Category
📺
TVTranscript
00:30These sooty terns are amongst the most aerial of birds, every one of them has spent the
00:50first three or four years of its life flying non-stop, feeding on the wind, swooping down
00:56picking up fragments of food from the surface of the sea, even sleeping on the wind. But
01:02there is one thing that compels them to come down to earth, this. Flying with an egg inside
01:22the body, let alone a clutch of three or four, makes huge demand on the energy of the bird.
01:28So every female bird in existence gets rid of that huge load just as soon as she can.
01:35She lays it free. In places like this island in the Seychelles, where there are no land
01:43predators to steal eggs, it's safe for the sooty terns to put those eggs directly on
01:48the ground. The birds sit as close to one another as they can without getting within
01:58the range of their neighbours' beaks, so they are equally spaced with almost mathematical
02:03precision. Other kinds of terns have other ideas about where to put their eggs. The
02:19fairy tern, for some reason, always puts them on a bare branch, though whether that is safer
02:26or more dangerous is debatable. The females seem wildly optimistic. A U-bend is one of
02:36their safer ideas. The dimple left when a branch breaks away is not bad either. But
02:42it seems a bit reckless to rely on a little dead twig like this, particularly when the
02:47trade winds blow as strongly as they do in the Seychelles. The fact is that fairy terns'
02:54eggs are easily dislodged if left unguarded. Skinks know that, and so do the fodies,
03:01the local sparrows. And that has solved the problem of how to crack it.
03:22Yolk is provided by a female bird to nourish her chick as it develops inside the egg. But
03:31it's equally good food for lots of other animals, so eggs are much sought after,
03:36and birds may have to go to great lengths to keep them safe.
03:52Swifts, living on the mainland, have to take greater precautions. The nests of palm swifts
03:59are minimal, but they put them in fairly inaccessible places, such as an isolated palm
04:04tree. They stick a few feathers on the underside of one of its dangling dead leaves, using their
04:12spittle as glue. To make sure the egg stays in this flimsy hammock of feathers, they stick
04:25that to the leaf as well.
04:37Finding places to take over incubation is a tricky operation when your nest is stuck
04:43to a vertical surface.
05:07The power of flight enables birds to put their nests in places that are beyond the
05:11reach of land-bound creatures. And there can be few more inaccessible spots than the rocks
05:17behind a great waterfall, like this one at Iguazu in South America, where Argentina and
05:23Swifts, once again, exploit their mastery of flight to go where no other bird can go.
05:51Dusky swifts roost for the night on the mist-drenched rocks alongside the falls. But this is not
05:58a safe enough place for their precious eggs. They will be deposited actually behind the
06:04curtain of water. And to do that, the birds must find the thinnest part of it.
06:34Behind the curtain, they still have an awkward climb before they reach a place where it's
06:38possible to put an egg.
06:57And this is it. A perpetually dripping ledge, just wide enough to accommodate a little muddy
07:03nest.
07:20Many birds choose cliffs for their nest sites, even when they're not screened by water.
07:26Seabirds around the world use them, but these aren't normal seabirds. They're parrots, and
07:34they're nesting on the coast of Argentina.
07:57The powerful hooked beak that all parrots have and use for cracking nuts and gouging
08:01fruit is also an excellent excavating tool. These cliffs are composed of a relatively
08:08soft sandstone, no problem for a parrot. Even so, digging a hole in them is hard work, so
08:15each hole, when it's finished, is a valuable property. And there is a great deal of competition
08:21and squabbling over any vacancy.
08:33Sandmartins are not nearly so well equipped for digging. They use their claws rather than
08:37their delicate beaks, and they can only tackle sandstone if it's soft and friable. But what
08:43they lack in equipment, they make up for with energy.
09:01Woodpeckers of course are expert carpenters, and regularly chisel their nest holes in trees,
09:07but this one is digging into softer material, an ant's nest.
09:13The ants are furious, but they quickly get used to their lodger sitting in the middle
09:17of their mansion, and then they act as her guardians, attacking any intruder that tries
09:22to steal her eggs.
