if I was a bird,
0:23:01 > 0:23:05I'd just be an honest worker and raise my own young.
0:23:11 > 0:23:14Just how many eggs a cuckoo can lay in any one season
0:23:14 > 0:23:20fascinated Edgar Chance, but not only for scientific reasons.
0:23:26 > 0:23:32There's no doubt that one of his motivations for discovering the cuckoo's laying procedure
0:23:32 > 0:23:34was so that he could collect
0:23:34 > 0:23:37the most number of eggs that a cuckoo had laid in a season.
0:23:40 > 0:23:45Chance was determined to beat a rival collector in Germany,
0:23:45 > 0:23:47who claimed the world record for the number of eggs
0:23:47 > 0:23:50laid by a cuckoo in one season.
0:24:00 > 0:24:02Edgar Chance did get his world record.
0:24:02 > 0:24:07He managed to get 25 eggs in one season from his beloved Cuckoo A.
0:24:12 > 0:24:16He collected every pipit and cuckoo egg that he could and
0:24:16 > 0:24:21his vast collection is now held at the Natural History Museum at Tring.
0:24:26 > 0:24:29We now know that a typical cuckoo
0:24:29 > 0:24:32will lay about ten eggs in any one season.
0:24:32 > 0:24:38But Chance intervened to make sure his female could lay more.
0:24:38 > 0:24:43Actually, this world record was achieved with Edgar Chance's help.
0:24:43 > 0:24:45And what Chance did was he used to
0:24:45 > 0:24:50farm the meadow pipit's nests in the sense that if incubation was already
0:24:50 > 0:24:53underway and the cuckoo had missed that nest,
0:24:53 > 0:24:56Edgar Chance would collect all the eggs and that would force
0:24:56 > 0:24:59those pipits to start a replacement nest,
0:24:59 > 0:25:04and by farming nests in this way and making more new nests available
0:25:04 > 0:25:07for the cuckoo at a suitable stage, he managed to get the world record.
0:25:10 > 0:25:15Egg collecting was key to many of Chance's discoveries about cuckoos.
0:25:18 > 0:25:20It wasn't illegal as it is now,
0:25:20 > 0:25:24but even back then, some naturalists disapproved.
0:25:30 > 0:25:34Chance was actually drummed out of the British Ornithologist Union
0:25:34 > 0:25:36because of his egg-collecting habits,
0:25:36 > 0:25:39so even back in the 1920s, many people regarded
0:25:39 > 0:25:43egg collecting as something which you simply shouldn't do.
0:25:45 > 0:25:48There's a final twist in this amazing story.
0:25:48 > 0:25:52Chance's trick of removing eggs to encourage the host bird
0:25:52 > 0:25:57to lay another clutch is actually just what a female cuckoo would do.
0:26:05 > 0:26:08The cuckoo is the only British bird to do this -
0:26:08 > 0:26:10her behaviour is unique.
0:26:21 > 0:26:24This cuckoo will eat whole clutches of eggs.
0:26:24 > 0:26:26Just like Chance,
0:26:26 > 0:26:30she wants to encourage the reed warbler to lay more clutches.
0:26:37 > 0:26:41Regardless of his methods, Chance's record stood and no one
0:26:41 > 0:26:46thought that any single cuckoo would ever lay as many eggs in one season.
0:27:00 > 0:27:0565 years after Edgar Chance, another amateur ornithologist
0:27:05 > 0:27:09appeared on the scene, but HE never took a single egg.
0:27:16 > 0:27:19I first became interested in cuckoos in June 1983.
0:27:19 > 0:27:24I was doing a water rail survey and to alleviate the boredom of sitting
0:27:24 > 0:27:26there I watched some reed warblers.
0:27:26 > 0:27:29When I found the nest, I was surprised to see a cuckoo egg in it.
0:27:29 > 0:27:34And I thought, "This is more interesting than watching water rails that aren't there,
0:27:34 > 0:27:37"so I'll see if there are any more reed warblers in the area".
0:27:37 > 0:27:39And I found another three pairs
0:27:39 > 0:27:42all of which contained cuckoo eggs of the same female.
0:27:42 > 0:27:46Now you might say I'd been bitten by the bug.
0:27:50 > 0:27:54Mike Bayliss proved himself to be every bit as skilful as Edgar Chance
0:27:54 > 0:27:56even though he had a full-time job
0:27:56 > 0:28:00and could only search for reed warbler nests in his time off.
