The bird within the animal kingdom

The animals in this group consist of many cells with a division of labour: one cell catches food, another anchors the body, a third supplies the sex cells. Between the cells sits a protein glue — collagen — and around them lies a shared outer skin, so that the whole behaves as one body. A body can therefore be far larger than a cell, and can have a quite different shape inside than out.

The body has a long axis, with a front end that goes first and a rear end that follows. Sense organs and nerve cells are piled up at that front end into something worth calling a head, and the gut runs all the way through from mouth to anus, so that eating and digesting happen at the same time. Between the outer layer and the gut lies a third germ layer, the mesoderm, and out of it muscle, skeleton and blood vessels are built.

Along the length of the body lies the notochord: a firm but flexible rod of fluid-filled cells inside a stiff sheath. Muscles pulling on either side of that rod do not simply fold the body double but make it wave from side to side like a whip, because the rod springs back every time. Swimming is therefore directed and fast instead of merely wriggling, and directly above the rod runs the nerve cord that sets the rhythm.

Around the notochord lie separate elements of cartilage or bone, linked up into a vertebral column: stronger than the rod alone, and with movable joints between the segments. At the front a skull closes around the brain, and the embryo has a tissue of its own, the neural crest, which supplies jaws, teeth and facial bones. The body thus has an internal frame that grows along with it, instead of an armour that has to be replaced again and again.

The foremost gill arches — cartilage hoops in the throat basket — do not stand upright here but tilt forward, and form hinged jaws. An upper and a lower jaw close against each other, carrying teeth made of the same tissue as the skin scales. An animal that can bite need not wait for whatever drifts past: it seizes prey, holds it and cuts it up, and hunting is therefore a way of life.

Inside the fleshy pectoral fin the bones lie in a fixed sequence: one bone against the shoulder, then two side by side behind it, and only then the fan of fin rays. That single bone is the humerus, the two behind it the radius and ulna — a bird's wing, a human arm. The head of the humerus is also convex and turns in a socket of the shoulder girdle, so the fin can prop and push instead of merely rowing.

At the end of the limb there is no fan of fin rays but a row of separate finger bones with joints between them, letting the animal push off against the ground. The shoulder girdle does not touch the skull, so there is a neck and the head can turn without the whole body turning with it. The ribs are heavy and hold the chest open, because out of the water a body presses down on its own lungs.

Three membranes lie around the embryo: the amnion with the fluid it floats in, the allantois that stores waste and handles breathing, and the chorion that encloses the whole. Around that is a leathery or chalky shell that keeps water in but lets oxygen through. Reproduction is therefore not tied to pond or pool, and the dry interior — dunes, steppe, high mountains — lies within reach as well.

Behind the eye socket sits a pair of windows in the skull roof on each side, with bony bars between them. The jaw muscles can bulge out through those openings as they contract, so that thicker muscles fit than the volume of the skull alone allows. The bars between the windows take up the pull: a light skull that bites hard and can gape wide.

In front of the eye socket sits another window, and one in the lower jaw as well: holes that keep the bone light without weakening it and that leave room for muscle and air spaces. The teeth stand deep in sockets of their own in the jaw bone instead of being fused to its rim, so they hold up to a struggle with prey that fights back. On the thigh bone sits a ridge where the great tail muscle takes hold, and that muscle pulls the leg powerfully back with every stride.

In the hip the socket that carries the head of the thigh bone is an open hole, and the head of the thigh bone itself is turned inward at a right angle to enter it from the side. The leg therefore does not sprawl sideways as in a lizard but stands straight under the trunk, so that weight runs down through the bones instead of being held up by muscle. Walking costs less effort, the stride is longer, and a body can be far heavier without collapsing.

Openings sit in the vertebrae, and further along in the ribs, the breastbone and the upper arm bone too, through which outgrowths of the lungs reach inwards; there is no marrow left there. What stands is a thin-walled tube of bone with air spaces inside it: as strong as solid bone, but so much lighter that a large animal can keep standing on two legs. Those outgrowths are the air sacs, and they make the lung not a bellows that breathes in and out but a pipe that air is drawn through in one direction — the flow-through breathing with which a goose crosses the Himalaya.

Hollow, flexible threads of keratin come out of the skin, branched at the base and packed so closely that they make a coat of down. They carry nothing and steer nothing: they hold body heat in and they give colour, and in Sinosauropteryx even the pigment granules of a chestnut-ringed tail have been recovered. Simple skin filaments may sit in wider groups as well, as far out as the pterosaurs, but the animals in this group unmistakably have feathers.

In the wrist two carpal bones sit fused into a single half-moon-shaped element with a smooth, curved rolling surface on top. The hand can therefore barely bend up and down, but it can snap sideways at great speed, in towards the body — exactly the movement with which an animal with outspread arms catches a struggling prey between its hands. In a bird that same half-moon bone is the wrist that folds the wing against the flank, and it is the same sideways sweep that throws the hand-wing forward on every stroke.

