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Explore the fascinating world of birds, from their unique skeletal structure to the intricate design of their feathers and muscles. Learn about the preen gland, air sac respiratory system, different types of feathers, and more.
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Characteristics of Class Aves • Forelimbs designed for flight in most birds. • Epidermis is covered in feathers, except the hind legs which are covered in scales. • No sweat or oil glands, but they do have a preen gland (uropygial gland) at the base of their tail, that they use to waterproof and protect their feathers. • Bony skeleton that is hollowwith air-filled cavities. • Skull with a beak made of keratin, no teeth.
Characteristics of Class Aves • 4-chambered heart, with completely separate pulmonary and systemic circulations. • Endothermic, warm-blooded. • Respiration by way of air sacs. Unidirectional flow. • No urinary bladder, semisolid urine as waste. • Separate sexes, internal fertilization. • Amniotic egg, with hard calcareous shell. • Eggs are incubated externally.
Feathers • Almost everything about birds, with the exception of the flightless birds, is designed on these two principles; more power, less weight. • Feathers being made of keratin, are lightweight, yet amazingly tough and resilient. • Most feathers are contour feathers, which are the feathers used for flight. • Contour feathers emerge from the skin at the quill (calamus), and turn into the shaft (rachis), which bears numerous barbs and barbules.
Feathers • The barbs branch out diagonally from the shaft (rachis) to form the vane. • There are several hundred barbs in a vane. • Each barb has separate parallel filaments that branch off from the barbs called barbules. • The barbules hook on to each other to keep the vane of the feather aligned. • Each barb can have up to 600 barbules on each side of the barb. • A single feather can have over 1 million barbules.
Different Types of Feathers • The three different types of feathers • Contour feathers (Flight feathers) • Down feathers (Insulation feathers) • Filoplume feathers (Sensory feathers)
Feathers • If the barbules become unattached, the birds preen their feathers to reattach them. • Feathers, like hair, are dead once fully grown. • Shedding or molting of feathers replaces old, worn out feathers. • Molting usually occurs once a year, usually in late summer, after nesting season. • Flight and tail feathers are usually lost gradually and in pairs, so the molting doesn't impair flight.
Bird Skeletal Structure • Bird's bones are pneumatic, which means they are hollow and are made of air-filled cavities. • The pneumatic bones make birds light, but they also play a role in the respiratory system of birds. • This bone structure makes them incredibly light. • The frigate bird has a 7-foot wingspan, but the skeleton only weighs a mere ¼ pound. • In order to increase the strength of the hollow bones, the inside of the bones have cross members or struts, like on an airplane.
Bird Skeletal Structure • The orbits (eye sockets) of birds are unusually large to accommodate their excellent vision. • Modern birds lack teeth. Some birds in the fossil record had teeth, like the famous archaeopteryx. • Teeth make the head too heavy for flight. • Instead, birds use a muscular gizzard to break down food, which is found in the center of their body. • The rib cage and sternum is rigid, not allowing expansion of the chest cavity for respiration. • The rib cage and sternum must support the powerful wing muscles on the breast.
Bird skulls • Archeopteryx Skull Raven Skull
Bird Skeletal Structure • The sternum is keeled, meaning it has a very large surface area for muscle attachment. • The clavicles are fused to form an elastic structure called the furcula, which stores elastic energy when the wings beat in flight. • The furcula is the Y-shaped bone that we call the wishbone at Thanksgiving. • The vertebrae and pelvis are also fused together to absorb the shock of landing.
Bird Skeleton • Note the unusually large keeled sternum that attaches to the breast muscles • Note the fused pelvis and vertebrae called the synsacrum, which absorbs the shock of landing.
Bird Muscular System • If you have ever eaten a bird you know that most of the muscle in the body is on the breast. • The largest muscle which pulls the wing down during flight is the pectoralis muscle. • The pectoralis muscle also has an antagonistic muscle that pulls the wing back up, but surprisingly it is not found on the back, where one might expect. If it was, it would impair flight by making the bird too top heavy.
Flight Muscles • Instead, there is another muscle found underneath the pectoralis muscle called the supracoracoideus muscle and it has an ingenious rope and pulley system.
Leg Muscles and the Perching Mechanism • Muscles of the lower leg are greatly reduced because large leg muscles would affect a bird's center of gravity and it would make the legs vulnerable to cold. • Birds have strong tendons that automatically clamp down when a bird is perched, allowing them to sleep while they are perched. • Also, the talons on raptors, like hawks, eagles, and falcons are assisted by these tendons. • The force of the strike causes the tendons to clamp down hard on its prey.
Birds Digestive System • Birds lack teeth because teeth are too heavy. • Instead, birds use a muscular gizzard that they fill with small stones and pebbles to grind food. • Storage of food takes place in the crop. • Another food chamber called the proventriculus is the actual stomach and it secretes gastric juice which chemically breaks down the food. • Birds of prey like owls, form pellets of undigestible material (bones and fur) in the proventriculus, then they regurgitate them.