09:25You might think that a hornbill would have the most powerful excavation tool of all,
09:30but in fact its huge beak is a relatively delicate structure, and no use at all as a
09:35chisel. So hornbills have to find holes that are either natural or have been dug by others.
09:50A pair do their house hunting together, and they're very choosy. Perhaps this one will
09:58do.
10:01With beaks over a foot long, these great Indian hornbills have to have a pretty big hole,
10:07but they like the entrance to be as small as possible, to deter intruders.
10:16Maybe this is just too small, but she can just make it.
10:25That's good, the search is over, this is it.
10:32The male regurgitates a little food for her.
10:44Now she's decided that this is for her, she won't leave again until her eggs are laid
10:49and hatched and the chicks well grown. She will depend entirely on her mate for food.
10:55She seals herself in, narrowing the entrance with a plaster made of chewed wood, mashed
11:01food and her own droppings. This will be her home, you could say her prison, for the next
11:08four months.
11:22The majority of birds, though, don't nest in holes. They build their nests, and builders,
11:28of course, need building materials.
11:58Having no hands, but just a beak, and at best one foot, seems bad enough, but these
12:04frigates on the Galapagos have got an additional problem. Their feet are so short and their
12:09wings are so wide that they find it difficult to land, so they much prefer to collect their
12:14building material on the wing.
12:19Boobies can settle and break off the branches they need for their nests. Frigates can't,
12:25they can steal.
12:30So for boobies, getting building material from its source to the building site is the
12:35most difficult part of the whole business.
13:55Stolen goods, it's true, but all the more precious for that.
14:11Eggs must not only be kept secure. Once they start to develop, they also have to be kept
14:16warm, and there is no better insulation than feathers. Ducks and geese line their nests
14:21with feathers they pluck from their own breasts. Other birds are not so self-sacrificing and
14:28prefer to use those that they find blowing about.
14:34Tree swallows compete with one another in collecting them.
14:41Miss, and the rival won't give you a second chance.
15:11Even when you've got it, you may not get away with it.
15:42Tackling a bird without the feather is all within the rules of this particular game.
15:50The golden-headed sesticular, a kind of Australian warbler, collects fibres and spider's webs
15:57not just for lining, but for stitching.
16:18There is no more skilled tailor in the whole of the bird world.
16:58There's little problem about concealing this nest, for the leaves she stitches together
17:08remain alive and green. When she's finished, you would barely notice that they're bent
17:13together in a slightly unusual way.
17:17The problem is greater when a nest sits on the bare branches of a tree. The sitella,
17:24an Australian equivalent of the nuthatch, constructs its nest from spider's webs and
17:29insect cocoons, and then covers the outside with rather coarser material. Two pairs are
17:36building here, close to one another. This one is in a tree covered in lichen.
17:44And this in one that has flaky bark.
17:53The sitellas are not rigidly-minded birds within flexible habits. They use lichen to
17:58cover the nest in the lichen tree, and bark on the one in the flaky bark tree.
18:14As a result, each is as well camouflaged as anyone could hope, and though both nests
18:26are plain for all to see, they're not easily recognised for what they are.
18:34The apostle bird uses mud mixed with grass.
18:54Nest building is a group activity. A pair with their grown-up young from previous seasons
18:59work together. It used to be believed that there was always a dozen in the team, which
19:05is why they're called apostle birds.
19:09Everyone seems anxious to contribute, and the team work so industriously and so harmoniously
19:15that their elegant cup is usually completed in a mere three days.
19:23A bird's beak, it seems, can serve just as well as a plasterer's trowel as a tailor's needle.
19:30Some birds build nests not just as cradles for their eggs and chicks, but as lodging
19:37houses for the whole year.
20:00Here in Namibia, in southern Africa, lives the sociable weaver, and very sociable it
20:05is too. As many as three or four hundred birds will live in a single apartment block.
20:13This haystack may be more than a century old. It's so heavy that part of it has broken
20:20the branch that supported it and fallen to the ground, and it's been built and maintained
20:25as a communal effort by all the inhabitants.
