FOUR

Meat-Eaters

The kea is a parrot. Its home is New Zealand and although it does not have the flamboyant colours of many of its Australian relatives, it is nonetheless a handsome bird with scarlet beneath its wings of muted green. Like all of its family, it has a powerful, sharply hooked beak which it uses to crack nuts, mash fruit and nibble at any odd bits and pieces lying around that it considers might perhaps be edible. It is a hardy bird, better able to tolerate cold than many other parrots, and lives happily in high mountain valleys, clambering around snow-covered boulders and gliding across the face of the great precipices.

One small community of keas haunts a desolate valley in New Zealand’s South Island, where the mountains run down steeply into the sea. There are colonies of sooty shearwaters here too. You never see these birds during the day, for they are fishing out at sea, but at night, they return to the nest holes they have dug for themselves in the turf among the boulders. The local people call the shearwaters ‘mutton birds’ because the young squabs, by the time they are four months old, have fed so well on the semi-digested fish brought back to them by their parents that they are full of fat and weigh a couple of kilos. The first human settlers to the island used to harvest them in great numbers. And so do the keas.

A kea with a fancy for such a meal, stalks through the warren of shearwater nest holes, bending down every now and then, head cocked, to listen. Within the burrows, shearwater chicks crouch silently in the darkness. But sometimes they call. Maybe they mistake the tread of the kea for that of their parent, returning with food. If one does make some sound, the kea reacts swiftly. It starts to dig. Using its beak like a mattock, it tears away the turf around the burrow’s entrance. It reaches inside, but the shearwater’s burrow can be a long one. The kea may have to dig for several minutes more, scattering the brown earth behind it, before it leans inside once again. It is now within reach of the chick. The young mutton bird is not entirely defenceless. Its stomach may still be full of the fish oil fed to it by its parents and it squirts it directly in the kea’s face. The kea is unlikely to be deterred. Doggedly, it reaches into the hole again until at last it is able to grab the chick by its neck or leg and drag it out. The young bird is still without feathers and covered only in down. It looks as fat and as vulnerable as a plucked duck and it squeals in distress. But not for long. The beak that was so effective a mattock now becomes a billhook and rips the young shearwater to pieces. The kea may be a parrot, but it has become a hunter.

Eating meat brings one great benefit. It is much more nutritious, pound for pound, than nuts or fruit so a meat-eater need only spend a small part of its day feeding. Nevertheless, collecting meat can be a major problem. Animals do not yield their flesh as easily as plants surrender their fruits and seeds or even their leaves. Keas probably took to hunting relatively recently in their evolutionary history and even now do not rely on it as the principal source of their food. Their hearing, on which they rely for finding their victims, is no more acute than that of most birds; and their beaks, so effective for cracking seeds, just happen to be also good for digging. Specialist meat-eaters, if they are to be successful, must have more refined senses with which to locate their prey than the kea possesses and more deadly weapons with which to despatch it.

The great grey owl that lives in the dark, cold, coniferous forests of the north has the most sensitive of ears. The range of frequencies it can hear is much the same as ours, but its sensitivity over that range is very much greater and it can detect sounds so faint that they are completely inaudible to us. It has two large ruffs on either side of its face, formed from fine hair-like feathers that collect the sound waves and deflect them into its ears, rather as an ear-trumpet does. The ruffs also shield each ear from the other, so that sound corning from the right side is received almost entirely by the right ear. Furthermore, its two ears are not symmetrically placed on its head. One is higher than the other. The owl is thus able to listen in super-stereo and can locate the source of any sound with great accuracy, even if it cannot see the creature that is making it.