Birds Respiratory System • The respiratory system of birds is arguably the most efficient in the animal kingdom. • It is no surprise, considering how hard they have to work in order to achieve flight. • For example, bar-headed geese migrate over the great Himalayan Mountains. • They have even been observed flying over Mount Everest at 29,000 feet elevation. • Not only that, but they do it all in one day without having to acclimate for hypoxic conditions.
Birds Respiratory System • The secret to their success is a one-way (unidirectional) flow of oxygen, created by nineair sacs distributed throughout the body, and tubes in the lungs called parabronchi. • Air sacs also connect to the bones via tubes. • It takes two full breaths for air to circulate the body. • On the first inhalation, the oxygen-rich air moves back to the posterior air sacs.
Birds Respiratory System • On the first exhalation, the air from the posterior air sacs moves into the lungs to deliver oxygen to the parabronchi. • On the second inhalation, the air from the parabronchi moves into the anterior air sacs. • On the second exhalation, the air from the anterior air sacs is removed from the body. • The result is a continuous flow of fresh air across the blood vessels surrounding the parabronchi.
Wing Shapes • There are four basic wing shapes found among birds to assist them in different ways. • Elliptical (low aspect ratio) designed for low speeds while maintaining high lift and excellent maneuverability. • High-speed (tapering or swept back wings) for extremehigh speeds with reduced turbulence. • Soaring (high aspect ratio) for high lift and moderate to high speed but low maneuverability. • High-lift for carrying heavy loads at low speeds with moderate maneuverability.
Wing Shapes • Two wing design modifications that reduce turbulence or stalling of an airfoil is wing slots (or alula)and a swept back tapering wing which reduces wing tip vortexes.
Wing Slots and Wing Tip Vortexes • Geese take advantage of the wing tip vortexes by flying in a V and catching the updrafts.
Bird Migration • Birds are the supreme migrators of the animal kingdom. The Arctic tern migrates to the Arctic Circle during the summer to breed. Then they migrate all the way back to Antarctica during the winter. A total round trip distance of about 22,000-30,000 miles. • The stimulus for migration is a change in the amount of daylight caused by changes in the seasons. • In spring, the seasonal changes stimulate an increase in gonad development, increase of fat storage, courtship and mating behavior and caretaking instincts.
Migration Navigation • Understanding exactly how birds navigate is not entirely understood by scientists. • Experiments have shown that some birds navigate by sight and recognizing landmarks. • Experiments with homing pigeons have shown that the earth's magnetic field may help some birds to navigate. • Small particles of magnetite have been detected in their neck muscles. • Other experiments have shown some birds use the position of the sun and stars to navigate.
Mating Strategies • There are two main types of mating strategies in animals; monogamy, in which mating occurs with only one partner, and polygamy, in which mating occurs with multiple partners. • There are many variations of the two types. • Monogamy is rare in most animals, but very common with birds; approximately 90%. • One reason why monogamy may be more common with birds is that birds lack milk, (except crop milk in some) so parental roles are more shared and equal.
Mating Strategies • The most common form of polygamy in birds is called polygyny. Polygyny is when one male has many female partners. This is a common practice with some members of the grouse family. • Male sage grouse defend their breeding territories (called leks) from other rival males. • The females choose the dominant male because he has the best genes.
Nesting and Care of the Young • Most birds build some type of nest in order to prepare a place to raise their young. • There are two ways the young are born. • Precocial young such as quail, grouse, geese, ducks etc. are born covered with down and can run or swimas soon as their plumage is dry. • Atricial young are born naked and helpless at birth and remain in the nest for a week or more. • Most birds (the passeriformes) are altricial.
Class Mammalia • Body covered in hair, reduced in some. • Skin glands including, sweat, scent, sebaceous, and mammary glands. • Turbinate bones in the nasal cavity that conserve water for endotherms and also give mammals an acute sense of smell. • Teeth in most mammals are diphyodont (deciduous or milk teeth), and also they are heterodont (vary in structure and function).
Class Mammalia • Fleshy external ears called pinnae. • 4-chambered heart and red blood cells are biconcave and nonnucleated. • Secondary palate, similar to crocodiles, which separates the air passages from the food. • A muscular diaphragm for breathing. • A highly developed, cerebral cortex. • Internal fertilization, with development via a placenta in most, young nourished from milk.
Hair • All mammals have hair. Humans have little, whales only have a few sensory hair bristles on their snout. • Hair is made from keratin, the same substance that makes up nails, claws, hooves, horns, feathers on birds, and scales on reptiles. • Mammals have two kinds of hair that make up their coat or pelage. They have dense, soft underhair for insulation, and coarse guard hairs used for protection and camouflage.
Hair • Most mammals undergo two annual molts. Many arctic animals like the weasels and snowshoe hare, have a brown to gray summer coat that is thin, and a white winter coat that is thick. • Many mammals have unusual color patterns like the zebra's which confuses predators, and the white spots on a fawn,imitating sunlight under the forest canopy. • Green is a rare color for mammals, possibly an adaptation to nocturnal behavior.
