Aves

The major characteristics of this class concern adaptations for flight, including appendages modified as wings, feathers, endothermy, a high metabolic rate, a vertebral column modified for flight, and bones that are lightened by numerous air spaces.

Horny bill and lack teeth.

Adaptive radiation has resulted in about 9,100 species of living birds, which are divided into about 27 orders.

The orders are distinguished from one another by characteristic behaviors, songs, anatomical differences, and ecological niches.

The covering of feathers on a bird is called the plumage.

Feathers have two primary functions essential for flight.

They form the flight surfaces that provide lift and aid steering, and they prevent excessive heat loss, permitting the endothermic maintenance of high metabolic rates.

Feathers also have roles in courtship, incubation, and waterproofing.

Only the inner pulp of feathers contains dermal elements, such as blood vessels, which supply nutrients and pigments for the growing feather,.

As feathers mature, their blood supply is cut off, and the feathers become dead, keratinized, epidermal structures seated in epidermal invagination of the skin called feather follicles.

The most obvious feathers are contour feathers, which cover the body, wings, and tail.

Contour feathers consist of a vane with its inner and outer webs, and a supportive shaft.

Feather barbs branch off the shaft, and barbules branch off the barbs.

Barbules of adjacent barbs overlap one another.

The ends of barbules are locked together with hooklike hamuli.

Interlocking barbs keep contour feathers firm and smooth.

Other types of feathers include down and filoplume.

Birds maintain a clean plumage to rid the feathers and skin of parasites.

Preening, which is done by rubbing the beak over the feathers, keeps the feathers smooth, clean, and in place.

Secretions from an oil gland at the base of the tail of may birds are spread over the feathers during preening to keep the plumage water repellant and supple.

Anting is a maintenance behavior.

Most colors in a bird's plumage are produced by feather pigments deposited during feather formation.

Other colors, termed structural colors, arise from irregularities on the surface of the feather that diffract white light.

Blue feathers - a porous, nonpigmented outer layer on a barb reflects blue wavelengths of light.

Mature feathers receive constant wear; thus, all birds undergo a periodic renewal of their feathers by shedding and replacing them in a process called molting.

The bones of most birds are lightweight yet strong.

Some bones, such as the humerus(forearm bone) have large air spaces and internal strutting which increase strength.

Birds also have a reduced number of skull bones, and teeth are replace with a lighter, keratinized sheath covering a beak.

Some aquatic birds have dense bones, which help reduce buoyancy during diving.

The cervical vertebrae have saddle-shaped articular surfaces that permit great freedom of movement.

In addition, the first cervical vertebra (the atlas) has a single point of articulation with the skull, which permits a high degree of rotational movement between the skull and the neck.

This flexibility allows the beak and neck to function a s a fifth appendage.

the thoracic region of the vertebral column contains ribs, which attach to thoracic vertebrae.

the ribs have posteriorly directed uncinate processes that overlap the next rib to strengthen the rib cage.

Posterior to the thoracic region is the lumbar region.

The synsacrum is formed by the fusion to the posterior thoracic vertebrae, all the lumbar and sacral vertebrae, and the anterior caudal vertebrae.

Fusion of these bones helps maintain the proper flight posture and supports the hind appendages during landing, hopping, an walking.

The posterior caudal vertebrae are fused into a typostyle, which helps support the tail feathers that are important in steering.

the sternum of most birds bears a large, median keel for the attachment of flight muscles.

The largest, strongest muscles of most birds ar the flight muscles.

The attach to the sternum and clavicles and run to the humerus.

Flight muscles must contract quickly and fatigue very slowly.

these muscles have large numbers of mitochondria and produce large quantities of ATP to provide the energy required for flight, especially long distance migrations.

Bird wings form an airfoil.

The anterior margin of the wing is ticker than the posterior margin.

The upper surface of the wing is slightly convex, and the lower surface is flat or slightly concave.

Air passing over the wing travels further and faster than air passing under the wing, decreasing air pressure on the upper surface of the wing and creating lift.

Lift can be increased by increasing the angle the leading edge of the wing makes with the oncoming air.

Most of the propulsive force of flight is generated by the distal part of the wing.

Because it is further from the shoulder joint, the distal part of the wing moves further and faster than the proximal part of the wing.

The tail of a bird serves a variety of balancing, steering, and braking functions during flight.

During horizontal flight, spreading the tail feathers increases lift at the rear of the bird and causes the head to dip for descent.

Closing the tail feathers has the opposite effect.

Tilting the tail sideways causes the bird to turn.

When a bird lands, its tail is deflected downward, serving as an air brake.

During gliding flight, the wing is stationary, and a bird loses altitude.

