The Insatiable Appetite The Life of Birds

*

If you travel by air, it's very important to keep your weight down to a minimum.

You can't afford to carry a lot of fuel around,

and what you do carry should be energy-packed and not too bulky.

And oak trees put just such a substance into their acorns.

It's to fuel the growth of their seedlings, and they protect it with a hard shell.

But jays know how to deal with that.

A beak is a concession to weight-saving.

It's much lighter than the jaws and teeth reptiles and mammals use to process food,

yet it's also very efficient and versatile.

A jay, using its beak like a pick, can cut through an acorn's armour without any difficulty.

Beaks are closely matched to diet.

A goldfinch uses its beak like a pair of tweezers.

It's just the right length for extracting the seeds from between a teasel's spines.

A blue tit has a stubbier beak. That gives it the strength to crack small seeds,

but it also prevents its owner from getting them from a teasel.

A greenfinch's beak is even stouter and stronger,

but it's far from clumsy.

Watch how, with help from its tongue, the bird delicately removes the outer shell from these rosehip seeds.

The strongest beak in the finch family belongs to the hawfinch.

It can even deal with a cherry stone,

but the bird doesn't simply rely on brute force.

First, it moves the cherry stone into just the right position for easy cracking.

It gets rid of the broken shell...

..and now it starts the fiddly operation of removing the papery husk that covers the kernel.

Pine trees - these are in California - protect their seeds by enclosing them in cones.

When they're green, the seeds within are beyond the reach of most birds.

But the crossbill has special equipment.

It's the only finch that can twist its upper and lower bill in opposite directions.

Now, right at the bottom, it can feel the soft young seed with its tongue.

Got it!

After a meal of pine seeds, these American crossbills regularly fly off to a bank of exposed clay.

They're in need of digestive tablets.

Green pine cones are very resinous and may cause stomach upsets,

but clay in the stomach will absorb the resin and so prevent any trouble.

A crossed bill, however, is not the best implement for digging -

you've to twist your head to one side to get your bill into the ground,

but they get enough to allow them daily meals from pine trees.

Seeds in the temperate parts of the world, whether pine cones, cherry stones or acorns,

all have a major disadvantage as a food for birds -

they're very seasonal.

There's none in spring and summer and a glut in autumn.

A single oak tree like this, in a season, can produce 90,000 acorns,

but there are lots of things, apart from birds, that eat acorns.

Squirrels do, for a start.

If a jay is to collect acorns in autumn, it will have to be quick,

before others grab them all.

It's carrying one in its beak because its crop is full,

as you can see from that bulge on its throat. There can be up to nine acorns in there.

But what is a jay going to do with such quantities? It can't eat them all.

They store them - by burial.

One jay, in a month, may bury as many as 3,000 acorns.

What is more, when winter comes and it needs food, it will remember exactly where most of them are.

In North America, oaks still form huge forests

and they produce acorns on an astronomical scale.

Among those that harvest them are woodpeckers.

A woodpecker's beak is a drill and a very efficient one, too,

so it's not surprising that it stores acorns by drilling.

There are as many as 60,000 acorns stored in the holes drilled in this one tree.

All the members of this woodpecker family - eight birds in all - use this one tree.

To start with, the birds deposit newly gathered acorns in a hole to allow them to dry off.

Then, when they've shrunk as much as they're going to do immediately, they're given individual storage.

This larder provides food for the family throughout the year, but the birds have to be vigilant

and ready at all times to repel raiders.

There's also a lot of maintenance work to be done.

As the acorns continue to dry and to shrink, they become loose in their sockets.

That would never do - they'd be easy for someone to steal, they might even drop out.

But if they're hammered into a hole that is too tight, the shell could crack and then the acorn would rot.

So maintenance is a never-ending, year-round labour, and it takes a lot of care and judgment.

The result is a great acorn treasury

that will last the whole family well into the next harvest.

These neat holes are made not for storage, but for theft.

They're made by another kind of woodpecker - a sapsucker.

They're just deep enough to tap the vessels along which the tree transports its sap,

and sap is largely what the sapsucker lives on.

The bird cuts its sap wells with an accuracy and symmetry that would do credit to the finest cabinet-maker.

Sap normally hardens quickly and seals a wound. This doesn't.

It could be that the sapsucker produces a kind of anticoagulant in its spittle,

but if it does, no-one yet has managed to identify it.

Even so, each little well eventually runs dry and the bird has to cut another.

These wells have been made in the trunk of a pine tree which produces sap throughout the year.

Other trees, such as these aspens, only produce sap in quantity during spring and summer.

When the sapsucker moves on to these, it cuts differently shaped wells.

With this spring increase in the sap supply, new birds appear in the woods.

