By Dr. BSc, BVSc, MRCVS, MACVSc(Avian health)

The natural world contains many wonders. Some can be difficult to see or experience. Imagine holding the tusk of a full grown bull elephant, or perhaps swimming with a whale shark or looking at the huge eyeball, the size of a basketball, of a deep sea squid. They would all be great experiences. Some wonders however are a bit more accessible. Incredibly , if you would like to see a dinosaur using its feathers to fly, all you have to do is go outside and look at a bird.

The evolution of birds is one of nature’s great success stories. The evolutionary line that went on to develop into birds and reptiles split off from the line that developed into mammals over 200 million years ago. Birds continued to evolve into a remarkable 9,000 species, much more than the approximate 4,400 mammals and 6,000 reptile species, yet despite this remarkable species diversity, there is a great morphological similarity in all birds. There is no mistaking a bird. It’s a winning design. There is less variation across all birds than there is in some quite small mammal groups that to us seem very similar. For example, there is more variation in the approximate 90 species of primate than there is in all birds.

Birds have colonized practically all global habitats. They range in size from the 1.6 gram Cuban Bee Hummingbird found hovering before flowers in the South American rainforest through to the 150kg African ostrich that speeds across the African veldt at 100km/h. Penguins thrive in sub-zero temperatures and ‘fly’ underwater, with some species (the Gentoo) holding their breath for up to 30 minutes. Some species of swifts and albatrosses sleep and mate in the air and are said to essentially come to earth to lay eggs and die. In some of these species the legs have become so reduced that they must throw themselves off an elevated point such as a cliff to become airborne. Albatrosses can circle the globe without landing. Some species of albatross and vulture have become so large and their weight/lift ratio so marginal that in an emergency they will vomit to reduce their weight in order to get off the ground. The South American Hoetzin, in early life, has claws at the end of its first and second digits which enable it to scramble through foliage while its crop can ferment plant material like the rumen of a cow.

Truly a remarkable group that have colonized the world. There are currently thought to be more than 100 billion birds on Earth.

By far the main characteristic that is associated with birds is their ability to fly. The entire forelimb has been modified to become a wing. This came at a cost. The forelimb could no longer be used to support weight, to restrain prey or for locomotion, but the ability to fly gave tremendous advantages. It gave the birds’ easy access to the trees and also to the sky – a huge uncontested ecosystem. They were no longer restrained by geographical barriers and could follow seasonal food sources. They could also more easily escape predation. The advantage that flight gives is also displayed in mammals. Bats are the second largest mammal group with approximately 1,000 species. The only larger group is the rodents. The only other vertebrates apart from birds and bats ever to have flown are the extinct pterosaurs.

Avian flight is remarkable for its endurance and efficiency. Arctic terms during their migration fly from pole to pole, a distance of 45,000km. American Golden Plovers fly 3,500km non-stop during their migration. The Australian Short tailed Shearwater flies from Australia to Japan at only a few months old. Planes have collided with mallard ducks at 12km high (a height at which humans can barely survive due to hypoxia).

Flapping flight is very expensive in its absolute demand for energy but because of its speed is still the most efficient way to get from point A to point B. Some birds are remarkable in their control of flight. Incredibly about 50 muscles are associated with wing movement. Hummingbirds can fly backwards and some birds can fly fast, very fast. Peregrine falcons can fly at nearly 300km/h. Even small finches can fly at 25-65km/h which is equivalent to 100 body lengths per second (about the same as a jet). To fly requires superior vision and motor coordination. This is reflected in the enlarged brain of birds with its big optic lobes and cerebellum.

Birds make flight look easy. However wing movements impart extremely high stress and strain on the avian skeletal frame and so as the ability to fly evolved, modifications had to be made to the bird’s body to make this truly efficient. Some bones became smaller and were lost while others became fused. Huge breast or flight muscles evolved to pull the wings down through the air and lift the bird’s body. In pigeons, 50% of the bird’s body weight is the flight muscles. The bones of the front of the chest formed into a strong bony plate called the sternum with a strong central ridge called the carina or keel for these muscles to attach. The anterior vertebrae in the chest fused into a single bone called the notarium. Without this the downward beat of the wing would simply set up a ripple effect along the spine. The spine becoming fused into a single structure meant the downward beat of the wing lifted the body as a whole. Also the bones holding the wings onto the body had to be reinforced. If you have ever held the body of a bird flapping its wings you will appreciate the tremendous forces generated here. Without re-enforcement the wing could almost be torn off the body. Where mammals have a single shoulder blade for each forelimb, birds have developed a second bone called the coracoid that acts a bit like a second shoulder blade and provides additional support to the shoulder and wing. In many birds the collar bones [ clavicles ] have become fused into a structure called the furcula (or ‘wishbone’). This provides additional rigidity and support. Also small processes are found on birds ribs. Called the uncinate processes they join one rib to another and help prevent the rib cage collapsing during the downbeat.