That is an essential talent during the winter, when the snow lies thick on the ground. The owl, perched on the branch of a pine tree, 6 metres up, suddenly detects the sound for which it has been waiting. Fifty metres away and half a metre beneath the surface of the snow, a vole is nibbling a frozen leaf and making just the faintest rustle. The owl swivels its head to face the direction from which the rustle comes and to locate it more accurately. Once it has fixed its position, it topples forward. Its downward glide is virtually silent for the primary feathers on its wings have comb-like fringes which deaden the sound of air passing across them. The little mammal beneath the surface of the snow hears nothing to warn it of what is to happen. Equally importantly, the owl can continue to hear the faint sounds that first attracted it. For a few moments, it hovers just above the snow, staring downwards. It is not focusing its eyes, for there is still nothing to be seen. It is focusing its ears. It pounces. There is a flurry of snow as its wings strike it and the owl reaches through it with its legs outstretched. With a powerful flap, it lifts itself into the air again carrying a still struggling vole transfixed in its talons. Silently, with a few slow flaps, it returns to its perch. It lifts the little furry body up to its beak and swallows it whole, in the way that all owls take their meals.

A few voles are quite enough to sustain it for some time. The rest of the night and the following day, it can sit immobile, well-nourished and quietly resting, digesting its meal. Inside its gizzard, the vole’s body is macerated. Meat and guts are separated from fur and bones. These inedible components are then rolled by the gizzard’s muscular walls into a pellet which, in due course, the owl will regurgitate and let fall to the ground.

The great grey owl has to depend primarily on its hearing, not only because the snow conceals its prey but because for half of the year, the Arctic forests are in partial or complete darkness. Farther south, owls can use their eyes for hunting. All birds must have excellent eyesight for it is essential in controlling their flight. But flying hunters have a special need for acute vision. The bigger the eye, the more light it can gather and owls, which so often hunt at twilight or by the light of the moon, have very big eyes indeed. The scops owl, which hunts in European and Asiatic forests, has such enormous eyes that they cannot revolve in their sockets. If it wants to look to one side, it has to turn its whole head. Owls are particularly good at that. They have a neck joint that allows them to turn right round and stare directly behind them if they wish. Some can even turn their heads beyond that and revolve through 270 degrees.

The retina, the screen at the back of the eye on which the lens of the eye casts its image, contains two different kinds of sensitive elements. One set, the rods, register shape; the other, the cones, register colour. Animals that sleep at night and are active mostly during the daylight hours have a mixture of rods and cones in their retinas. But colour is largely invisible at night and nocturnal creatures have eyes with retinas packed almost entirely with rods. So we can be reasonably sure that many owls perceive the world largely in monochrome but are able to do so in light so dim that other creatures would not be able to see at all. At such low light levels, it is movement more than shape that catches the attention. A mouse, crouched motionless on the forest floor, may be almost invisible to an owl. But if it moves, then it will immediately reveal itself. And since a mouse must move if it is to gather its food, it cannot be safe from the owls that hunt in the stillness of the night-time forest.

Birds that hunt during the day sacrifice low-light perception of shape for sensitivity to colour and have both rods and cones in their retinas. The acuity of their vision is nonetheless extraordinary, for their retinas, in the case of some eagles, are physically even bigger than ours; and the rods in them are more densely packed. The retina of a human eye contains some two hundred thousand rods. That of a buzzard contains about a million. If our eyesight is particularly good, we may be able to spot a rabbit flicking its ears from a hundred metres away. A buzzard is able to do so from a distance of more than 3 kilometres.

Some species are able to see over an even wider colour spectrum than we can. Humans have three different kinds of cones in our retinas and we detect different shades by combining signals from all three in rather the same way that printers reproduce different tints by using three different inks in varying proportions. Birds, on the other hand, have five or six different kinds of cones and some, certainly, can perceive ultraviolet light. The kestrel is one and it has been suggested that this might have value for the bird. Voles are one of its main prey. The little animals run along regular tracks, collecting their vegetarian meals, and mark these paths with little squirts of urine. This helps them find their way and conveys messages of ownership and sexual availability. But it also reveals their paths to a kestrel, for urine reflects ultraviolet light. So the bird, hovering above, knows just where it should look to spot a giveaway movement.