A yellow-rumped warbler. They're quick to drink from the wells cut by the sapsuckers.

Food is short so early in the year.

The birches are now in leaf

and the sapsucker moves on to them and makes wells of yet a different shape.

A northern oriole - another hungry migrant only too willing to benefit from the labours of others.

And a hummingbird.

It used to be thought that hummers timed their arrival to coincide with the opening of the spring flowers.

But they arrive well before that. Their appointment is with the rising of the sap and the sapsucker's work.

Sap is an energy food, easily picked up with the tongue,

but it can only be collected from many northern trees during part of the year,

so when winter approaches, warblers and hummingbirds fly south again to where it's winter all year long.

Here, in Mexico, the sap is taken not only by birds, but by insects.

The trunks of many trees seem to be sprouting long hairs.

At the end of each, there's a tiny drop of liquid.

The hair is a tube, projecting from the rear of an insect, lying beneath the bark, drinking sap.

But the insect gets more sugar than it needs, so it excretes the excess.

That is what the hummingbirds, with exquisite accuracy, manage to collect.

Many different warblers take it, too.

The liquid, rather flatteringly called honey dew, is so sought after

that some birds take up residence in a particular tree, and will drive away any others that try to feed.

Their meals, however, come in such small instalments that feeding has to be almost continuous.

It takes about an hour for a drop to accumulate at the end of a tube,

so to get enough to sustain themselves, the hummingbirds have to travel round throughout the day.

Plants produce other edible things as well as seeds and fruit and sap.

They sprout leaves, but leaves are not very good food. They're bulky and need a lot of digesting.

So animals that live on leaves, like these cows, for example,

tend to be rather hefty creatures with massive batteries of grinding teeth and special capacious stomachs.

Cows, having grazed, lie down and bring up each mouthful for a second grinding chew.

No bird does that - you can't chew with a beak. Geese have to use a different technique.

They're big birds, as they have to be to accommodate such bulky meals.

Rather than digest grass intensively they eat a great deal of it,

and get rid of what they can't digest almost immediately.

The appetite of geese is apparently never-ending,

and a flock of them, like these barnacles, will work its way across a meadow nibbling non-stop

and pooping all the way.

It's a terrible mess, but it means that after feeding for several hours

the geese are not laid down by great quantities of undigested grass...

and can get into the air without much difficulty.

In South Africa, there's a rather smaller leaf-eater, the mousebird.

They do make some attempt to digest their meals a little more thoroughly.

They start feeding early in the day.

Then, with as much nibbled leaves on board as they can manage,

they sit for hours with their distended stomachs turned towards the sun to help with digestion.

You can't beat a siesta after a heavy meal.

In all the bird kingdom, there's only one species that is really specialised for leaf eating -

the hoatzin of South America.

Like a cow, it has two compartments to its stomach,

the second of which is full of bacteria that help ferment its meals.

In consequence, it's a bulky bird and positively clumsy in the air,

more a lumbering cargo plane than a super jet.

Birds, by stripping leaves, eating seeds and drinking sap, are exploiting plants,

stealing from them.

But many plants exploit birds by using them as couriers.

The arrangement is such an ancient one

that both have evolved special ways of transacting their business.

The plants attract their couriers with flowers and pay with nectar,

which is easy and cheap to produce - it's water with a dash of sugar -

and that's what I've got in here.

These Australian rainbow lorikeets love it.

Here, in Australia, there are some plants that are in flower all year,

so it's possible for birds to specialise as nectar-feeders, as these lorikeets do.

Their tongues, instead of being hard and leathery,

have a feathery, brushy tip so they can lap up the nectar.

And the plants, when they have a need for a messenger, advertise the fact

by producing flowers with particularly bright petals.

Having collected all the nectar immediately available on one tree,

the lorikeets move off to another,

carrying the pollen they collected with them.

But if, as here, the next plant they visit happens to be a different one, the pollen will be wasted.

The plant's way of reducing that risk

is to recruit an exclusive service with couriers who, during flowering season, will visit them alone.

Here, in South Africa, this species of heather encloses its nectar in a kind of floral safe

which only a particularly shaped beak can unlock.

This orange-breasted sunbird has a beak of that shape,

but, even so, it has to probe really deeply for the nectar,

and every time it does, it triggers a little explosion of pollen.

When the bird drinks at another heather plant, some of that pollen will be brushed off,

and the heather will have achieved its end.

But bird and flower can fit one another more closely than that.

On Mount Kenya, there's a sunbird with an even more strongly curved bill - the golden-winged sunbird.

And this is its employer - the lion's claw flower.

The feathers on the sunbird's head look golden, like those on its wings,

but not so - they're black.

