Life of Birds The 8 - The Demands of The Egg
These sooty terns are amongst the most aerial of birds.
Every one of them has spent the first three or four years of its life flying non-stop.
Feeding on the wing,
swooping down picking up fragments of food from the surface of the sea,
even sleeping on the wing.
But there is one thing that compels them to come down to earth.
Flying with an egg inside the body, let alone a clutch of three or four,
makes huge demands on the energy of a bird.
So every female bird in existence gets rid of that huge load
just as soon as she can. She lays it.
In places like this island in the Seychelles,
where there are no land predators to steal eggs,
it's safe for the sooty terns to put those eggs directly on the ground.
The birds sit as close to one another as they can
without getting within the range of their neighbours' beaks,
so they are equally spaced with almost mathematical precision.
Other kinds of terns have other ideas about where to put their eggs.
The fairy tern, for some reason, always puts them on a bare branch,
though whether that is safer or more dangerous is debatable.
The females seem wildly optimistic.
A U-bend is one of their safer ideas.
The dimple left when a branch breaks away is not bad either.
But it seems a bit reckless to rely on a little dead twig like this,
particularly when the trade winds blow as strongly as they do in the Seychelles.
The fact is that fairy terns' eggs are easily dislodged if left unguarded.
Skinks know that... ..and so do the fodies, the local sparrows.
And that has solved the problem of how to crack it.
Yolk is provided by a female bird to nourish her chick as it develops inside the egg.
But it is equally good food for lots of other animals,
so eggs are much sought after,
and birds may have to go to great lengths to keep them safe.
Swifts living on the mainland have to take greater precautions.
The nests of palm swifts are minimal, but they put them in fairly inaccessible places,
such as an isolated palm tree.
They stick a few feathers on the underside of one of its dangling dead leaves,
using their spittle as glue.
To make sure that the egg stays in this flimsy hammock of feathers,
they stick that to the leaf as well.
Changing places to take over incubation is a tricky operation
when your nest is stuck to a vertical surface.
The power of flight enables birds to put their nests in places
that are beyond the reach of land-bound creatures.
And there can be few more inaccessible spots than the rocks behind a great waterfall,
like this one at Iguacu in South America,
where Argentina and Brazil meet.
Swifts, once again, exploit their mastery of flight
to go where no other bird can go.
Great dusky swifts roost for the night
on the mist-drenched rocks alongside the falls.
But this is not a safe enough place for their precious eggs.
Theywill be deposited actually behind the curtain of water
and to do that the birds must find the thinnest part of it.
Behind the curtain, they still have an awkward climb
before they reach a place where it is possible to put an egg.
And this is it, a perpetually dripping ledge,
just wide enough to accommodate a little muddy nest.
Many birds choose cliffs for their nest sites
even when they are not screened by water.
Seabirds around the world use them.
But these aren't normal seabirds.
They're parrots and they are nesting on the coast of Argentina.
The powerful hooked beak that all parrots have and use for cracking nuts and gouging fruit
is also an excellent excavating tool.
These cliffs are composed of a relatively soft sandstone.
No problem for a parrot. Even so, digging a hole in them is hard work,
so each hole, when it is finished, is a valuable property,
and there is a great deal of competition and squabbling over any vacancy.
Sand-martins are not nearly so well equipped for digging.
They use their claws rather than their delicate beaks
and they can only tackle sandstone if it is soft and friable.
But what they lack in equipment they make up for with energy.
Woodpeckers, of course, are expert carpenters
and regularly chiseltheir nestholds in trees.
But this one is digging into softer material, an ants' nest.
The ants are furious,
but they quickly get used to their lodger sitting in the middle of their mansion,
and then they act as her guardians, attacking any intruder that tries to steal her eggs.
You might think that a hornbillwould have the most powerful excavation tool of all,
but in fact its huge beak is a relatively delicate structure
and no use at all as a chisel.
So hornbills have to find holes that are either natural or have been dug by others.
A pair do their househunting together and they are very choosy.
Perhaps this one will do.
With beaks over a foot long,
these Great Indian hornbills have to have a pretty big hole.
But they like the entrance to be as small as possible to deter intruders.
Maybe this is just too small.
But she can just make it.
That's good. The search is over.
This is it.
The male regurgitates a little food for her.
Now she has decided that this is for her,
she won't leave again until her eggs are laid and hatched and the chicks well-grown.
She will depend entirely on her mate for food.