The weapons used by hunting birds are simple, unique – and lethal. Many other kinds of birds can and will, on occasion, kill other animals. Jays and magpies will eat the chicks of other birds and herring gulls catch mice when foraging on refuse tips. But such omnivorous birds kill with blows of their beaks. The specialist hunters – owls and eagles, hawks and falcons – kill with their talons. These are long, sharp and so deeply curved that when they stab into an animal’s body, only a deliberate action by their owner will release their terrible grip. Three talons point forwards; a fourth, longer than the others, extends backwards. A hawk, clutching the head of its victim, will often slide that rear claw into the back of the skull and into the brain. It is sometimes called the killer claw.

The talons of the eagles of tropical forests are so massive and so sharp that they can stab right through the body of a monkey. The African crowned eagle catches monkeys that weigh as much and sometimes more than itself. As it dives on them, it swings its legs and pelvis forward, and strikes its victims with such force that the blow may kill them outright. If it does not, then the wounds inflicted by the huge talons stabbing into their internal organs are sufficient to do so. The harpy eagle of South America is even said to take sloths, which are the size of a sheepdog, grabbing them as they hang somnolently from the branches of rainforest trees. Indeed, eagles are said to be the only creatures that can reach these bizarre animals to prey on them. In the Galapagos Islands, an endemic hawk, closely related to the buzzards, hunts marine iguanas, seizing the black, 60-centimetre-long reptiles in its talons and eventually, often after a long struggle, killing them by tearing them apart with its beak.

Each kind of hunter has its own technique for deploying its weapons. A golden eagle, hunting rabbits, is unlikely to be successful if it dives directly from the sky on to its target. The rabbit will see it coming and scamper to safety in its hole before the eagle can reach it. So instead, an eagle will soar at altitude over its hunting territory, scanning the ground beneath. Having spotted a grazing rabbit, it will flap away and gently lose height. Then it flies back swiftly, only a few feet above the ground, and comes out of nowhere to pounce before the rabbit has time to get back to its burrow. The sparrowhawk also attacks from low-level. It will fly fast along one side of a hedge and then suddenly vault over to catch small birds unawares perched on the other side. The merlin takes small birds in the air. Larks are a favourite prey and the merlin tries to catch them by dropping down from above. A lark, therefore, if it becomes aware of the merlin’s presence, seeks safety by climbing. It rises nearly vertically, frantically pumping its wings and seemingly standing on its tail. The merlin gives chase, circling up after it. As long as the lark remains uppermost, it is safe, but if the merlin seems to be climbing faster, then the lark will have to change tactics. Suddenly, it closes its wings and drops like a stone. If it manages to pitch into a bush or thick grass, then as long as it keeps completely motionless it will be safe.

The peregrine, closely related to the merlin but significantly larger, uses the same tactic of diving on its victim from high altitude but does so with even greater speed. Split second timing and perfect flight control are essential. Somehow, the peregrine must keep its eyes fixed on its victim, to judge its velocity and course, and adjust its own trajectory accordingly. What happens in the final stages is far too fast for the human eye to follow. It can only be seen with the help of slow-motion film. In the last few moments, the peregrine actually slows down slightly. A fraction of a second before impact, it brings its legs and talons forward and strikes. The rear talon, the killer claw, is the main contact. It rips along the back of the target and up to its neck, breaking its spine and sending it tumbling from the sky.

Sometimes, not surprisingly, the peregrine overshoots. Then, almost unbelievably, it may pull out of its dive and strike its prey from the rear on the way up again. This sudden change of direction must produce such a gravitational pull on its body that a human fighter pilot might well wonder why the peregrine does not black out. The skills required to hunt in this way are so finely tuned that even a misjudgement by a microsecond will cause failure and the fact is that a hunting peregrine only succeeds in less than one in ten of its attacks.