20:36Weavers are closely related to sparrows. Though some of their relations do indeed weave, the
20:44sociable weaver builds in a rather simple way. It just pushes straws one by one into
20:50this gigantic bale.
20:58A large communal apartment block has a considerable advantage over small, isolated nests.
21:06When you live in a desert like this, during the day it gets ferociously hot, but that
21:11thick roof of thatch keeps the apartments beneath nice and cool.
21:21As the sun sets, the weavers, having been away feeding, come back to their homes.
21:28At night, it can get very cold in the desert, as much as seven or eight degrees below freezing,
21:38and then the thatch is probably at its most valuable, because it acts as insulation for
21:44the nest chambers, so that they retain much of the heat that they had during the day,
21:48and the birds that roost inside remain snug and warm.
21:56Not all the chambers are for nesting. Some are not nurseries, but bedrooms, in which
22:04several of the colonies snuggle together for warmth.
22:09So, at last, some kind of receptacle, simple or complex, is prepared for the egg, and it's
22:18time for the female to produce one. The male frigate welcomes his partner back. The crucial
22:24moment has arrived.
22:54Mated female birds have been feeding intensively, and their ovaries are much enlarged. Each
23:16egg, emerging from one as a microscopic cell, is planted on a growing globule of yolk, which
23:23will provide it with food. It's fertilized by one of the sperm that have been lying awaiting
23:29it in the duct. Albumin is wrapped round it to provide it with the water necessary for
23:35development. It travels onwards for several hours. Eventually, it reaches the section
23:43in the duct where there are glands that produce lime. Here, it lingers for twenty-four hours
23:49while the shell is added. Pigment glands squirt little spots of color on it. And so, an avocet
24:01produces her egg.
24:08The blotches and speckles on such an egg as this, laid on the ground, serve primarily
24:22to camouflage it. These are the eggs of a golden plover. They, too, are laid directly
24:32on the ground and are practically invisible, as indeed the bird that laid them will be
24:37once she settles down.
24:57Plovers nearly always lay four eggs. A few birds, however, have adopted a rather more
25:03risky policy. They lay just two or even one. If they do that, they must lay it in a nest
25:10that is really secure, hidden, for example, deep in a burrow, as the kiwi does. Her egg
25:18is gigantic, the biggest laid by any bird, and a quarter of her total body weight. It
25:26contains so much yolk that the chick will be fully developed when eventually it breaks
25:31out of its shell. Expelling such an egg is obviously a huge effort.
26:02The owner of this nest, a blue tit, adopts a very different strategy.
26:14Her egg is tiny. It weighs no more than a gram. But she lays lots of them, one a day,
26:23day after day, for a fortnight or so. But the chicks, after they hatch, will face so
26:28many hazards that many will die, and the survival rate in the end will be not unlike the kiwis.
26:41Few eggs are totally safe from hungry raiders, no matter how skilfully protected and artfully
26:47concealed they are. Those lying in the bottom of these dangling nests of caciques in South
26:53America are certainly difficult to reach. But the red-breasted toucan has a long beak.
27:24This toucan's bill is just not long enough for these particular nests. But the toco toucan
27:35has an even longer one.
27:42The caciques are extremely agitated, and with good reason.
28:12If the caciques are to defend themselves against these powerful predators, they will
28:43have to build even longer nests in the future.
28:52In Australia, the prime egg thief is the currawong. An unguarded nest. An obvious target.
29:13Another clutch gone. Few eggs are safe from currawongs, and the Australian birds, like
29:28birds everywhere, have developed many different strategies to try and foil burglars.
29:35This is the nest of a yellow-rumped thornbill, but in fact this neat little cup is empty.
29:42You might think, therefore, that it has been robbed of its eggs, but in fact this part
29:46of the nest has never had any eggs in it. There's another entrance to the nest. It's
29:55down here.
29:57You might never notice it until you watch the parent return.
