Adaptations for Flight
Birds have evolved to fly and have many unique features in their compact and lightweight skeleton. The skeleton makes up only about 5% of their total weight.
Many bird bones are hollow and filled with air sacs, linked to the lungs. These hollow bones are strengthened inside by a thin criss-cross structure, like honeycomb; the wing bones are like this. Other bones, like the skull and pelvis, are thin and full of holes.
Birds do not have heavy teeth and the associated jaw muscles; instead food is torn up by the beak, swallowed in larger pieces and broken down in the stomach, by the muscles in the gizzard. This adaptation moves the muscles and weight to the bird's centre of gravity, away from the head. In fact, the bulk of the bird's body is in the centre, underneath the wings. This makes the bird more stable in flight; it is unlikely to roll over, upside down, if its weight is all below it.
Many bones have been fused together to create rigid structures, which are light but strong. Birds have fused bones in the pelvis, chest, wings and legs.
The breastbone in birds is distinctive, with a large keel-shaped blade projecting forward to act as an anchor for the enormous flight muscles, which are not situated on the wings, but below the body, in the chest.
The keel beneath the centre of the bird's body also helps provides stability in flight against conflicting air currents from the sides, like a keel which stabilises a boat in rough water.
There is very little flexibility along the bird's body from the start of the shoulder blades to the tail. This prevents the bird's body being pulled and twisted out of shape, during flight.
Bird skulls are made of light, delicate bone. You can see by the size of the eye sockets, that sight is their most important sense. The beak also dominates the skull.
In bird skulls, there is little room for the brain. However, it has been discovered that birds have many more brain cells, or 'neurons', than mammals do, in the part of the brain which deals with intelligence; so it may be that their brain cells are just more densely packed.
All birds have a ring of overlapping, vertical, bony plates which surround and protect the eyes. The eye ring is called the 'sclerotic ring'.
Dinosaurs also had sclerotic rings and snakes still do; a shared adaptation, connecting birds to their distant past and to their reptilian relatives.
Bird skulls are light and strengthened by struts of bone, as you can see in the photo, below.
The round hole at the base of the skull, the 'foramen magnum', is where the spinal cord goes into the brain case.The small ball, directly below the hole, is where the first neck vertebra joins onto the skull, with a ball and socket joint. (This is the ball.) This is what a Jackdaw's head spins on.
Birds have a tough layer of keratin around their upper and lower 'mandibles', or beak. Keratin is what our nails and hair are made from. It is also what feathers are made of. This sheath, called the 'rhampotheca', protects the bones at the tip of the beak, where they get the most wear and tear, from feeding, preening and nest building.
The holes in the top of the beak are called the 'nares' or nostril. This is where air passes into the bird's lungs.
Beak shapes and sizes vary enormously between different bird species.
Birds have a stiff tongue for moving food to the back of the mouth; they even have bones inside the tongue! They have no teeth. Instead, food is broken down and processed in the 'gizzard'.
Birds do not have ears, or flaps of skin and cartilage like mammals do, which lead to the ear canal. They just have small ear holes which lead directly to the inner ear. The ear holes are located immediately behind the eyes.
The sections of a bird's spine are examined more closely, in their appropriate regions, such as the rib cage and pelvis, but this is a general introduction to the spinal column.
Animals with a backbone, or 'spine', are called vertebrates. All birds, mammals, fish, amphibians and reptiles have a spine.
The spine is a line of small bones, called 'vertebrae', which link up to form the central column in the animal's skeleton. The spine connects at the top, to the skull and extends to the tip of the tail.
The spinal cord runs along a tunnel, called the 'neural canal'. It is formed inside the small vertebrae. Nerves branch out from the spinal cord, through gaps between the bones.
The shape of the vertebrae varies, depending on whether they are in the neck, chest, back, hips or tail. The different vertebrae are neck (cervical), chest (thoracic), lower back (lumbar), hip (pelvic) and tail (caudal).
The first type of vertebrae in the spine are the neck, or 'cervical' vertebrae. Different species of bird have different numbers of neck vertebrae.
The chest, or 'thoracic' vertebrae connect to the ribs. A large ridge of bone projects up from each of the thoracic vertebrae.
In birds, the spine then becomes very different to a mammal spine as all of the lower back or' lumbar' vertebrae, the hip or 'sacral' vertebrae and some of the tail or 'caudal' vertebrae are fused with the bones of the pelvis to form one of the largest bones in the skeleton, called the 'synsacrum'. This fusion takes away the flexibility which many other animals have, in the middle of their spine. The extra stability it provides helps to strengthen the bird's body against the pressures imposed by flight. It also keeps the bird's body streamlined in flight.
A few tail, or 'caudal' vertebrae extend beyond the fused bones in the pelvic region.
Birds have such large eyes, they cannot move their eyeballs within the sockets. To compensate for this, they have evolved long, flexible necks, so they can twist their neck, instead.
Birds also use the beak for preening and manipulating food, which is necessary as they cannot use their forearms, which have evolved into wings.
Birds have surprisingly long necks which are usually tucked away and not visible beneath all their feathers!