The structure of the bones themselves also had to change. Bones needed to be light and elastic and yet strong. Some bones became what is called pneumatized – here extensions of the airsacs (themselves extensions of the lungs) extended into the bones replacing marrow with lighter air filled bags. In albatrosses the bones, except those in the legs ,are thinner than paper and have even developed internal struts that act as supports to stop them collapsing. In Jabirus (Australian black necked Stork) even the toe bones are pneumatized.

The transformation of the forelimb into a wing –capable of lifting its owner into the air and giving the power of flight is one of nature’s miracles. As mentioned earlier, mammals diverged from birds (and reptiles) about 200 million years ago. In both birds and bats the wings are modified forelimbs. The primate forelimb developed into a hand. A bats wing is essentially a primate hand with the fingers stretched apart and then covered by fine skin. In the avian forelimb some bones fused, some elongated and others were lost but the common ancestry is still readily discernable. The avian wing is like a pointing human hand.

A bird is not a mammal with wing. Quite apart from modifications directly associated with the ability to fly other changes to the body had to occur to cope with this new ability. Because the forelimb was just used for flight, the neck, head and beak had to take over various functions. The neck became long to assist with grooming. With few exceptions most mammals (even giraffes) only have 7 neck vertebrae. Birds have up to 25. Vertebrae in the chest became neck vertebrae. In the chest these bones had ribs associated with them. Even today the lower neck bones of birds have tiny little bumps on each side of them that are the remnants of these ribs. They are called cervical ribs. Because the forelimbs could not be used to restrain or tear prey and many species had specialized beaks, food often had to be swallowed whole. To do this the jaw had to open wide. In mammals, like us, the top jaw is part of the skull and cannot be moved independently. Birds developed a joint between the top jaw and the skull. Also in birds the lower jaw is made up of three bones. The smaller vertical part of our lower jaw is made up of two separate bones in birds (the quadrate and pterygoid bones) each with their own joint. The result is increased flexibility and function.

As wings evolved and became non-weight bearing this meant that the whole of the birds weight had to be supported on just two legs. Particularly in big birds that have to run to take off the amount of force generated is significant. A series of amazing changes occurred to compensate for this. The vertebral bones of the lower back, pelvis and anterior tail fused into a single strong bone called the synsacrum. This bone then fused to a part of the pelvis called the illium. To add additional support birds also have a secondary ‘hip’. Like us, birds have a small ball at the top of the thigh bone that fits into the hip socket. Just behind this ball is a bony knob called the trochanter. This fits into a second small depression in the pelvis just behind the main hip joint. Called the anti-trochanter it increases support to the main hip, even enabling birds to comfortably stand on one leg.

Balance was also an issue. It’s not easy to balance on two legs. Except for penguins that stand like us, all birds lean forward. To allow for this birds have developed long splayed toes, one or two of which point backwards. A number of other fascinating changes have also occurred. As in the wings, some bones fused, others elongated and some were lost. Birds don’t have ankles. The lower bones of the ankle (tarsus) have fused to those of the foot (metatarsal bones) to form a new bone called the tarsometatarsus while the upper bones of the ankle have fused to the shin (tibia) to form another new bone, the tibiotarsus. Both the tarsometatarsus and tibiotarsus have increased in length and rotated forward. This and the fact that the legs are held in a slightly bent position (like a human skier) means that the knees are located directly below the birds centre of gravity in-line with the heavy flight muscles and abdominal organs. With these modifications a bird actually expends little energy in a standing position.