The gold colour is entirely due to pollen which is stamped on it when the bird thrusts deep in the flower.

The devices used by plants to restrict their payments to their employees

may, if taken to extremes, defeat the object of the exercise.

This South American plant has gone to great depths to shield its nectar from all but its established partner,

and that has encouraged burglary.

This is the black flower piercer.

It knows exactly where the nectar is stored and it knows a quick way of getting it too.

Its tongue is flicking into the nectary at the top of the flower's trumpet,

so that the nectar is channeled down its lower bill into its throat.

The datura has even longer flower trumpets,

but they are robbed just as easily.

In S America, hummingbirds are the main collectors of nectar,

and they will collect it any way they can -

utilising a flower piercer's break-in is as good a way as any other, as far as they're concerned.

The trumpet of the datura is so long you might think nothing could drink from it legally, as it were,

and only one bird can - the sword-billed hummer,

which has the longest beak, proportionately, of any bird.

A plant only flowers for a short period each year,

so a nectar drinker has to have a succession of suppliers.

The swordbill also drinks from passionflowers.

The S American climate is so equable and the number of plants so huge, there are always flowers to be found.

Accordingly, hummingbirds have been able to evolve highly specialised equipment for nectar feeding.

They developed a unique way of flying that lets them hang in the air while they drink from a blossom.

Their tongues have become threads that flick in and out a dozen times a second,

but they're virtually useless for collecting any other kind of food.

Where most plants tend to bloom at the same time of the year,

neither the suppliers nor the drinkers of nectar can afford to be so specialised.

The coral tree, in Thailand, has no alternative but to offer its nectar in a free and open way,

and this delectable seasonal treat attracts all kinds of birds from far and wide.

Such a large and varied clientele is pretty well bound to do the job required of them.

After they are pollinated, plants produce seeds and then many engage other birds to distribute them.

That, by and large, is heavier work and the payments they offer for that are made with a different currency -

fruit.

Hornbills are on their way to do a job for a fig tree in the Indonesian rainforest.

In Northern Europe and America, waxwings gorge themselves on autumn berries.

A plant wraps its seeds in the minimum flesh needed to persuade a bird to swallow them.

These berries have so little that it's quickly stripped off in the stomach,

and then the waxwing can get rid of the indigestible seed.

The story is the same the world over.

In New Zealand, kokakos are great berry-eaters, distributing the seeds of the plants of their native forest.

In South America, tiny wild avocados are the special favourite of one of the most dazzling birds, the quetzal.

The avocados may be small, but they're still too big for the quetzal to swallow,

so the stones are ejected, not from the back end, but from the front.

The bird has had a good meal, and the avocado has had some of its seeds carried to a new site.

There are other things to eat apart from the products of plants.

They may be difficult to find and even more difficult to catch, but they're well worth having,

because they're full of nutrition - things, for example, like this.

The morpho - a big and powerful butterfly.

The jacamar - a cousin of the kingfisher's.

A butterfly's wings aren't very digestible

and have to be stripped off before the bird can swallow the fat nutritious body.

Winged termites erupting from their holes in the ground and flying away to establish new colonies -

a whole host of birds relish these.

Ants are trickier meals - they, after all, can sting.

It takes a specialist to deal with them.

This is the rufous woodpecker of Southern India.

It's as good at drilling into an ants' nest as it is at drilling wood,

and the bird seems totally indifferent to the ants' stings.

The feathers of its tail, like those of all woodpeckers, are particularly stiff,

so that they can serve as a prop.

The most nutritious morsels are the soft, fat, stingless grubs that can be found in the centre of the nest.

Insects are almost everywhere on every tree -

on twigs, in buds, crawling around in crevices of the bark,

and many birds find quite enough to sustain themselves just by looking carefully.

But some work harder - and get greater rewards.

The nuthatch, in European woods, is indefatigable.

It will eat many things, including seeds in autumn and winter, which they crack with a workmanlike beak,

but, in summer, insects are a major part of its diet.

Its beak serves equally well for picking them out of the bark.

The great spotted woodpecker is a little more specialised.

It particularly likes the grubs of wood-boring beetles. First, it has to chisel away the bark.

Its tongue extends for an inch-and-a-half beyond its beak

and has a harpoon at its tip.

When that hits a grub fair and square, it sticks.

Tree-boring insects are never safe when woodpeckers are around.

But woodpeckers never got to the Galapagos, far away from anywhere in the Pacific.

Insects did, though, and their grubs bore into trees here just as they do everywhere else.

But no Galapagos birds have the physical adaptations with which to reach them.

Galapagos finches, however, are both intelligent and ingenious.

Their beaks are perfectly adequate for stripping away bark.

There's a grub under there somewhere. It can hear it.