30:05The cup at the top is a dummy, and it seems that many currawongs have not yet discovered
30:16the fact.
30:19This wren in Costa Rica has another way of protecting its eggs.
30:27Its nest is pretty obvious.
30:37And equally obvious is another one close by, a wasp's nest. The wren nearly always builds
30:44within a yard or so of these formidable insects. It's a brave thief that risks being attacked
30:50by these.
30:53But Coatimundis are brave, sometimes to the point of recklessness.
31:51Out here, on the cold, windswept plains of Patagonia, there are no trees in which a bird
32:01could build a nest. So plovers, like plovers everywhere, lay their eggs on the ground and
32:09trust to their camouflage. But if a stranger wanders too near it, one of the adults immediately
32:16responds.
32:18Ahead, I can just see a bird crouching on her eggs. And away she goes. And now she starts
32:28a most bizarre pantomime.
32:36This hardly looks like any kind of bird, and whatever it is, it seems to be crippled.
32:50If I was a fox, or maybe a hawk, I might quite well think that this extraordinary performance
32:58represents an injured bird, or maybe a little rodent. Either way, it looks like an easy
33:06meal, and I might follow it and be led away from the nest.
33:28And now she's taken me so far from her eggs that she can abandon her play-acting. There's
33:37nothing the matter with her. Having deflected me, she returns to her nest. She has to go
33:48back to prevent her eggs getting chilled, and the young within from dying of cold. Keeping
33:53eggs warm, indeed, is a continuing problem for most nesting birds.
34:14The Malio has a very labour-saving way of dealing with its heating problems. It lives
34:19on an island in Indonesia. In this one unshaded patch of the beach, the sand is kept so hot
34:29by the sun that the eggs will hatch by themselves, and the whole of the local Malio population
34:34know it. The birds are accurate judges of temperature. They have to be. If they don't
34:42dig deep enough, their eggs will bake, and if they go too far, they won't even develop.
34:48That's probably about right. Now, all that's needed is to fill in the hole.
35:12And then they abandon it.
35:20These eggs up in Alaska must be tended much more assiduously. They belong to a snowy owl.
35:28As she returns, you can see that the feathers on her belly are rather long and floppy. There's
35:33even a glimpse of pink naked skin.
35:39Her body has to be particularly well insulated with dense plumage to prevent it losing heat
35:44in these near-freezing conditions, yet somehow she has to transfer some of that heat to these
35:50eggs. This naked brood patch on her belly will enable her to do just that.
35:58Her mate is away hunting for lemmings, not only for himself, but for her.
36:15She cannot leave her eggs for more than a minute or two, so her mate looks after her,
36:23and he will have to do this for almost two months.
36:41The chilly lakes of North America. Here, some birds trick others into looking after at least
36:49some of their eggs. This female canvasback duck is sitting contentedly incubating and
36:56minding her own business.
37:00Out on the water, another female canvasback is mating with a drake.
37:08The mated female has not yet built a nest of her own, so she makes her way to the one
37:20who has.
37:28The sitting female clearly doesn't like this intrusion, but equally she's not going to
37:40abandon her eggs.
37:58The intruder pushes her to one side and quickly lays. Sometimes the sitting bird doesn't seem
38:05to realise what has happened and accepts the egg.
38:12But not this time.
38:17There's a different kind of duck here, too. It's a little smaller, and the male has a
38:23head and neck, the redhead duck. His female has also cast her eye on the canvasback's
38:29nest. Perhaps she can try the same trick.
38:40A canvasback egg is pale green, a redhead's white, so the redhead female tries to make
38:47the success obvious by placing it in the middle of the canvasback's clutch.
38:54The male redhead awaits.
39:01And the deed is done.
39:32The female canvasback leaves her nest for a meal and reveals that this last intrusion
39:42was not the first. There are three white redhead eggs in her nest.
39:49Meanwhile, the redheads sail away to make a nest of their own.
39:57Distributing eggs between several nests, as both canvasbacks and redheads do, is like
40:03taking out an insurance policy. And there are plenty of hazards on the lakes against
40:08which a duck needs to insure, if she possibly can.