In many hunting species, there is a considerable difference in size between the sexes. The male sparrowhawk is almost half the size of the female. This is because the female, as the breeding season approaches, has to accumulate bodily reserves from which to make her eggs. Her ovaries swell and her weight increases by as much as 13 per cent. This increased weight makes it difficult for her to achieve the speed and agility that she needs in order to catch sparrows and other small birds. So there is a division of labour between a pair. She, with her added bulk and size, mounts guard over the nest and defends it against predators. He – small, swift and agile – does the bulk of the hunting and brings back his catch to her and, in due course, to their chicks. Later, when the chicks are bigger with increasing appetites, she may help to feed them by snatching pigeon squabs from their nests, something that he is too small to manage without great difficulty.

Falcons, hawks and eagles, unlike owls, do not swallow their prey whole. Often it is far too big for them to do so, for a peregrine may take a grouse and an eagle kill a hare. It is now that their beaks come into action. Falcons have a distinctive little tooth halfway along the cutting edge of their upper beak which they use to give their prey the coup de grâce, jabbing it between the vertebrae of their victim’s neck, but for the most part, hunting birds use their beak for butchery. Holding the body of their prey on to the perch with their talons, they rip it apart, taking the meat and favoured parts of the entrails such as the liver, and discarding the bones, feathers and fur. They do not, therefore, need to regurgitate pellets of inedible material to the same extent that owls do.

Some hunting birds even dare to tackle terrestrial animals on the ground. The European short-toed eagle takes on particularly dangerous prey – snakes. It goes into battle with its wings outstretched and uses them in the same way as a bullfighter uses his cape – to deceive its opponent about where its body is and where an attack might come from. The snake, a grass snake or even a viper, strikes repeatedly and accurately enough at what it sees, but all it manages to hit are feathers and any venom discharged is wasted. It might, with luck touch the eagle’s legs, but even if it did, it would inflict little damage, for the eagle’s bony legs are protected by particularly stout scales and have few blood vessels in them that would carry the snake’s poison to vital parts of the bird’s body. The eagle prances back and forth, keeping just out of range of the snake’s striking head until, in a split second, it sees its chance and grabs one of the snake’s coils in its talons. The snake is now at close quarters and still has enough freedom of movement to stab the eagle’s body with its fangs. The bird must act fast with its other weapon, its beak. With one stabbing bite, it severs the vertebrae just behind the snake’s skull. The battle is over.

If the eagle has chicks, it will need to get its capture back to the nest. That is indeed a problem. A long snake, dangling from its claws, would not only be unwieldy in the air but would be an open invitation to others to try to steal it. The solution is simple. The snake is swallowed. Inch by inch, the eagle gulps down the still writhing coils until only the tip of the tail is left protruding from its beak. Back at the nest, the snake’s body takes almost as long to come out as it did to go in. The eagle tries to pull it out with its foot but it cannot get a proper grip on it. One of the chicks tries to help. It grabs the snake’s tail and pulls. Out comes the whole snake. The chick seems in no way dismayed by the sight but sets about swallowing a meal that looks to be almost bigger than itself.

Three birds in particular have become so skilled at snake-hunting that they have largely abandoned flight. Red-legged and black-legged seriemas, distant South American relatives of the crane, will pick one up by the tail and kill it by repeatedly beating its head against the ground. The African secretarybird, which feeds on all kinds of ground-living creatures from insects to rats, makes its kills by stamping on its victims.

All meat-eaters, however, do not do their own killing. They find what they want by waiting beside the hunters’ tables. In Africa, marabou storks, bald of head and scabrous of bill, stand impassively beside the kills of lions, waiting for the moment when they may be allowed to pick scraps of meat from the bones; in the Antarctic, sheathbills scuttle through penguin colonies seeking dead chicks and the yolk of broken eggs; in the Falkland Islands, where other food is in extremely short supply, even yellow-billed pintail ducks will rip scraps of meat from the carcasses of seals that so often litter the beaches. But the specialist scavengers of the bird world are the vultures.