How, without the woodpecker's long tongue, can it get it out?

It needs a tool - a spine from a cactus.

A success, but only a partial one.

It has only extracted little bits of the grub.

Nearby, a bird from another clan of finches uses a slightly different technique.

It selects a rather stouter tool that can be used not so much for stabbing as for levering.

That has shifted the grub a little nearer the hole.

It's not quite within reach, but it still has its lever.

Give it another go.

And that's got the rest of it.

Another remote and isolated island on the other side of the Pacific - New Caledonia.

It gets a lot of rain, so it has a much bigger and richer forest than the Galapagos.

But even so, it's so far away from any of the major continents that woodpeckers have not got here either.

This fallen tree trunk is studded with holes,

the work of wood-boring beetles.

Their size suggests that they're made by much bigger insects than their equivalent on the Galapagos.

The New Caledonian crow - and crows are among the most intelligent of birds.

Once again, the sound of a grub, gnawing away in its burrow, betrays its presence.

And once again, since this grub-hunter hasn't got the woodpecker's long tongue,

a tool is needed.

To contact this grub, the stick will have to be thrust in really deeply.

A spectacular catch.

Some of these crows become so attached to one particular tool that they carry it about with them.

This log is clearly a good source of grubs,

and a whole group of crows have come here to feed.

Their technique is neither to stab nor to harpoon,

but something more subtle - to irritate.

This grub has got big jaws and, if attacked, it can give a powerful bite.

And that's what the crows rely on.

A younger bird joins an experienced adult to see how things are done.

Now the pupil has a chance.

It hasn't got all the details exactly right.

It will be about a year before it masters the skill.

There are insect grubs everywhere of course.

The only problem for insect-eating birds is getting at them.

Sometimes, other creatures help.

You might think that this is a recent partnership,

but I'll bet, when our prehistoric ancestors first dug for tubers and planted seeds in Europe,

one of these little robins appeared within a couple of days.

Other animals must have done the same job for them before human beings did.

Once, not so long ago, wild pig were common all over Europe,

and they are great diggers and rootlers.

So maybe the robin's boldness and friendliness with other kinds of animals

started in prehistory, even before human beings arrived in Europe.

Such partnerships exist everywhere, even in the most unlikely places.

This little bird lives on a small island in the Seychelles,

so small and so isolated that few mammals got here before human beings.

It's not, in fact, closely related to the European robin, but it behaves like one.

When Europeans first came to the Seychelles, they called it a robin because of its similar habits.

But what partner did it have before human beings came along?

Could it be this?

Once, there was a large population of these giant tortoises on several of the Seychelles islands.

They weigh several hundredweight,

and those huge legs dig into the ground with every step.

There we are!

These little birds, rarer now than the tortoises, are still their regular companions.

A swamp in South America -

an abundance of water and a warm tropical sun make it a paradise for insects of all kinds.

A kind of flycatcher - a cattle tyrant -

and another obliging partner, a capybara,

a large semiaquatic rodent.

As the capybara moves around, it inevitably disturbs insects of one kind or another,

and what better place for an insect-eater to spot them than sitting on the back of one?

Would the view be any better from there?

Perhaps it would be.

A few of these partnerships between birds and other animals have become very intimate indeed.

The hide of a hippo may not seem a particularly rich insect source,

but there are little ticks to be had in the various cracks and crannies,

and oxpeckers go there to search for them.

They have extremely sharp claws with two toes pointing forwards and two backwards,

so they can cling at any angle - even on a slippery hippo.

Land animals with hair on their hide are likely to be more productive.

Oxpeckers pay particular attention to their ears.

That's the sort of place you might find a tick, and the oxpecker will remove it.

It also eats earwax.

Dandruff is another part of their diet.

Their beaks are flattened so that, with their head held sideways, they can comb through their hosts' hair.

Oxpeckers spend all their lives on or closely beside their animal hosts.

They court and mate on their backs.

When they fly off to make a nest, as they must necessarily do,

they pluck hair from their hosts' backs with which to line it. But do they do anything in return?

It's true that they remove irritating, even damaging insects their hosts can't dislodge,

but the birds' main diet is blood.

Sometimes they get it by swallowing ticks that are bloated with blood,

but they also take it directly, pecking at an animal's wounds to keep them open.

When their host gets irritated, they go back to their toiletry duties, before once again having a sip.

So, in spite of having such a specialised life, living on the bodies of mammals,

oxpeckers manage to get quite a varied diet -

a maggot or a tick, a little sip of blood, some tasty earwax -

but there are some birds that literally live on mammals, alive or dead.

They eat them, and those are the birds we'll be looking at in the next programme in this series.

Subtitles by Martin Maguire BBC Scotland 1998