Arctic Life/Arctic Animals/Birds/Anatomy
The anatomy of birds is quite fascinating. Below you will visually explore anatomy features of birds and contrast them to those of mammals and reptiles. We will cover avian skull, ears, jaws, eggs and different types of feathers.
Because flight requires a highly centralized body mass with light extremities, a bird's head must be very light. Teeth, and the jaws that hold them, are cumbersome and, for birds, no longer necessary. Instead of using teeth to tear food into pieces, birds use one of two strategies. They will either eat only food items of a size they can swallow whole, or, as in the case of raptors, they will have a beak adapted for tearing food. In general, a bird's beak is adapted to the kinds of food it eats.
A bird's skeleton is designed for flight; many bones are fused or reduced in size, reducing the mass of the skeleton, and redistributing mass towards the centre of the body.
Many bones in a bird's body are pneumatized, meaning that they contain large air pockets that are connected to the respiratory system. Bird bones are therefore very light. Small bone struts in the air pockets provide strength. Different species of birds show differing degrees of pneumatization. As a rule, small birds tend to show less pneumatization than large birds, and in birds adapted for diving, there is hardly any pneumatization at all.
The bones of the wing show various flight adaptations. The humerus is very short and thick, to support the major flight muscles. It is also light because it is pneumatized. The radius and ulna support the middle of the wing; the ulna has small tubercles to which the muscles controlling the position of the secondary flight feathers are attached. The carpals and metacarpals are fused, and there are only three digits. This structure keeps the primary flight feathers in the right positions during flapping flight.
Occipital condyles are knobs that enable the skull to articulate with the vertebral column. Reptiles and birds have only one occipital condyle, mammals have two.
This drawing shows the single middle ear bone, the columella, which is found in birds and reptiles, compared to the three (stapes, incus, malleus) found in mammals.
The jaws of reptiles and birds are composed of five fused bones, while the mammalian jaw is formed from a single bone, the dentary.
The red blood cells of birds and reptiles are nucleated, while those of mammals lack nuclei.
Reptilian and avian eggs share the same membranes and structures. Avian eggs are more rigid, but this is simply the result of a larger amount of calcium deposited in the shell.
Both hatchling reptiles and birds have an egg tooth for breaking through the eggshell at hatching.
Feathers are presumed to have evolved from reptilian scales, are composed of keratin, and are complex in structure. Different kinds of feathers have different functions.
The feathers of the wing are divided into primary, secondary, and tertiary flight feathers, alula, and primary, secondary, and tertiary coverts. The primary flight feathers are responsible for propulsion during flight, while the secondaries form an airfoil. The tertiary flight feathers may either increase the airfoil or assist in the streamlining of the wing to the body. The alula acts like an aerodynamic slot and spoiler. All three sets of coverts are made up of less rigid contour feathers that smooth the shape of the wing, making it highly aerodynamic.
Contour feathers cover the body and wings. They are often asymmetrical and provide an aerodynamic shape to the wings. The microscopic hooks and barbules shown in the diagram, give the contour feather both flexibility and strength. One of the purposes of preening is to "zipup" the hooks and barbules, repairing the structural integrity of the contour feathers. If you have a large contour feather handy, you can try this yourself. Separate the vane, then zip it up again.
Down feathers are simple in structure, and have no hooks or barbules. Their primary function is as insulation. The amount of down present on nestlings and adult birds varies among species. There is also a special kind of down, the "powder down". The ends of the feathers disintegrate easily, and the powder produced is thought to be an aid in waterproofing and grooming. Powder down is common in birds such as herons, parrots, and hawks.
Filoplumes are long and hair-like and are sensory in function. Sensory corpuscles at the base of each filoplume let a bird in flight know the position of each of its feathers.
Semiplumes look like a cross between a contour feather and a down feather. Semiplumes lie underneath the contour feathers and smooth out body contours, providing insulation, and making the bird more aerodynamic.
Bristles have only a few hooks and barbules. Bristles are found around the eyes and nostrils, and at the comers (or ricta) of the mouth. Insectivorous birds may use their rictal bristles to sense the position of prey.
Muscles and Tendons of the Wing
The muscles and tendons of the wing are designed for flight. The tendons are very wiry, and can withstand a large amount of force. All of the muscles labeled, function together to move the wing up and down, forwards and backwards, steady it, and extend and flex it. The major force for flight is actually a pair of muscles, the pectoralis.
Aside from bones, the tissues and organs of birds have the same mass as mammalian size equivalents, but more of a bird's overall mass is found in its torso. The centre of mass in a bird is found between the wings. The limbs are far less muscled, compared with those of a typical mammal; major flight muscles are located in the pectoral region, rather than on the wings themselves. Birds also have very small heads, and short, stubby tails, again reducing the mass of the extremities.
Birds have the highest metabolic rate for their size of any vertebrate. A high metabolic rate allows them to fly efficiently. The fastest metabolic rates are found among the smallest birds, the hummingbirds. Also, high metabolic rates allow some birds to fly at very high altitudes. Migrating geese and swans have been observed flying at 9000 m!
The avian respiratory system is unlike any other. It is composed of a spongelike lung, and a number of membranous air sacs. In the lung, the air channels, or bronchioles, run cross-current to the circulating blood. The transfer of oxygen from air to blood is extremely efficient, and because the lung does not expand when air passes through it, there is no change in the aerodynamics of a bird in flight. The air sacs fill and empty in a two-breath cycle, ensuring a constant flow of air through the lungs.
- "The Evolution of Birds". Canada's Aquatic Environments. University of Guelph. Retrieved July 28, 2019.