Different species of bird have different numbers of neck bones, between thirteen and twenty five; humans have just seven.
The bones in the neck are called cervical' vertebrae. In mammals, the neck vertebrae are tightly stacked and have projections, called 'processes', which extend out to the side; these restrict the range of movement and make the mammal neck stronger.
In birds, the shape of the cervical vertebrae allows much more movement and the processes are much smaller; these two factors give the neck much greater flexibility.
The first vertebra below the skull is the 'atlas'. It is a ring-shaped bone, which allows the head to nod.
The second vertebra is the 'axis'. It has a ball shape on which the skull can swivel. The atlas, which lies above the axis, has a socket which this ball slots into.
The shape of the remaining cervical vertebrae below the atlas and axis, opens up, further down the neck.
The main photo shows the pectoral girdle of a Woodcock.
The pectoral girdle fits around the rib cage and anchors the bird's wings to the central core of the body.
The bones which form the 'pectoral girdle' are crucial when it comes to flight. The girdle consists of four bones: the breastbone, or 'sternum'; the two vertical, pillar-like 'coracoid' bones; the two horizontal, elongated shoulder blades or 'scapulae'; and the fused collar bones, the 'furcula'.
The first major bone on the wing, the 'humerus', connects to this girdle of bones, at the point where the coracoid, scapula and furcula meet.
The breastbone or 'sternum'. In birds which can fly, the breastbone is enlarged with a bony projection, shaped like the keel of a boat; this is where the flight muscles attach.
The depth of the keel varies in different species of bird; it usually indicates how strong a flier, the bird is. Birds which migrate long distances have an extended keel and birds which rarely fly like the pheasant or chicken, have a much flatter breastbone. Bats also have a keeled sternum.
The two 'coracoid' bones are like pillars, linking the sternum to the shoulder blades. This is an extra bone in birds, which braces the shoulder blades against the stresses of flight. They prevent the chest being squeezed, as the wings move up and down.
The long shoulder blade, or 'scapula', forms a large surface area on each side, for the shoulder muscles to attach onto. They lie back along the top of the ribcage, parallel with the vertebrae. They glide over the surface of the ribcage, as the bird's wings move up and down in flight.
The collar bones, or 'clavicles' are fused to form a U shaped bone, called the 'furcula' in front of the rib cage. This is the 'wishbone' in a roast chicken! The breast muscles attach here. As a bird flies, the furcula acts like a spring, flexing in and out, reacting to the flapping of the wings. When the wings come down, the furcula expands outwards and when the wings rise, the furcula springs back into its normal position.
A tiny canal runs through the point where the scapula joins the coracoid bone. The large muscle which is used to raise the wings in flight, lies on the breastbone below the wings. A tendon connected to this muscle, runs through this canal; it then attaches to the humerus, the first bone of the wing. The tendon acts like a pulley and enables the humerus to raise the wing, using muscles which are beneath it. Birds have the mass of their flight muscles located centrally, below their body, not spread out along the wings. This helps stabilise the body in flight.
The main photo shows the rib cage of a Song thrush.
Birds have a short, deep rib cage. They have seven pairs of ribs.
The first two pairs are tiny and attach to the bottom two cervical vertebrae. They are called 'cervical' ribs. They are floating ribs and do not connect with the 'sternum', or breastbone.
Each of the next five ribs attaches to the chest, or 'thoracic' vertebrae. These ribs have two sections: the 'vertebral rib' above, joined to the spine, and the 'sternal rib' below, connected to the sternum or breastbone. The final rib does not attach to the breastbone.
In birds, the five central ribs each has a small flap of bone which overlaps onto the next one; these are called 'uncinate processes'. These assist with expanding the ribcage for breathing and strengthen it against the rigours of flight when the wings beat up and down.
A vertical ridge of bone projects up from each of the thoracic vertebrae. This forms a ridge along the back of the body. This is called the 'neural spine'. Back muscles attach here. The chest or 'thoracic' vertebrae are fused together, in some bird species such as the wood pigeon.
As you saw in the previous section, six of the tail or 'caudal' vertebrae are fused into the synsacrum. Five or six caudal vertebrae continue out from the synsacrum, as individual bones.
Finally, at the tip of the tail is a flat blade of bone called the 'pygostyle'. This is a fusion of the last four caudal vertebrae. The muscles which attach to it, help the bird to steer, take off and land, by adjustments of the tail.
The spinal cord runs all the way along the spine from the skull to the tail. It is enclosed inside the central tunnel, the 'neural canal', formed by the vertebrae. See the tiny round hole in the photo below, which is almost at the tip of a House sparrow's tail; the spinal cord would have run through this hole.
The tail feathers on the left, are from a Magpie. They are iridescent when they catch the light!
Birds' wing bones have a similar structure to the human arm and hand. When not in flight, the wings fold up over the sides of the bird's body into three, in line with these three sections.
The first bone is a long upper arm bone, the 'humerus'. The major flight muscles on the breast attach to the humerus, which drives the wings.
