The African spurred tortoise lives at the southern edge of the desert.
The third largest tortoise in the world is this one.
By the end of this section, you will be able to describe the evolutionary history of birds and describe the derived characteristics in birds that facilitate flight.
Research shows that birds evolved from dinosaurs about 150 million years ago.
Even though the presence of feathers seems to set birds apart, we now know that feathers were common in the ancestors of dinosaurs.
According to the most recent research, feathers in these clades are related to reptilian scales and mammalian hair.
Birds rely on feathers and wings, along with other modifications of body structure and physiology, for flight, as we shall see.
They usually have an elevated and constant body temperature, which is much higher than the average body temperature of most mammals.
This is due to the fact that active flight requires enormous amounts of energy, which in turn necessitates a high metabolism.
Like mammals, birds have several different types of feathers that keep the heat out of the core of the body, away from the surface where it can be lost by radiation.
There are two main types of feathers in modern birds.
The barbs have tiny branches that are linked together by minute hooks, making the vane of a feather a strong, flexible, and unimpeded surface.
Newly hatched birds are covered almost entirely in down, which serves as an excellent coat of insulation, increasing the thermal boundary layer between the skin and the outside environment.
The lift and thrust necessary for flying birds to become and stay airborne is provided by feathers.
The drag on the wing is reduced by the fact that the feathers at the end of the wing are flexible.
The air flowing over the flight feathers creates lift.
Two types of flight feathers are found on the wings.
In contrast to primary and secondary feathers, the body has contour feathers that help reduce form drag during flight.
They create a smooth, aerodynamic surface so that air moves quickly over the bird's body, preventing turbulence and creating ideal aerodynamic conditions for efficient flight.
The contraction of the chest muscles moves the wings downward, whereas the contraction of the supracoracoideus muscles moves the wings upward.
The back of the bird offers little meat because the muscles that humans eat on holidays are attached to the keel of the sternum.
Birds have a higher percentage of body mass than most mammals.
The flight muscles attach a blade-shaped keel to the body.
The large flight muscles of birds are accommodated by the deeper sternum.
Oxygen-storing myoglobin is found in the flight muscles of birds who are active flyers.
The furcula provides support to the shoulder girdle during flapping.
A low body weight is important for flight.
The muscle output required for flying increases as body weight increases.
The largest bird is the ostrich, which is much smaller than mammals and flightless.
Reducing body weight makes flight easier for birds.
Pneumatization of bones is one of the modifications found in birds.
Large birds have more pneumatic bones than small birds.
The skulls of almost all birds are pneumatic.
The jaw is lightened by the replacement of heavy jawbones and teeth with a beak made of keratin, just as hair, scales, and feathers are.
The hollow, pneumatic bones of many birds make flight easier.
The lack of a urinary bladder is one modification that reduces weight.
Water can be absorbed back into the bloodstream with the help of the cloaca.
Water is not held in a urinary bladder, which would increase body weight.
The females of most bird species only have one functional ovary, which reduces body mass.
The respiratory system of birds is very different from that of mammals and reptiles.
The air sacs are connected to the path of air through the bird's body.
Air flowing in and out of bird lungs is the same as air flowing in and out of mammals.
Air capillaries are small air passages within the lungs.
The countercurrent exchange system is created by the arrangement of air capillaries in the lungs.
In a counter-current system, the air flows in one direction and the blood flows in the opposite direction, creating a favorable diffusion and efficient means of gas exchange.
The upper part of the trachea is not related to the syrinx.
There is an efficient system of gas exchange with air flowing.
A full cycle of breathing takes two cycles.
The second inhalation moves the air in the lungs to the anterior air sacs, and the second exhalation moves the air in the anterior air sacs out of the body.
Each inhalation moves air into the air sacs, while each exhalation moves fresh air through the lungs and out of the body.
The air sacs are connected to the bones.
Birds are also unusual because of a number of other features.
A short tail is produced from the fusion of the caudal vertebrae.
Birds have a single occipital condyle that allows them to move their head and neck.
They have a closed circulatory system with two atria and two ventricles, but unlike mammals, they have nucleated red blood cells.
The unique and highly derived characteristics of birds make them one of the most successful groups of animals in the world, filling a range of ecological niches and ranging in size from the tiny bee hummingbird of Cuba to the ostrich.
Their large brains, keen senses, and the ability of many species to imitate vocalization and use tools make them some of the most intelligent vertebrates on Earth.
Even though bird bones do not fossilize as well as those of other mammals, the evolutionary history of birds has become clearer thanks to new fossil discoveries in China.
The skulls of modern birds are so specialized that it is difficult to see any trace of the original diapsid condition.
The Saurischia and Ornithischia are two groups of birds that evolved within the Dinosauria.
It was not the bird-hipped dinosaurs that gave rise to modern birds.
One group included the longnecked herbivorous dinosaurs, such as Apatosaurus.
Theropods gave rise to birds.
There are many similarities between theropod fossils and birds, specifically in the structure of the hip and wrist bones, as well as the presence of the wishbone, formed by the fusion of the clavicles.
One of the most important fossils of an animal that was intermediate between dinosaurs and birds is Archaeopteryx.
Both modern birds and dinosaurs have characteristics of Archaeopteryx.
Some scientists think it's a bird, while others think it's a dinosaur.
The jaw and tail of Archaeopteryx were similar to those of a dinosaur.
Like birds, it had feathers modified for flight, both on the forelimbs and on the tail, a trait associated only with birds among modern animals.
Modern flight feathers may not have had the characteristics of older feathered dinosaurs.
The fossil has flight feathers like modern birds, but it has cuplike thecodont teeth.
The claws on the wings are still found in a number of birds.
There are two basic hypotheses that explain how flight may have evolved in birds.
In this scenario, wings could be used to capture prey.
The large flightless birds hold their wings out when they run.
This condition may be a relic of the flying birds that were their ancestors.
It seems more likely that small feathered dinosaurs were capable of gliding from tree to tree and branch to branch, improving the chances of escaping enemies, finding mates, and obtaining prey such as flying insects.
The opportunity for species dispersal would have been greatly increased by this early flight behavior.
We have a good idea of how feathers and flight have evolved, but the question of how endothermy evolved in birds remains unanswered.
If body heat is being produced internally, feathers are beneficial for thermoregulatory purposes.
If insulation is present, internal heat production can be viable for the evolution of endothermy.
feathers or endothermy may be evolved first in response to some other pressure, such as the ability to be active at night, repel water, or serve as signals for mate selection.
It seems probable that feathers and endothermy coevolved together, the advancement and evolution of feathers reinforcing the advancement of endothermy.
The term "opposite birds" refers to the fact that certain bones of the shoulder are joined differently than the way the bones are joined.
These birds had teeth in their jaws, but had a shortened tail, and at least some fossils have preserved fans of tail feathers.
These birds did not survive past the end of the last century.
The evolutionary line includes modern birds.
This clade was present in the past.
Our current understanding of the relationships among living birds is the result of analysis based on large data sets.
The three major clades are the Paleognathae, the Galloanserae, and the Neoaves.
The Paleognathae are a group of birds that are flightless.
Birds in the Galloanserae include pheasants, ducks, geese and swans.
All other birds are included in the Neoaves.
The Neoaves have been distributed among five different clades.
Despite the current classification scheme, it is important to understand that the current birds are not the last to be revised.
Next-Generation DNA Sequencing has led to the creation of a comprehensive phylogeny of birds.