Feathers are uniquely avian, all birds have them and yet no other creatures do. The story of their development and how this development was discovered is fascinating. For some time it was thought that feathers evolved first in small scurrying four legged reptiles leaping from tree to tree that initially started to glide and then fly as feather development occurred. However, in 1970 researchers noticed the similarities in the skeletons of theropods (meat eating dinosaurs, the same shape as and including T. rex) and birds. As recently as the mid 1990’s the initial fossil theropods with feathers were found. Hundreds have now been found representing a number of species. The gradual transition of forms leaves no doubt that these animals are the ancestors of birds. In many cases their feathers are fairly primitive, being little more than filaments. And so how did the long hollow filaments on theropods evolve from the flat scales of reptiles. Fortunately there are theropod like animals with thread like feathers alive today – baby birds. The feathers in a baby bird begin as bristles and only later develop into more complex shapes. In a bird embryo these bristles arise from tiny patches of skin cells called placodes. A ring of fast growing cells develop on top of the placode that builds a cylindrical wall to become a bristle. Reptiles have placodes too but here only half of the circle is activated so that instead of building up a hollow tube that becomes a feather, a flat plaque of tissue that becomes a scale forms. It appears that the difference between a feather and a scale is not so great after all and may only depend on a few genes activating parts of the placode ring. Up until the last few years it was thought that feathers originally evolved in a lineage of early theropods that eventually lead to birds. As recently as 2009, feathers have been found on even more primitive dinosaurs. This raises the possibility that the ancestors of all dinosaurs had hair like feathers and that some species lost them later in evolution. In those that kept them some went onto give rise to birds.

There is still continuing debate as to whether feathers initially evolved for flight or for other purposes such as display, camouflage or insulation. However what is apparent is that in certain lines of dinosaurs as feathers increased in length and developed a more sophisticated structure, including asymmetrical vanes, their owners were able to lift into the air and fly.

One fascinating point is that only in the last two years it has become possible to determine the colour, in real life, of fossilized feathers. Fossilised feathers contain microscopic sacs, called melanosomes, that correspond precisely in shape to structures associated with specific colours in the feathers of living birds. The melanosomes are so well preserved that scientists can actually reconstruct the colour of dinosaur feathers. Some fossilized feathers have been shown to be shades of grey, brown, white and red.

In addition to the evolution of feathers, remodelling of the skeleton and the development of flight many, many other changes have occurred to make birds the truly unique, mind-boggling creatures they are today –