She seals herself in,
narrowing the entrance with a plaster made of chewed wood,
mashed food and her own droppings.
This will be her home, you could say her prison, for the next four months.
The majority of birds, though, don't nest in holes.
They buildtheir nests. And builders, of course, need building materials.
Having no hands, but just a beak and at best one foot, seems bad enough,
but these frigates on the Galapagos have got an additional problem.
Their feet are so short and their wings are so wide that they find it difficult to land,
so they much prefer to collect their building material on the wing.
Boobies can settle and break off the branches they need for their nests.
Frigates can't, but they can steal.
So for boobies, getting building material from its source to the building site
is the most difficult part of the whole business.
Stolen goods, it's true, but allthe more precious for that.
Eggs must not only be kept secure.
Once they start to develop, they also have to be kept warm
and there is no better insulation than feathers.
Ducks and geese line their nests with feathers they pluck from their own breasts.
Other birds are not so self-sacrificing
and prefer to use those that they find blowing about.
Tree swallows compete with one another in collecting them.
Miss... and a rivalwon't give you a second chance.
Even when you have got it you may not get away with it.
Tackling a bird without the feather is allwithin the rules of this particular game.
The golden-headed cisticola, a kind of Australian warbler,
collects fibres and spiders' webs, not just for lining, but for stitching.
There is no more skilled a tailor in the whole of the bird world.
There's little problem about concealing this nest
for the leaves she stitches together remain alive and green.
When she has finished, you will barely notice that they are bent together
in a slightly unusualway.
The problem is greater when a nest sits on the bare branches of a tree.
The sitella, an Australian equivalent of the nuthatch,
constructs its nest from spiders' webs and insect cocoons,
and then covers the outside with rather coarser material.
Two pairs are building here, close to one another.
This one is in a tree covered in lichen.
And this, in one that has flaky bark.
The sitellas are not rigidly-minded birds with inflexible habits.
They use lichen to cover the nest in the lichen tree,
and bark on the one in the flaky bark tree.
As a result, each is as well camouflaged as anyone could hope,
and though both nests are plain for allto see,
they are not easily recognised for what they are.
The apostle bird uses mud mixed with grass.
Nest building is a group activity.
A pair with their grown-up young from previous seasons work together.
It used to be believed that there was always a dozen in the team
which is why they are called apostle birds.
Everyone seems anxious to contribute,
and the team works so industriously and so harmoniously
that their elegant cup is usually completed in a mere three days.
A bird's beak, it seems, can serve just as well as a plasterer's trowel,
as a tailor's needle.
Some birds build nests not just as cradles for their eggs and chicks,
but as lodging houses for the whole year.
Here in Namibia in Southern Africa lives the sociable weaver,
and very sociable it is, too.
As many as three or four hundred birds will live in a single apartment block.
This haystack may be more than a century old.
It is so heavy that part of it has broken the branch that supported it
and fallen to the ground.
And it's been built and maintained as a communal effort by allthe inhabitants.
Weavers are closely related to sparrows.
Though some of their relations do indeed weave,
the sociable weaver builds in a rather simple way.
It just pushes straws, one by one, into this gigantic bale.
A large communal apartment block
has a considerable advantage over small isolated nests
when you live in a desert like this.
During the day it gets ferociously hot,
but that thick roof of thatch keeps the apartments beneath nice and cool.
As the sun sets, the weavers, having been away feeding, come back to their homes.
At night, it can get very cold in the desert, as much as 7 or 8 degrees below freezing.
And then the thatch is probably at its most valuable,
because it acts as insulation for the nest chambers,
so that they retain much of the heat that they had during the day,
and the birds that roost inside remain snug and warm.
Not allthe chambers are for nesting.
Some are not nurseries, but bedrooms
in which several of the colony snuggle together for warmth.
So at last some kind of receptacle, simple or complex, is prepared for the egg,
and it is time for the female to produce one.
The male frigate welcomes his partner back.
The crucial moment has arrived.
Mated female birds have been feeding intensively
and their ovaries are much enlarged.
Each egg, emerging from one as a microscopic cell,
is planted on a growing globule of yolk which will provide it with food.
It is fertilised by one of the sperm that has been lying awaiting it in the duct.
Albumen is wrapped around it to provide it with the water necessary for development.
It travels onwards for several hours.
Eventually, it reaches a section in the duct where there are glands that produce lime.
Here it lingers for 24 hours while the shell is added.
Pigment glands squirt little spots of colour on it.
And so, an avocet produces her egg.