Flapping flight generates the power for flight and is the most common type of flying.

Soaring flight allows some birds to remain airborne with little energy expenditure.

Most birds have ravenous appetites.

This appetite supports a high metabolic rate that makes endothermy and flight possible

Bird beaks and tongues are modified for a variety of feeding habits and food sources.

In may birds, a divertidculum of the esophagus, called the crop, is a storage structure that allows birds to quickly ingest large quantities of locally abundant food and then seek safety while digesting their meal.

The stomach of birds is modified into two regions.

The proventriculus secretes gastric juices that initiate digestion.

The ventriculus (gizzard) has muscular walls to abrade and crush seeds or other hard materials.

Sand and other abrasive may be swallowed to aid digestion.

The bulk of enzymatic digestion and absorption occurs in the small intestine, aided by secretions from the pancreas and liver.

Paired ceca may be located at the union of the large and small intestine.

These blind - ending sacs contain bacteria that aid in the digestion of cellulose.

Undigested food is usually eliminated through the cloaca; however, owls form pellets of bone, fur, and feathers that are ejected form the ventriculus through the mouth.

The circulatory system of birds is similar to that of reptiles, except that the heart has completely separated atria and ventricles, resulting in separate pulmonary and systemic circuits.

In vertebrate evolution, the sinus venosus has undergone a gradual reduction in size.

it is a separate chamber in fishes, amphibians, and turtles and receives blood from the venous system.

In other reptiles, it is a group of cells in the right atrium that serves as the pacemaker for the heart.

In birds, the sinus venosus also persists only as a patch of pacemaker tissue in the right atrium.

The bird heart is relatively large (up to 2.4% of total body weight) and its rate of beating is rapid.

Rates in excess of 1000 beats per minute have been recorded for hummingbirds under stress.

Larger birds have relatively smaller hearts and slower heart rates.

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A large heart, rapid heart rate, and complete separation of oxygenated from unoxygenated blood are important adaptations for delivering the large quantities of blood required for endothermy and flight.

Because of high metabolic rates associated with flight, birds have a greater rate of oxygen consumption than any other vertebrate.

When other vertebrates inspire and expire, air passes into and out of respiratory passageways in a simple back and forth cycle.

Ventilation is interrupted during expiration and there is a considerable quantity of dead air in the lungs because to all air is forced out during expiration.

Because of their unique structure, bird lungs provide a nearly continuous movement of fresh air over respiratory surfaces during inspiratory and expiratory cycles.

the quantity of dead air in the lungs is sharply reduced compared with other vertebrates.

Most air is taken in through external nares, which lead to nasal passageways and the pharynx.

the trachea is supported by bone and cartilage.

A larynx is undifferentiated, but a special voice box, called the syrinx, is located where the trachea divides into bronchi.

The lungs of birds are made of small air tubes called parabronchi.

Air capillaries about 10µm in diameter branch from the parabronchi and provide gas-exchange surfaces.

Inspiration and expiration are accomplished by the expansion and compression of thin-walled air sacs that ramify throughout the body cavity and even penetrate some bones, such as the humerus of the wing.

During inspiration, air moves into the abdominal air sacs.

At the same time, air already in the lungs moves through parabronchi to the thoracic air sacs.

During expiration, the air in the thoracic air sacs moves out of the respiratory system, and the air in the abdominal air sacs moves into parabronchi.

At the next inspiration, the air moves into the thoracic air sacs, and is expelled during the next expiration.

it takes two ventilatory cycles to move a particular volume of air through the respiratory system of a bird.

Birds regulate their body temperatures between 38 and 45 C.

Lethal extremes are lower than 32 and higher than 47 C.

On a cold day, resting birds fluff their feathers to increase their insulating properties, as well as the dead air space within them.

They also tuck their beaks into their feathers to reduce heat loss from the respiratory tract.

The most exposed parts of a bird are the feet and tarsi which gave neither fleshy muscles or a rich blood supply.

Temperatures in these extremities are allowed to drop near freezing to prevent heat loss.

Shivering is also used to generate heat in extreme cold.

Some birds become torpid and allow their body temperatures to drop on cool nights.

Excess heat can be dissipated by panting.

The forebrain of birds is much larger than that of reptiles due to the enlargement of the cerebral hemispheres, including a region of gray matter, the corpus striatum.

The corpus stratum functions in visual learning, feeding, courtship, and nesting.

A pineal body is located ont eh roof of the forebrain.

It appears to play a role in stimulating ovarian development and in regulating other functions influenced by light and dark periods.

the m along with the corpus striatum, plays an important role in integrating sensory functions.

The midbrain also receives sensory input from the eyes.