There are two slimmer, forearm bones, the 'radius and ulna', which can slide over each other, as ours do. This enables a bird to twist its wing for steering, during flight. The mid wing, or 'secondary' flight feathers attach onto the ulna, the broader of the two bones. You can see slight bumps along the ulna, which is where the feathers attach.
Birds have two small wrist bones between the forearm and bones at the wing tip. See photo, below.
The wrist and hand have fused into a blade-like bone called the 'carpometacarpus', which has an oval window in it. These bones carry the the main flight feathers, called the 'primaries'.
The thumb bones have become one small bone which projects out at the base of the 'carpometacarpus'. (Above the 'carpometacarpus', in this picture.) This thumb bone supports the feathers of the 'alula', or 'false wing''; it is raised when a bird comes in to land, to prevent it stalling, like the flaps which are raised on aeroplane wings.
The bones of the second and third fingers have fused into one at the very tip of the wing. The bones of the fourth and fifth fingers have been lost completely, as bird skeletons have evolved.
The joint between the carpometacarpus and the radius and ulna, (which is like the wrist), allows a considerable range of movement, so that the bird can fold this outer section of the wing, back underneath the second section, close in to the body, when not in flight. (We cannot bend our hands back, underneath our forearms.)
The relative lengths of the three sections of the wing, vary for different birds according to their flight style and how far they migrate.
Legs & Feet
Like most vertebrates, birds have a long thigh bone or 'femur', which connects to the pelvis or hip, with a ball and socket joint.
Below that the lower leg bone, the 'tibia' has fused with some of the foot bones to create the 'tibiotarsus'. The 'fibula' is like a thin pin, running alongside this bone.
The bones of the lower foot have fused to form another medium-length bone, the 'tarsometarsus', which appears to give a bird's leg, three sections.
The extended, third section of the leg helps birds take off and land. It also gives extra leverage when they jump forward, or paddle in the water.
In birds, the knee joint is usually covered in feathers and so is rarely seen. This makes it look as if a bird's knee bends backwards, but in fact the leg joint, which is visible below the body and feathers, is the ankle or heel.
Birds actually walk on their toes, not on their feet. This is described as being 'digitigrade'; they stand on their toes with their ankles in the air. They generally have four toes, three forward-facing and one, equivalent to our thumb, backward-facing.
The skin on the toes is scaly, offering protection against the wear and tear of walking, feeding and landing on branches.
The feet vary according to the habitat, lifestyle and diet of the bird.
The Collector, Photographer & Author
I am Susanna Ramsey and I have a unique collection of natural history objects relating to British Wildlife. Over the last ten years, I have assembled an extensive range of skulls, skeletons, bones, skins, feathers, wings, antlers, insect specimens and taxidermy, all from animals in the UK.
During 2010-2018, I took my Nature Collection into local primary schools to display and run workshops for the children, linking the exhibition to science topics in the National Curriculum such as Adaptations, Bones, Classification, Food Webs, Habitats, Life Cycles and Local Wildlife.
In 2018-2020, I worked with the leading schools' catalogue, TTS ,to create a range of Educational Resources for primary schools, nurseries, after school clubs and families. To find out more about these products, click here or see below.
In the school workshops, children and teachers were always completely fascinated to see what is inside the animals we see everyday, in the garden or local parks. On these web pages, I want to continue to share my enthusiasm for the skeletons of our local wildlife. I hope you find these intricate structures as beautiful and amazing, as I do.
The more I examine the bones in my Collection, the more amazed I am at the miracle of every living creature. Each bone is so intricately-shaped and different to the rest; each fits perfectly in just one place on the skeleton like the overlapping tiers of feathers on a bird's wing. The skeleton provides the framework for the entire body from the tip of the nose to the tip of the tail and out along the limbs, to the tips of the claws. The joints allow the parts of the body to move in different directions so that each animal can do everything it needs to.
When you interweave through this complex skeleton, the network of muscles, tendons, veins, arteries, nerves and sense organs, plus the heart, brain, digestive and respiratory systems, waste and reproductive organs, all fully functioning, you can't help but be in awe.
The miracle too, is that every different species is a variation on a theme, with the same basic skeletal structure and individual bones, each evolved into a slightly different shape and size.
Among my favourite items in the Collection are the fragile rib cages of birds and small mammals. They are so delicate, yet despite death and cleaning by flesh-eating beetles, they still form a rigid, basket-like structure, sufficient to protect a beating heart and lungs. The rib cages of a Field mouse and a Robin are so similar, yet the animals live and move so differently! It is fascinating to compare the skeletons of birds and mammals and to discover how each species has adapted to its own habitat and way of life.
BRITISH WILDLIFE PRODUCTS
If you know children who are interested in nature, are a teacher, or would like to learn more about British Wildlife yourself, explore the range of British Wildlife products recently created by The Nature Collection and the leading schools' catalogue, TTS.
The Classification: Natural History pack features 40 small photos of animal skeletons, skulls, feathers, insect specimens and much more, all from The Nature Collection!
The photo opposite shows some of the the Blue Tit pack, in the Birds Discovery Bags.
The products are perfect for use in primary schools, nurseries, after school clubs, forest schools or at home with friends and family. Click on the links below to find out about each product.