The blotches and speckles on such an egg as this, laid on the ground,
serve primarily to camouflage it.
These are the eggs of a golden plover. They too are laid directly on the ground
and are practically invisible; as indeed the bird that laid them will be
once she settles down.
Plovers nearly always lay four eggs.
A few birds, however, have adopted a rather more risky policy.
They lay just two, or even one.
If they do that, they must lay it in a nest that is really secure,
hidden, for example, deep in a burrow as the kiwi does.
Her egg is gigantic, the biggest laid by any bird,
and a quarter of her total body weight.
It contains so much yolk that the chick will be fully developed
when eventually it breaks out of its shell.
Expelling such an egg is obviously a huge effort.
The owner of this nest, a blue tit, adopts a very different strategy.
Her egg is tiny. It weighs no more than a gram.
But she lays lots of them;
one a day, day after day, for a fortnight or so,
but the chicks after they hatch willface so many hazards that many will die
and the survival rate in the end will be not unlike the kiwi's.
Few eggs are totally safe from hungry raiders,
no matter how skilfully protected and artfully concealed they are.
Those lying in the bottom of these dangling nests of caciques in South America
are certainly difficult to reach.
But the red-breasted toucan has a long beak.
This toucan's bill is just not long enough for these particular nests.
But the toco toucan has an even longer one.
The caciques are extremely agitated.
And with good reason.
If the caciques are to defend themselves against these powerful bandits,
they will have to build even longer nests in the future.
In Australia, the prime egg thief is the currawong.
An unguarded nest.
An obvious target.
Another clutch gone. Few eggs are safe from currawongs
and the Australian birds, like birds everywhere,
have developed many different strategies to try and foil burglars.
This is the nest of a yellow-rumped thornbill,
but in fact this neat little cup is empty.
You might think therefore, that it has been robbed of its eggs.
But in fact this part of the nest has never had any eggs in it.
There's another entrance to the nest. It's down here.
You might never notice it, untilyou watch the parent return.
The cup at the top is a dummy,
and it seems that many currawongs have not yet discovered the fact.
This wren in Costa Rica has another way of protecting its eggs.
Its nest is pretty obvious...
..and equally obvious is another one close by, a wasps' nest.
The wren nearly always builds within a yard or so of these formidable insects.
It is a brave thief that risks being attacked by these.
But coatimundis are brave, sometimes to the point of recklessness.
Out here on the cold windswept plains of Patagonia
there are no trees in which a bird could build a nest.
So plovers, like plovers everywhere, lay their eggs on the ground
and trust their camouflage.
But if a stranger wanders too near it, one of the adults immediately responds.
Ahead, I can just see a bird crouching on her eggs. And away she goes.
And now she starts a most bizarre pantomime.
This hardly looks like any kind of bird, and whatever it is, it seems to be crippled.
If I was a fox or maybe a hawk,
I might quite wellthink that this extraordinary performance represents an injured bird
or maybe a little rodent.
Either way it looks like an easy meal and I might follow it and be led away from the nest.
And now she has taken me so far from her eggs that she can abandon her play-acting.
There is nothing the matter with her.
Having deflected me, she returns to her nest.
She has to go back to prevent her eggs getting chilled
and the young within from dying of cold.
Keeping eggs warm, indeed, is a continuing problem for most nesting birds.
The maleo has a very labour-saving way of dealing with its heating problems.
It lives on an island in Indonesia.
In this one unshaded patch of the beach, the sand is kept so hot by the sun
that the eggs will hatch by themselves,
and the whole of the local maleo population know it.
The birds are accurate judges of temperature. They have to be.
If they don't dig deep enough, their eggs will bake,
and if they go too far, they won't develop.
That's probably about right.
Now allthat is needed is to fill in the hole.
And then they abandon it.
These eggs, up in Alaska, must be tended much more assiduously.
They belong to a snowy owl.
As she returns, you can see that the feathers on her belly are rather long and floppy.
There is even a glimpse of pink naked skin.
Her body has to be particularly well-insulated with dense plumage
to prevent it losing heat in these near-freezing conditions,
yet somehow she has to transfer some of that heat to these eggs.
This naked brood patch on her belly will enable her to do just that.
Her mate is away hunting for lemmings, not only for himself, but for her.
She cannot leave her eggs for more than a minute or two,
so her mate looks after her, and he will have to do this for almost two months.
The chilly lakes of North America.
Here some birds trick others into looking after at least some of their eggs.