  1. Birds have two stomachs. Digestion is very efficient in birds. This helps to explain why, if they swallow a metal particle, heavy metal poisoning is so common in them. Food moves into the first or non-glandular stomach (proventriculus) where it mixes with acids and digestive enzymes before passing into the second or glandular stomach (ventriculus) where it is physically ground up.
  2. Birds almost have six kidneys – each of two kidneys is divided into three large divisions. Each division is in turn divided into lobules that contain both mammal and reptile components. Birds therefore produce two types of urine a clear liquid, like a mammal, and a white paste, uric acid, like a reptile.
  3. Birds have one external opening, the vent, for both the elimination of wastes and reproduction. The vent has a bag inside called the cloaca into which the bowel, the ureters {from the kidneys} and either the oviduct or vas deferens { from the testis } open. Both urine and faeces collect in this. When the cloaca empties, both are passed together.
  4. Birds do not have a urinary bladder – urine travels directly to the cloaca where liquid can be resorbed back into the body through the cloacal wall or even reflux into the lower bowel and be resorbed there. Unlike mammals, birds can therefore alter the concentration of urine after it is produced.
  5. Bird testes are carried inside the body but are still kept cool by air in the surrounding airsacs. Testis can increase and decrease in size up to 50 times quite normally depending on the season and birds health.
  6. Birds lay eggs. The only other animals to do this are the monotremes- the echidna and platypus.
  7. Birds can regenerate central nervous system tissue. If a mammals brain or spinal cord is damaged, this damage is permanent. Clinical improvement occurs if adjacent nervous tissue takes on the function of the lost area. In birds the damaged area can grow back.
  8. Birds have no external ear flap (pinna) and only one internal ear bone or ossicle called the columella. Mammals have three.
  9. Birds have generally larger hearts and faster heart rates than mammals. On average most small birds breathe about 100 times a minute and have heart rates of about 1000beats/minute. As a general rule smaller birds have higher metabolic rates and faster heart rates than bigger birds. Blood pressure in birds is higher than in mammals of similar size. Avian blood is similar to mammalian blood except the red blood cells have nuclei.
  10. Birds have airsacs. Airsacs are fine, balloon like bags that extend from the lungs and fill most of the abdominal cavity and may even extend into some bones. Reptiles and insects also have airsacs and in fact those in the chameleon are the most similar to birds. Airsacs make the birds’ body light for its size, helping with flight and also through an amazing two respiratory cycle ‘shunting’ mechanism deliver fresh air to the lungs during both inspiration and expiration. Respiration is very efficient in birds. 60% of inspired oxygen is removed from the blood compared with only about 27% in humans. This helps birds fly at altitudes where a human would die. Unlike mammals, birds do not have a fully developed diaphragm but must breathe by expanding their ribs (this is why a bird will die if held too tightly around its chest). Also the cartilage rings in the windpipes of birds are complete. Different species have specialised adaptations, for example, in diving birds like penguins and petrels there is a small median support strut in their windpipe to stop it collapsing underwater.
  11. Most birds do not have a penis. Most water birds however do.
  12. Birds eyes are very special – because birds travel at speed and often hunt for small objects like seeds and insects, birds need to have great vision. They need to be able to detect movement and avoid collisions. Birds can see colour. They can see in bright and dim light and even underwater. In some species the combined weight of the eyes is bigger than that of the brain. The avian eye itself has a number of special features. 11-15 overlapping bony plates are embedded in the sclera (the white part of our eye) around the cornea to protect the eye from injury. The outer surface of the lens is covered in a protective coating called the annular pad. This is in turn attached to the lining of the eye. Birds focus probably by a combination of altering their lens shape (like us) but also by moving the lens backward and forward within the orbit via this annular pad. A small fleshy knob (approximately 2mm wide in the budgerigar) is found in the back of birds eyes and helps to nourish the retina (the membrane that gives sight). The beautiful colour of many birds’ eyes comes from a combination of pigments and fat droplets in the iris. When birds get sick and lose weight these fat droplets resorb which is why the eyes of sick birds lose their vibrancy. The whole of the birds eye can be covered by a third eyelid called the nictitating membrane that protects the eye and releases secretions that lubricate and cleanse the eye. Special glands on the edge of the eyelids release a mucus that makes tears stick to the eyeball surface. This is why birds can fly fast in dry air with their eyes wide open.
  13. Birds are clever. The cleverest parrots have been said to be as clever as the cleverest dog. Dogs have been shown to know the meaning of perhaps 5 words such as walk, ball, etc. Some African Grey parrots are thought to know the meaning of approximately 80 words and they can say them too!
  14. Birds have a beak but do not have teeth. They therefore don’t need strong jaws or chewing muscles. This helps keep them light for flight.
  15. Birds do not have sweat glands and in fact have very few glands in the skin. Some birds have a uropygial or preen gland near the base of the tail. A few glands are found around the entrance to the ear and a few oil producing glands are scattered over the body. Because birds don’t have sweat glands, they cool themselves by evaporating moisture from the lining of their airsacs or by exposing non-feathered parts of their body. This is why hot birds pant and may extend their wings exposing areas of skin.
  16. Birds have comparatively few taste buds.
  17. Many birds (not ratites) have a crop. This is a distensible bag located about halfway between the throat and stomach. It is designed to store food, particularly if eaten quickly, that can then be moved into the stomach and digested at leisure in a safe place.
  18. Birds have fast metabolic rates. This means they heal quickly and metabolise veterinary medicines more quickly than mammals. It also means they need to take in more energy by eating more food and more often. They also generate more heat. Their average body temperature is 41.7C compared to 37.8C for humans.

It sometimes seems that the more we learn about birds the more fascinating they become. The story of how birds became what they are today has been a long one perhaps starting 200million years ago when the line of living things that was to become the mammals diverged from the line which was to become the reptiles and birds. 50million years later, 150million years ago the line that was to become the reptiles in turn split off from the developing birds. The birds then continued to evolve in their own line. Modern wing design had been perfected by about 125million years ago and fully modern birds began to appear about 70 million years ago, or put another way, 700,000 centuries, a very long time ago. The end result is a unique set of creatures, the birds we know today, whose dinosaur origins and anatomical and physiological links with both the mammals and reptiles are readily apparent. This amazing blend of characteristics has enabled them to become the most ubiquitous vertebrates on earth.