The down filaments here sit on a single firm central shaft, and the side branches lock together with thousands of tiny hooks into a closed, springy surface. A quill feather of that kind is stiff enough to hold air back and light enough to carry dozens of them side by side; they stand in a row along the trailing edge of the arm and in a fan on either side of the tail. The animals in this group do not fly with them — the feathers serve for display, for covering a clutch, perhaps for grip while running up a slope — but the lifting surface is there.

The forelimb is longer than the hind limb, which is the case nowhere in the wider groups above, and on the hand and forearm the feathers are asymmetrical: a narrow vane along the leading edge, a broad one behind. That is exactly the cross-section that speeds air over the upper surface and so pulls at it, and dozens of such flight feathers side by side make one closed lifting surface. With it the animal can glide and soar, and because the arms can also beat down hard it pushes itself forward: the same grasping arm serves here as a wing.

The bony tail is short: a handful of vertebrae fused into a single pygostyle, on which the tail feathers can swivel, spread and close like a fan instead of trailing stiffly behind. There are no teeth in the jaws; the bone around them carries a covering of horn: a bill weighs a fraction of a set of teeth, and at the front of the body every gram counts double. The breastbone has a tall ridge, the keel, on which the two flight muscles are anchored that together make up a fifth of a bird's weight — pygostyle and keel are there in wider groups along the stem as well, so it is only the lack of teeth that sets the animals in this group apart.

In the palate the bones are broad and largely fused, with a heavy vomer holding everything in place down the middle: the upper bill can barely move against the skull. In the ratites something more drastic comes with it: the breastbone carries no keel and is a flat raft — ratis, raft, is exactly where the word ratite comes from. Without a keel there is nothing to attach the flight muscles to, and those are duly absent; the skeleton does not have to be light, so the bones are thicker-walled and heavier instead, solid enough to carry an ostrich at fifty kilometres an hour.

The vomer is no more than a thin spike and the palate bones lie loose, linked by a series of slender rods and small joints. When the lower jaw opens, the quadrate pushes those rods forward, the palate slides with them, and the upper bill tilts up around a thin hinge at the forehead: the beak opens at both ends at once. Small differences in the length and angle of those same rods yield tweezers, a chisel, a sieve or a nutcracker, and much of the sheer variety of birds rests on that.

The flamingo turns its head over to feed, so that the upper bill comes to lie underneath and the kink in the bill cuts horizontally through the water. Along the edges of both jaws run rows of thin horny plates, the lamellae, which mesh like the teeth of two combs and make a sieve with gaps a few tenths of a millimetre wide. The tongue — thick, fleshy and set with spines — slides back and forth like a piston in the trough of the lower jaw several times a second, pumping water in through that sieve and out again while brine shrimps and diatoms stay behind.

In the crop, the storage pouch at the foot of the gullet, the lining of a pigeon starts to thicken and fill with fat in the last days before the eggs hatch. The shed cells make a thick, yellow-white paste that the young take from the parent's throat: crop milk, almost a third protein and almost a third fat, far richer than mammal milk and without a trace of sugar. Both parents produce it, driven by the same hormone, prolactin, that switches on a mammal's milk gland, and a young pigeon eats nothing else in its first week.

In a hummingbird the upper arm and the forearm are no more than stumps while the hand bones and above all the flight feathers are extravagantly long: almost the whole wing is hand, and that hand stays stretched out in flight as one stiff blade. The shoulder joint turns so far that the wing can flip over on the upstroke, so that it pushes air down on the way back as well instead of merely springing back. The wingtip therefore traces a figure of eight and every half stroke yields lift — the bird hangs motionless in the air, slides sideways, and is the only group of birds that truly flies backwards.

In a penguin the bones of arm and hand are flattened into slats and the elbow and wrist can hardly bend at all: the wing is a single stiff, solid paddle that hinges only at the shoulder. The bone is not filled with air but dense and heavy, so that the animal is not pushed back to the surface but can hold its depth. The feathers are short, stiff and curved and lie so close together that they look more like scales than feathers; underneath them is a layer of down that traps the air in which the bird stays warm — and the same stroke that drives a swift through the air drives a penguin along twenty metres down.

In the woodpeckers and in the parrots the fourth toe points backwards, so that the foot has two toes forward and two back: zygodactyly, a pair of pincers instead of the usual three-against-one. A woodpecker clamps itself to a vertical trunk with it and, with the stiff tail feathers as a third point of support, holds on so firmly that it can drive its head into the wood twenty times a second. A parrot uses the same pincers to pluck a nut from the branch and carry it to the bill, which makes it one of the few birds that eat with a hand.

The syrinx does not sit high in the throat but deep in the chest, right at the fork where the windpipe divides into the two bronchi, and each bronchus holds a vibrating membrane of its own. In the songbirds that organ is strung with a set of small muscles — counts differ, but around seven pairs is usual — that can tension and retune each half separately. Left and right are under separate control, so that one bird can produce two notes at once, run them against each other or alternate them without pausing for breath: that is what a blackbird does and a chicken does not.