Vultures have broad, blunt-ended wings that enable them to circle on the currents of warm air that rise above the grassy savannahs; and naked heads, for they feed by thrusting their heads inside corpses and feathers would quickly become soiled by blood and guts and become the source of infections.

The bare heads may also bring an additional advantage. The temperature at the altitudes where the birds spend so much of their time is very low and the plumage on their bodies is consequently particularly dense so that they remain warm as they soar. Down on the ground, however, it can be very hot indeed. Many vultures cool themselves by squirting urine over their legs which, as it evaporates, cools them – and also, incidentally, leaves them looking as though they have walked through whitewash. But on the ground, the bare skin of their heads may also be of great help in getting rid of unwanted heat.

It has long been a matter of argument as to how vultures, circling high in the sky, are able to detect a corpse lying on the grasslands below. Do they do so by sight or by smell? This apparently innocent question sparked off one of the most spectacular and vitriolic feuds in natural history. It started in 1825 when Charles Waterton, a choleric Yorkshire squire and an ardent naturalist, published an account of his explorations in the forest of Guyana. In it he reported, with no particular claim to originality, that turkey vultures there were attracted to carrion by smell. The book brought him national celebrity. The following year, however, another naturalist from the Americas, Jean-Jacques Audubon, appeared in Britain, bringing with him his spectacular drawings of American birds for which he was seeking a publisher. He too made a great impact on British society. He wore buckskin jackets, had hair down to his shoulders and told thrilling stories of adventures on the American frontier. Two such outspoken and extravagant characters both claiming expertise in the natural history of the New World could hardly fail to cross swords. Audubon picked on Waterton’s account of the turkey vulture and gave a lecture ‘exploding the opinion generally entertained of its extraordinary power of smelling’. Waterton took up the challenge and published a letter saying that Audubon’s claim was ‘lamentably false at almost every point’.

So battle was joined. Each side of the argument attracted followers. In America, college professors and lecturers joined together to sign an affidavit supporting Audubon. One supporter conducted an experiment in his back garden in which he presented a vulture with a painting of a dead sheep and a barrow load of fresh offal. The bird attacked the painting and ignored the offal, thus proving to the experimenter’s satisfaction that it relied on visual not olfactory stimuli. Waterton stuck to his guns and declared that Audubon should be horsewhipped for his ‘gross exaggerations and errors in ornithology’.

The controversy continued, almost unbelievably, throughout the rest of the century. Reading the arguments now, it seems that no one had allowed for the possibility that even the closely related black and turkey vultures could have different sensitivities. Audubon’s supporters seemed prepared to attribute any sense to any vulture as long as it was not an olfactory one. One man claimed that a vulture was brought down from high in the sky by sound – the buzzing of flies around carrion; another that it was the sight of small flesh-eating rodents converging on a corpse that caught the birds’ attention. Yet a third invoked an ‘occult sense’ which he explained disarmingly was so occult that there was no way that an experimenter could detect it.

While naturalists argued, more pragmatic people came to their own conclusions. In the 1930s, engineers in California were having trouble with locating leaks on a 50-kilometre stretch of pipeline carrying gas across rough country. Someone maintained that turkey vultures could not only smell but were particularly attracted by mercaptan, the pungent chemical put into commercial gas supplies which are otherwise odourless and therefore dangerous. The percentage of mercaptan in the pipeline gas was significantly increased and groups of vultures assembled wherever it escaped, showing the engineers exactly where repairs were needed.

Even so, ornithologists did not resolve the matter to their own satisfaction until 1964 when experiments were conducted with turkey vultures that rigorously and painstakingly excluded any other possibility of their being guided by any clue other than smell and demonstrated beyond any doubt that this is how turkey vultures are guided to their meals.