0:28:09 > 0:28:13It wasn't as if I was trying to break his record, or even equal it.
0:28:13 > 0:28:14It was only when I passed the 20 mark
0:28:14 > 0:28:17that I realised it was attainable.
0:28:20 > 0:28:23You did hear one singing across here, didn't you?
0:28:23 > 0:28:25Let's just cruise along here.
0:28:25 > 0:28:30Mike observed reed warblers in the reed beds along the Thames.
0:28:30 > 0:28:33And, like Chance, he got lucky.
0:28:33 > 0:28:37One female, he called Cuckoo X,
0:28:37 > 0:28:39returned to the same site year after year.
0:28:42 > 0:28:47I'd say the best year was obviously when Cuckoo X, in 1988,
0:28:47 > 0:28:52achieved a world record under natural conditions when she laid 25 eggs.
0:28:52 > 0:28:55Er, this had previously been done by Edgar Chance under
0:28:55 > 0:28:58experimental conditions in 1922.
0:28:59 > 0:29:03Mike had shown just how productive one cuckoo could be
0:29:03 > 0:29:05under natural conditions.
0:29:07 > 0:29:11Cuckoo X returned to Oxford for eight seasons
0:29:11 > 0:29:13and laid a total of 113 eggs.
0:29:17 > 0:29:22Again, like Chance, Mike used his detailed knowledge to get amazing
0:29:22 > 0:29:25footage of a cuckoo laying her eggs.
0:29:33 > 0:29:38This is Cuckoo X laying her egg in a reed warbler's nest in 1989.
0:29:39 > 0:29:42First, the female cuckoo removes a warbler egg.
0:29:42 > 0:29:45Holding it carefully in her bill,
0:29:45 > 0:29:49she then slips forward to lay her own egg, now.
0:29:53 > 0:29:55It only takes a few seconds.
0:29:59 > 0:30:03So why does the cuckoo have to be so quick?
0:30:05 > 0:30:08You can test this by experiment, and what we've done is
0:30:08 > 0:30:12we've put a stuffed cuckoo next to a reed warbler nest
0:30:12 > 0:30:16to simulate, if you like, a female who's very slow at laying.
0:30:30 > 0:30:33Not surprisingly, if the reed warblers see this cuckoo they mob
0:30:33 > 0:30:38it like mad - they've got this harsh scolding crrr, crrr, noise like this.
0:30:38 > 0:30:40LOUD CHIRPING
0:30:44 > 0:30:46What is surprising though
0:30:46 > 0:30:49is that not all reed warblers react in the same way.
0:30:49 > 0:30:55Naive birds tend to treat a cuckoo like a dangerous bird of prey.
0:30:55 > 0:30:59It looks rather similar and so they won't get too close.
0:31:04 > 0:31:09Experienced parent reed warblers will close right in -
0:31:09 > 0:31:12they know they aren't in any danger.
0:31:17 > 0:31:21This shows Nick that whilst reed warblers instinctively
0:31:21 > 0:31:23know to reject certain eggs,
0:31:23 > 0:31:27they actually have to learn to recognise the cuckoo.
0:31:27 > 0:31:31The interesting result is that when you take the cuckoo away,
0:31:31 > 0:31:34the reed warblers seem much more alert to any change
0:31:34 > 0:31:36in their nest contents.
0:31:36 > 0:31:38Our experiments show that they now
0:31:38 > 0:31:42are very likely to reject even a good matching model egg from their nest.
0:31:46 > 0:31:51The arms race is very much alive at this stage.
0:31:51 > 0:31:54The warblers can fight back and their defences can work.
0:31:56 > 0:31:59For the cuckoo's trickery to be successful,
0:31:59 > 0:32:00there is a lot she has to do.
0:32:08 > 0:32:11She must first remove one or more warbler egg
0:32:11 > 0:32:13to make room for her egg.
0:32:25 > 0:32:28She has to a lay a similar looking egg
0:32:28 > 0:32:30so it's not recognised and thrown out
0:32:30 > 0:32:35and she has to do this quickly so she doesn't alert the warblers.
0:32:38 > 0:32:42If the cuckoo gets all this right, the trap is set.
0:32:53 > 0:32:57Warblers who have been tricked can have no idea of what is to come.