40:17Night is the time for burglary.
40:26A raccoon.
40:57This clutch is finished, but there's still a chance that one or two of this female's
41:11eggs laid elsewhere will survive. There really is sense in not putting all your eggs in one
41:18basket.
41:23The redheads, however, don't care for any of their eggs. As parents, they are totally
41:28irresponsible, and the most famous, of course, are the cuckoos. This is the nest of an Australian
41:35fantail. The two little eggs belong to the fantails, and the very big one is a cuckoo's.
41:41It's so different that you would think that the fantails would immediately recognise that
41:48it's a cuckoo.
41:55The fantail female is around, but it's the male who comes back first. He seems quite
42:02unaware that anything is wrong.
42:10The female cuckoo is also keeping an eye on things. The fantail has accepted the egg,
42:17and that will be disastrous, because when the bigger cuckoo chick hatches, it will push
42:22out the baby fantails. Maybe cuckoos have only just started to do this, and fantails
42:28haven't yet developed a defence.
42:32In North America, the cowbird is also playing this game. It has put an egg in the nest of
42:38a gnatcatcher. It's slightly bigger, but very similarly marked.
42:46Will the gnatcatchers notice the difference?
43:06They have. They are destroying their own nest. There is no future for their own chicks in
43:12this one, but nesting material is too valuable to waste, so they are going to start again.
43:26They begin a new nest quite close by.
43:32Day after day they labour, destroying the old and building the new.
43:51And there goes the alien egg.
43:57The cowbird has lost this particular duo.
44:04Africa. The duos are being fought out here too. This is a colony of lesser-masked weavers,
44:11and sitting in the trees nearby are once again cuckoos. These are didgerid cuckoos.
44:19The cuckoos, if they can, will dump their eggs in the weaverbird's nest and leave them
44:26there for the weaverbirds to rear, and the weavers seem well aware of the danger.
44:33They are taking precautions, adding long entrance tunnels to their globular nests, as they must
44:40have been doing for many centuries.
44:45The cuckoo watches for a recently finished nest in which a female has just started to lay.
45:16There's one.
45:27But the cuckoo is having a lot of trouble getting in.
45:33In the past, cuckoo eggs have been frequently found in the nests of these weavers, but none
45:38seem to be getting into this colony. Maybe the weavers are beginning to make those entrance
45:44tubes just a little bit narrower.
45:50The battle seems to be swinging the weavers' way.
45:58Nearby, there's a colony of a slightly different kind of weaver, the marginally bigger masked
46:03weaver.
46:06The weavers seem to be getting a little bit closer to the nest.
46:12Nearby, there's a colony of a slightly different kind of weaver, the marginally bigger masked
46:18weaver.
46:23They don't put entrance tubes to their nests, perhaps because they themselves are nearer
46:28the size of a cuckoo, so any entrance they can get into, a cuckoo could also. They have
46:34a different defence.
46:38The colours of their eggs are extraordinarily variable, ranging from pure white to blue
46:43and speckled. But any one cuckoo can only lay one kind of egg, and it has no way of
46:49knowing what colour the eggs are in any particular nest, so the odds are against the eggs matching.
46:56Now, I happen to know that this nest contains speckled eggs. Let's see what happens if I
47:01put a pale egg in it.
47:09Back comes the owner.
47:24No doubt about who's winning here either, this time.
47:31The battle between cuckoos and other birds is a continuing one. The cuckoo is developing
47:37new stratagems and perhaps finding new victims, and the victims finding new defences. Soon
47:44in those nests behind me, eggs will start hatching. Most will produce young weaver birds,
47:50but some, equally certainly, will be cuckoos.
48:00Whichever they are, the young chicks will be faced with a whole set of problems that
48:06they have to solve if they're to go into adults. And the ingenious and sometimes touching way
48:12in which that is done is what we'll be looking at in the next programme, In the Life of Birds.
48:35In the Life of Birds.
49:05In the Life of Birds.