As in reptiles, the hindbrain includes the cerebellum and the medulla oblongata, which coordinate monitor activities and regulate heart and respiratory rates, respectively.

Vision is an important sense for most birds.

Similar to that of other vertebrates, they are much larger in proportion to body size than in other vertebrates

Birds have a unique, double focusing mechanism.

Padlike structures control the curvature of the lens, and ciliary muscles change the curvature of the cornea.

Contains both rods and cones.

Rods are active under low light intensities and cones under high light intensities.

Cones are especially concentrated at a focal point called the fovea.

Unlike other vertebrates, some birds have two foveae per eye.

The one at the center of the retina is sometimes called the search fovea because it gives the bird a wide angle of monocular vision.

The other fovea is at the posterior margin of the retina.

It functions with the posterior fovea of the other eye to allow binocular vision.

The posterior fovea is called the pursuit fovea, because binocular vision is necessary for depth perception, which is necessary to capture prey.

Like reptiles, birds have a nictitating membrane that is drawn over the surface of the eye to cleanse and protect the eye.

Olfaction apparently plays a minor role in the lives of most birds.

Exceptions include turkey vultures, which locate their dead and dying prey largely by smell.

In contrast, hearing is well developed in most birds.

The external ear opening is covered by loose, delicate feathers called auriculares.

The sensitivity of the avian ear 100 - 15000 Hz is similar to that of the human ear 16 - 20000 Hz

Like reptiles, birds excrete uric acid, which is temporarily stored in the cloaca.

Water reabsorption also occurs in the cloaca.

In addition, some birds have supraorbital salt glands that drain excess sodium chloride through the nasal openings to the outside of the body.

These are especially important in marine birds that drink seawater and feed on invertebrates containing large quantities of salt in their tissues.

These activities include establishing territories, finding mates, constructing nests, incubating eggs, and feeding young.

All birds are oviparous.

Gonads are located in the dorsal abdominal region, next to the kidneys.

Testes are paired, and coiled tubules conduct sperm to the cloaca.

An enlargement of the vasa deferentia, the seminal vesicle, is a site for temporary storage and maturation of sperm prior to mating .

Birds have no intromittent organ, and sperm transfer occurs by cloacal contact during brief mounts by the male on the female.

Usually only the left ovary fully develops.

A large, funnel-shaped opening of the oviduct envelopes the ovary and receives eggs after ovulation.

Fertilization of the egg occurs in The upper portions of the oviduct, and the zygote is gradually surrounded by albumen secreted rom glandular regions of the oviduct wall as the egg completes its passage.

A shell is added by a shell gland in the lower region of the oviduct.

The oviduct opens into the cloaca.

Territories.

Over 90% of birds are monogamous.

Monogamy is common when resources are widely and evenly distributed, and one bird cannot control access to resources.

Monogamy is also advantageous because both parents usually participate in nest building and care of the young.

Some birds ar polygynous.

Males mate with more than one female.

polygyny tends to occur in species whose young are less dependent at birth and in situation where patchy resource distribution may attract many females to a relatively small breeding area.

Prairie chickens are polygynous.

A few bird species are polyandrous, and the females mate with more than one male.

polyandry occurs in spotted sandpipers.

If a male loses his eggs to a predator, the female replaces those eggs.

Polyandry results in the production of more eggs than monogamous matings.

It is thought to be advantageous when food is plentiful but, because of predation or other threats, the chances of successfully rearing young is low.

The nesting behavior of birds id often species specific.

The number of eggs laid by a female is usually variable.

Those that are entirely dependent on their parents are said to be altricial, and they are often naked at hatching.

they grow rapidly, and when they leave the nest they are nearly as large as their parents.

Precocial young are alert and lively at hatching.

they are usually covered by down, and can walk, run, swim, and feed themselves, although one parent is usually present to lead the young to food and shelter.

Life is usually brief for birds.

Most birds, if kept in captivity, have a potential life span of 10 to 20 years.

Natural longevity is much shorter.

The average american robin lives 1-3 years.

Mortality is high in the first year from predators and inclement weather.

Migration refers to periodic round trips between breeding and nonbreeding areas migrations are annual.

Birds migrate in response to species specific physiological conditions.

Innate clocks and environmental factors influence preparation for migration.

The photoperiod is often cited as an important migratory cue for may birds, particularly for birds in temperate zones.

Two forms of navigation are used by birds.

Routebased navigation involves keeping track of landmarks on an outward journey so that those landmarks can be used in a reverse sequence on the return trip.

Location based navigation involves establishing the direction of the destination from information available at the journey's site of origin.

It involves the use of sun compasses, other celestial cues, and or the earth's magnetic field.