0:33:01 > 0:33:04Those whose nests have remained safe from the cuckoo
0:33:04 > 0:33:08are ready for a brood of their own hungry chicks to emerge.
0:33:13 > 0:33:17The long days of summer, with endless supplies of insects,
0:33:17 > 0:33:20are bountiful for the warblers of Wicken Fen.
0:33:27 > 0:33:32In a good year, a pair of warblers can raise two broods each
0:33:32 > 0:33:34of up to four or five chicks.
0:33:41 > 0:33:45Those that have been tricked by the cuckoo will have
0:33:45 > 0:33:46no time for a second brood.
0:33:46 > 0:33:50It takes as much effort to raise one cuckoo
0:33:50 > 0:33:52as ten of their own chicks.
0:34:00 > 0:34:02The cuckoo chick has just hatched.
0:34:02 > 0:34:05And now the reed warblers have lost everything.
0:34:09 > 0:34:13Their lives will be totally dominated by this imposter,
0:34:13 > 0:34:16and there is nothing they can do about it.
0:34:22 > 0:34:26Just 24 hours old and still naked and blind,
0:34:26 > 0:34:29the cuckoo chick instinctively pushes out
0:34:29 > 0:34:31any other eggs in the nest.
0:34:31 > 0:34:34So why is it left to the new-born hatchling
0:34:34 > 0:34:37to take on this Herculean task?
0:34:40 > 0:34:43You might think that one of the things the female cuckoo
0:34:43 > 0:34:49could do is simply remove all of the host eggs and leave her egg instead.
0:34:49 > 0:34:53Well, the host will always desert a single egg, so she can't do that.
0:34:53 > 0:34:57And that explains very nicely why it's the young cuckoo that has to
0:34:57 > 0:35:00take on this task of rejecting the host eggs,
0:35:00 > 0:35:03because although the host will always desert a single egg,
0:35:03 > 0:35:05they never desert a single chick.
0:35:06 > 0:35:10The cuckoo chick is astonishingly strong and has
0:35:10 > 0:35:11a distinctive hollow back
0:35:11 > 0:35:14that helps balance the host's egg or chick
0:35:14 > 0:35:17before throwing it out of the nest.
0:35:24 > 0:35:28Nothing the little ogre does alarms the foster parents.
0:35:28 > 0:35:31Even when their own eggs are being forced out of the nest
0:35:31 > 0:35:32from right beneath them.
0:35:39 > 0:35:43The simple fact is that a warbler nest won't be big enough
0:35:43 > 0:35:48to hold both reed warbler chicks and the growing cuckoo chick.
0:35:48 > 0:35:53The imposter will need all the food that its adopted parents can bring.
0:36:16 > 0:36:18Sometimes, the reed warbler's eggs
0:36:18 > 0:36:24are more advanced and both warbler and cuckoo chicks hatch together.
0:36:44 > 0:36:50Again, it falls to the blind cuckoo chick to deal with the situation.
0:37:38 > 0:37:42You might think the cuckoo's cruel and of course in a way it is,
0:37:42 > 0:37:45but it's no more cruel than the reed warbler.
0:37:45 > 0:37:48If I was a fanatic of damselflies and dragonflies,
0:37:48 > 0:37:50I might get very upset
0:37:50 > 0:37:54when I see a reed warbler murder a damselfly and feed it to its chicks,
0:37:54 > 0:37:57just as I would when I see a cuckoo chick eject reed warbler
0:37:57 > 0:37:58eggs or reed warbler young.
0:37:58 > 0:38:02In nature, individuals are cruel, they're all
0:38:02 > 0:38:06out for what they can get, exploiting others as food or hosts or whatever.
0:38:11 > 0:38:15Those reed warbler pairs who managed to escape the attention
0:38:15 > 0:38:19of the cuckoo, or spotted the egg and ejected it, are now busy
0:38:19 > 0:38:21looking after their own brood.
0:38:28 > 0:38:30These reed warbler chicks are
0:38:30 > 0:38:34nine days old and demand to be fed whenever there's daylight.
0:38:43 > 0:38:47In the nest nearby, the cuckoo chick is about eight days old.
0:38:47 > 0:38:51Having cleared the nest of all competition, there's now just one
0:38:51 > 0:38:56hungry mouth devouring all the food the warblers can bring to the nest.
0:39:09 > 0:39:14Just two days later and the cuckoo chick has grown massively.