The turkey vulture is, nonetheless, exceptional. Its nostrils are larger and internally more complex than those of its relations and the section of its brain that interprets the signals conveyed from its nostrils is also considerably bigger. The black vulture, closely similar in appearance except that its naked head is black and not scarlet like the turkey vulture, has it seems no sense of smell. Neither have the other two South American species, the king vulture and the condor. Consequently, turkey vultures are nearly always the first to discover carrion. They feed as quickly as they can, but black vultures will soon notice them doing so. They arrive in force, usually greatly outnumbering the turkeys, and are so aggressive that the turkeys are pushed aside. They thrust their long scrawny necks in through the dead animal’s mouth and up its anus to rip out the entrails. But neither blacks nor turkeys have the strength to tear a hole in the hide of a large animal such as a cow. Before long, however, king vultures descend on the black scrum. As they land, the smaller vultures defer and allow them to rip the hide. Last of all come the huge condors. They strut towards the corpse shouldering aside smaller rivals. The carcass is torn apart, its entrails spill and the feast becomes a frenzy.

No African vulture has the power of smell to rival that of the turkey vulture. They do indeed rely primarily if not entirely on sight – and not only to spot a minuscule speck lying prostrate on the plains below. They also keep a sharp watch on the behaviour of other birds circling in other thermals. As soon as one tips sideways and starts to descend, the others realise that there is food to be had and glide across, either directly in a long shallow glide or indirectly from the top of one thermal to the bottom of another and up again until at last they come down on their target.

Just as there is an order in which different species feed on a carcass in South America, so there is in Africa. Griffon and white-backed vultures, are usually the first on a corpse. The biggest of the African vultures, the lappet-faced, does not usually arrive until the feast has already started, but it is so large and powerful that the smaller species give way to it. It struts towards the body with exaggerated steps, displaying its power and aggressive temper to lesser breeds and begins to rip at the hide. In fact, it prefers to eat the hide and sinews rather than the entrails. The smallest of the group, the hooded and the Egyptian, wait on the fringes of the feeding scrum, picking off scraps and waiting until the bigger birds have eaten their fill. They will do the final clearing up.

But African vultures do not necessarily have the carcass all to themselves. Hyenas also scavenge. They will have kept a sharp eye on the birds in the sky and noticed if a succession of them start to descend and land. The birds can glide down faster than the hyenas can run across the ground so they are likely to get there first. They eat so quickly and gluttonously, stuffing their capacious crops with gobbets of flesh and entrails that much of the carcass may have disappeared before the hyenas get to it. But its flesh may not be entirely lost to the hyenas. The vultures by now have eaten so much that they have real difficulty in getting off the ground. The hyenas chase them and if the birds fail to get into the air, they will disgorge some of their meat. The hyenas stop to grab it – and the vultures, with somewhat lighter loads, manage to fly up laboriously into the lower branches of the nearest tree.

Before long, little will be left of the kill except bones. The hyenas will finish these off, using their heavy grim jaws to crack them open and expose the succulent marrow. But in more temperate parts of Africa and in southern Europe, a bird is able to do even that. The lammergeier, the bearded vulture, positively prefers bones to meat. It pulls a skeleton apart and removes the long limb bones. They contain marrow and that is so rich that their diet, consisting of 70 per cent of marrow bones with a little meat and skin, is calculated to be 15 per cent richer in energy than one which consists entirely of meat.

The lammergeier does not have a cracking tool that can compare for power with the hyena’s jaws. It has another way of solving the problem. It picks up a bone and flies up high above a patch of bare rock. Then it drops it. It will do so as many as fifty times until eventually the bone hits bare rock in just the right way to split it. The lammergeier then picks up the splinters and, holding one vertically in its beak, takes it into its throat like a circus sword-swallower. The bone disappears downwards, for the bird has exceptionally powerful digestive acids in its stomach, and the bottom end is crumbling away, even as the top is entering its beak. It can totally digest a cow’s vertebra in two days.

There are not many sources of food left untapped by birds, either on the earth or above it.