This female canvasback duck is sitting contentedly,
incubating and minding her own business.
Out on the water, another female canvasback is mating with a drake.
The mated female has not yet built a nest of her own,
so she makes her way to the one who has.
The sitting female clearly doesn't like this intrusion,
but equally, she's not going to abandon her eggs.
The intruder pushes her to one side and quickly lays.
Sometimes the sitting bird doesn't seem to realise what has happened
and accepts the egg.
But not this time.
There's a different kind of duck here, too.
It is a little smaller and the male has a redder head and neck, the redhead duck.
His female has also cast her eye on the canvasback's nest.
Perhaps she can try the same trick.
A canvasback egg is pale green, a redhead's white,
so the redhead female tries to make hers less obvious
by placing it in the middle of the canvasback's clutch.
The male redhead awaits.
And the deed is done.
The female canvasback leaves her nest for a meal
and reveals that this last intrusion was not the first.
There are three white redhead eggs in her nest.
Meanwhile, the redheads sail away to make a nest of their own.
Distributing eggs between several nests, as both canvasbacks and redheads do,
is like taking out an insurance policy.
And there are plenty of hazards on the lakes against which a duck needs to insure,
if she possibly can.
Night is a time for burglary.
This clutch is finished,
but there is still a chance that one or two of this female's eggs laid elsewhere will survive.
There really is sense in not putting allyour eggs in one basket.
Some birds, however, don't care for any of their eggs.
As parents, they are totally irresponsible and the most famous, of course, are the cuckoos.
This is a nest of an Australian fantail.
The two little eggs belong to the fantails, but the very big one is a cuckoo's.
It is so different that you would think that the fantailwould immediately recognise
that something is wrong. But watch.
The fantailfemale is around, but it's the male who comes back first.
He seems quite unaware that anything is wrong.
The female cuckoo is also keeping an eye on things.
The fantail has accepted the egg, and that will be disastrous,
because when the bigger cuckoo chick hatches, it will push out the baby fantails.
Maybe cuckoos have only just started to do this
and fantails haven't yet developed a defence.
In North America, the cowbird is also playing this game.
It has put an egg in the nest of a gnatcatcher.
It's slightly bigger, but very similarly marked.
Willthe gnatcatchers notice the difference?
They have. They are destroying their own nest.
There is no future for their own chicks in this one.
But nesting material is too valuable to waste,
so they are going to start again.
They begin a new nest quite close by.
Day after day they labour, destroying the old and building the new.
And there goes the alien egg.
The cowbird has lost this particular duel.
Africa. The duels are being fought out here, too.
This is a colony of lesser masked weavers
and sitting in the trees nearby are, once again, cuckoos.
These are diederick cuckoos.
The cuckoos, if they can, will dump their eggs in the weaver birds' nest
and leave them there for the weaver birds to rear.
And the weavers seem well aware of the danger.
They are taking precautions, adding long entrance tunnels to their globular nests
as they must have been doing for many centuries.
The cuckoo watches for a recently finished nest in which a female has just started to lay.
But the cuckoo is having a lot of trouble getting in.
In the past, cuckoo eggs have been frequently found
in the nests of these weavers.
But none seem to be getting into thiscolony.
Maybe the weavers are beginning to make those entrance tubes just a little bit narrower.
The battle seems to be swinging the weavers' way.
Nearby, there's a colony of a slightly different kind of weaver,
the marginally bigger masked weaver.
They don't put entrance tubes to their nests,
perhaps because they themselves are nearer the size of a cuckoo,
so any entrance they can get into, a cuckoo could also.
They have a different defence.
The colours of their eggs are extraordinarily variable,
ranging from pure white to blue and speckled.
But any one cuckoo can only lay one kind of egg.
And it has no way of knowing what colour the eggs are in any particular nest.
So the odds are against the eggs matching.
Now I happen to know that this nest contains speckled eggs.
Let's see what happens if I put a pale egg in it.
Back comes the owner.
No doubt about who's winning here either, this time.
The battle between cuckoos and other birds is a continuing one.
The cuckoos developing new stratagems and perhaps finding new victims,
and the victims finding new defences.
Soon, in those nests behind me, eggs will start hatching.
Most will produce young weaver birds, but some equally certainly, will be cuckoos.
Whichever they are, the young chicks will be faced with a whole set of problems
that they have to solve if they are to grow into adults.
And the ingenious and sometimes touching way in which that is done
is what we will be looking at in the next programme in The Life ofBirds.
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