0:39:17 > 0:39:20It's about half-way through it's time in the nest now
0:39:20 > 0:39:23and can barely still fit inside.
0:39:28 > 0:39:32When I was a young student I saw, in the fens here actually,
0:39:32 > 0:39:35a little reed warbler feeding an enormous cuckoo chick
0:39:35 > 0:39:40and this just amazed me. I think this is one of the most astonishing things
0:39:40 > 0:39:42you can see in the whole of nature.
0:39:44 > 0:39:47Its foster parents are lavishing
0:39:47 > 0:39:52as much care and attention upon it as they would for their own brood,
0:39:52 > 0:39:54instinctively caring for whatever hatches from an egg
0:39:54 > 0:39:56they assume is their own.
0:40:06 > 0:40:08The cuckoo chick is well fed
0:40:08 > 0:40:13and it's huge in comparison to reed warbler chicks of the same age.
0:40:19 > 0:40:24It's a very strange-looking youngster and bears no resemblance
0:40:24 > 0:40:27at all to a reed warbler chick.
0:40:27 > 0:40:30It has fooled a pair of adult warblers into working
0:40:30 > 0:40:34as hard as they possibly can, 16 hours a day.
0:40:34 > 0:40:37An extraordinary feat of manipulation.
0:41:00 > 0:41:04Reed warblers are programmed to treat any chick in their nest
0:41:04 > 0:41:09as one of their own, but why do they feed it so well?
0:41:09 > 0:41:12It's a question that has puzzled bird watchers and academics
0:41:12 > 0:41:15since it was first observed.
0:41:21 > 0:41:23The cuckoo chick does have a
0:41:23 > 0:41:26problem and the problem is how on earth does it stimulate the
0:41:26 > 0:41:28reed warblers to bring as much food
0:41:28 > 0:41:32as they would to a whole brood of their own hungry young?
0:41:37 > 0:41:40Nick believes he's discovered the answer.
0:41:40 > 0:41:44It's a trick the cuckoo chick uses from the very moment it hatches,
0:41:44 > 0:41:48to make sure it gets as much food as it needs.
0:41:51 > 0:41:56If you listen to the cuckoo's begging call, it is absolutely remarkable.
0:41:56 > 0:41:59Most chicks when they're hungry just go cheep cheep,
0:41:59 > 0:42:02but the cuckoo's got the most incredibly rapid call.
0:42:02 > 0:42:06It goes cheep cheep cheep cheep cheep - very fast.
0:42:07 > 0:42:11When it's a week of age actually it sounds like a whole brood of hungry
0:42:11 > 0:42:14reed warbler chicks, and by two or three weeks of age
0:42:14 > 0:42:17it sounds like two broods of hungry reed warbler chicks,
0:42:17 > 0:42:21so we thought maybe it's this very rapid begging which simulates lots
0:42:21 > 0:42:26of hungry young, which spurs the reed warbler foster parents
0:42:26 > 0:42:27on to extra work.
0:42:30 > 0:42:34Nick and his colleague Becky Kilner tested this idea
0:42:34 > 0:42:36with an ingenious experiment.
0:42:38 > 0:42:42First, they carefully borrowed a blackbird chick the same size
0:42:42 > 0:42:46as a cuckoo chick and temporarily swapped them around.
0:42:46 > 0:42:48So now a blackbird chick
0:42:48 > 0:42:53is in the nest that was occupied by a similar-sized cuckoo chick.
0:42:53 > 0:42:54Next to this nest,
0:42:54 > 0:42:58they placed a tiny loudspeaker connected to a tape player.
0:42:58 > 0:43:01They measured how much food the warblers brought to the
0:43:01 > 0:43:04blackbird chick on its own and compared this with how
0:43:04 > 0:43:08much food the warblers brought in when they played cuckoo chick
0:43:08 > 0:43:10begging calls through the speaker.
0:43:10 > 0:43:13Their results were astonishing.
0:43:13 > 0:43:16I think the reed warblers are coming, I can see the reeds moving.
0:43:16 > 0:43:19- OK, tape's on.- Here we go. Right, blackbird begging now.
0:43:19 > 0:43:21OK.
0:43:21 > 0:43:24Still begging.
0:43:26 > 0:43:30- OK, feeding now.- The size of the chick is not important.
0:43:30 > 0:43:33A big chick alone doesn't encourage the reed warbler
0:43:33 > 0:43:34parents to bring more food.
0:43:34 > 0:43:38A cuckoo's begging call is vital.
0:43:39 > 0:43:42- And now she's gone. Stop begging. - Stopped begging.
0:43:43 > 0:43:47The warblers will feed any chick in their nest,
0:43:47 > 0:43:51but only with the cuckoo's begging calls will they bring enough food
0:43:51 > 0:43:53to satisfy a chick this big.
0:43:55 > 0:44:00The female cuckoo uses the visual trickery to get her egg accepted,
0:44:00 > 0:44:05and the cuckoo chick uses vocal trickery to get enough food.
0:44:11 > 0:44:15It's now the beginning of July - just ten weeks since the adult
0:44:15 > 0:44:18cuckoos arrived in Britain and they are already leaving for home.
0:44:18 > 0:44:23With no more new reed warbler nests being built and no new opportunities
0:44:23 > 0:44:26for the cuckoo, they set off.
0:44:28 > 0:44:30HE IMITATES CUCKOO CALL
0:44:38 > 0:44:42They have the shortest breeding season of any British migrant
0:44:42 > 0:44:45and the birds can be back under African skies
0:44:45 > 0:44:49before the last of their offspring has even left the nest.
0:45:00 > 0:45:03The giant cuckoo chick is 20 days old.
0:45:05 > 0:45:09The nest can hardly hold it any longer.
0:45:09 > 0:45:11Soon it will have to leave the nest,
0:45:11 > 0:45:13but still the reed warblers will
0:45:13 > 0:45:18continue to feed it for another two weeks, until it becomes independent.
0:45:23 > 0:45:27This reed warbler's season has been wasted
0:45:27 > 0:45:29raising another species' offspring.
0:45:31 > 0:45:34In a way, it's surprising there are not more cheats
0:45:34 > 0:45:35to exploit honest workers.
0:45:35 > 0:45:37The big question is why?
0:45:39 > 0:45:43There's only one parasitic bird in Britain - that's the cuckoo.
0:45:43 > 0:45:48In fact, only about 100 birds out of the 10,000 species in the world are
0:45:48 > 0:45:50professional cheats like the cuckoo.
0:45:50 > 0:45:53And I think the reason is simply that cheating seems
0:45:53 > 0:45:58a wonderful thing to do until those who are duped begin to fight back.
0:45:58 > 0:46:02And I think that it's the fact that the hosts fight back which
0:46:02 > 0:46:03really limits the cuckoo's options
0:46:03 > 0:46:06and is the reason why cheating doesn't really prosper
0:46:06 > 0:46:08so well in the very long term.
0:46:13 > 0:46:19It's hard to believe that in three to four weeks this clumsy chick
0:46:19 > 0:46:25will begin its very first journey - a 3,000 mile flight to Africa.
0:46:27 > 0:46:31If it survives the long and testing flight, it will return to the fen
0:46:31 > 0:46:35next year, ready and able to trick the reed warblers yet again.
0:46:54 > 0:46:58I think people often like the idea of individuals who make
0:46:58 > 0:47:02a living in a rather unusual way - don't follow the crowd.
0:47:02 > 0:47:04I think they admire cuckoos cos
0:47:04 > 0:47:07they wonder how on earth they can get away with it.
0:47:07 > 0:47:10They equate the cuckoo's behaviour
0:47:10 > 0:47:14with tremendous cunning and wit, if you like.
0:47:14 > 0:47:16There's an old saying, I think by Edward Topsell,
0:47:16 > 0:47:20who says, "The grand creator has given the cuckoo extra wit
0:47:20 > 0:47:26"to make up for the fact that it lacks maternal affection".
0:47:28 > 0:47:31The cuckoo will need all the wit it can find,
0:47:31 > 0:47:34for its future is uncertain.
0:47:37 > 0:47:39Nick's research will be vital for saving it.
0:47:41 > 0:47:45For not only is the cuckoo in an arms race with all the host species,
0:47:45 > 0:47:48but the cuckoo has also had to cope
0:47:48 > 0:47:51with huge changes in our countryside.
0:47:51 > 0:47:56We should treasure the brief summer visit of the cuckoo
0:47:56 > 0:47:58and listen out for that delightful call.
0:47:59 > 0:48:01I, for one,
0:48:01 > 0:48:04hope that it continues to announce spring for years to come.