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BIRD NOTES EXTERNAL STRUCTURE AND MOVEMENT • The covering of feathers on a bird is called the plumage. • Feathers have 2 primary functions that are essential for flight. – They form the flight surfaces that provide lift and aid in steering. – They prevent excessive heat loss. EXTERNAL STRUCTURE • Feathers are also important in courtship, incubation, and water proofing. • As feathers mature, their blood supply is cut off, and the feathers become dead. EXTERNAL STRUCTURE • The most obvious feathers are contour feathers, which cover the body, wings, and tail. • These feathers consist of a vane 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 lock together with hamuli, which are like little hooks. • Interlocking barbs keep contour feathers firm and smooth. EXTERNAL STRUCTURE • Other types of feathers include down feathers, which act as insulation, and filoplume feathers, which have sensory functions. EXTERNAL STRUCTURE • Birds keep a clean plumage to rid the feathers and skin of parasites. • Preening, which is done by rubbing the bill over the feathers, keeps the feathers smooth, clean, and in place. EXTERNAL STRUCTURE • Secretions from an oil gland at the base of the tail of many birds are spread over the feathers during preening to keep the plumage water repellant. • Secretions also lubricate the bill and legs to prevent chafing. • Anting is a behavior common to many songbirds and involves picking up ants in the bill and rubbing them over the feathers. • The formic acid that ants secrete is toxic to feather mites. EXTERNAL STRUCTURE • Feather pigments deposited during feather formation produce most colors in a bird’s plumage. • Color patterns are involved in cryptic coloration, species and sex recognition, and sexual attraction. EXTERNAL STRUCTURE • All birds periodically shed and replace their feathers in a process called molting. • A typical molting pattern for songbirds begins after hatching when the chick is covered with down. • Juvenile feathers replace the down at the juvenile molt. • A postjuvenile molt in the fall result in plumage similar to that of an adult. • Once sexual maturity is reached, a prenuptial molt occurs late in winter or early spring, prior to the mating season. • A postnuptial molt usually occurs between July and October. EXTERNAL STRUCTURE • Flight feathers are often lost in a certain order so that birds are not completely deprived of flight during molt periods. • Many birds, however, such as ducks, coots, and rails cannot fly during molt periods and will hide in thick marsh grasses. The Skeleton • The bones of most birds are lightweight but very strong. • Some bones, like the humerus (forearm), have large air spaces and internal strutting (reinforcing bony bars), which helps increase strength. • Birds also have a reduced number of skull bones, and a lighter structure, the bill, which replaces the teeth. • Some aquatic birds, like the loon, have dense bones, which helps reduce buoyancy during diving. The Skeleton • The appendages involved in flight cannot manipulate nesting materials or feed young. • The bill and very flexible neck and feet make these activities possible. • The cervical vertebrae have a saddle-shaped surface that allows great freedom of movement. • This flexibility allows the bill and neck to function as a 5th appendage. The Skeleton • The pelvic girdle, vertebral column, and ribs are strengthened for flight. • Most ribs overlap the next rib to help strengthen the rib cage. • Fusion of the thoracic, lumbar, and sacral vertebrae helps maintain the proper flight posture, and supports the hind appendages during landing, hopping, and walking. The Skeleton • The posterior caudal vertebrae are fused into a pygostyle, which helps support the tail feathers that are important in steering. The Skeleton • The sternum of most birds is very large for the attachment of flight muscles. The Skeleton • The appendages of most birds have also been modified. • Some bones of the front appendages have been lost or fused, and are points of attachment for flight feathers. • The rear appendages are used for hopping, walking, running, and perching. The Skeleton • Perching tendons run from the toes across the back of the ankle joint to muscles of the lower leg. • When the ankle joint is flexed, as in landing on a perch, these tendons contract and the foot grips the perch. • This automatic grasp helps a bird perch even while sleeping. Muscles • The largest, strongest muscles of most birds are the flight muscles. • Muscles of most birds are adapted physiologically for flight. • These muscles must be able to contract quickly and fatigue slowly. • They also have many mitochondria and produce large amounts of ATP to provide energy needed for flight, especially long-distance migrations. Muscles • Domestic fowl have been selectively bred for massive amounts of muscle (white meat). • This is good for food, but poorly adapted for flight because it doesn’t have enough mitochondria for energy. Flight • The wings of birds are adapted for different kinds of flight. • Bird wings form an airfoil, which is a surface that provides lift. Flight • Air passing over the wing travels farther and faster than air passing under the wing, decreasing air pressure on the upper surface of the wing and creating lift. Flight • The lift must overcome the bird’s weight, and the forces that propel the bird forward must overcome the drag that the bird moving through the air creates. • Increasing the angle that the leading edge of the wing makes with the oncoming air (angle of attack) increases lift. • As the angle of attack increases, the flow of air over the upper surface becomes turbulent, reducing lift. • Turbulence is reduced if air can flow rapidly through slots at the leading edge of the wing. • Slotting the feathers at the wing tips and the presence of an alula (groups of small feathers that bones of the wing support) help reduce turbulence. Flight • The tail of a bird helps with many things such as steering and braking during flight. • During horizontal flight, spreading the tail feathers increase lift at the rear of the bird and causes the head to dip for descent. • Tilting the tail sideways turns the bird. Flight • When a bird lands, its tail deflects downward, serving as an air brake. • In the males of some species, like sunbirds and widow birds, tails have dramatic ornamentation that attracts females and improves reproductive success. Flight • Different birds have different kinds of flight. • During gliding flight, the wing is stationary, and a bird loses altitude. • Waterfowl come in for a landing using gliding flight. Flight • Flapping flight generates the power for flight and is the most common type of flying. • Soaring flight allows some birds to remain airborne without using too much energy. • Ocean soarers, such as albatrosses and frigate birds, have long, narrow wings that provide maximum lift at high speeds. Flight • Hummingbirds perform hovering flight. • They hover in air by fanning their wings back and forth 50-80 beats per second. • The wings move in a figure 8 pattern. NUTRITION AND DIGESTIVE SYSTEM • Most birds have huge appetites!! • This appetite supports a high metabolism that makes endothermy and flight possible. • Bird bills and tongues are modified for many different feeding habits and food sources. • For example, a woodpecker tongue is barbed for getting grubs from tree bark. • Sapsuckers make holes in trees and use a brushlike tongue for licking the sap that builds up in these holes. NUTRITION AND DIGESTIVE SYSTEM • The tongues of hummingbirds and other nectar feeders roll into a tube for getting nectar from flowers. NUTRITION AND DIGESTIVE SYSTEM • Birds’ bills are used for feeding, preening, nest building, courtship, and defense. • Modifications of the bill reflect specific functions. • The bill of an eagle is modified for tearing prey, the bill of a cardinal is specialized for cracking seeds, and the bill of a flamingo is used to strain food from the water. NUTRITION AND DIGESTIVE SYSTEM • In many birds, the crop is a storage structure that allows birds to quickly ingest large quantities of locally abundant food. • They can then seek safety while digesting their food. NUTRTION AND DIGESTIVE SYSTEM • The crop of pigeons produces “pigeon’s milk”, a cheesy secretion that young pigeons (squabs) feed on until they can eat grain. • Crops are less developed in insect eating birds because they typically eat all day long. NUTRITION AND DIGESTIVE SYSTEM • The stomach of birds is modified into 2 regions. • The proventriculus secretes gastric juices that start digestion. • The ventriculus (or gizzard) has muscular walls to help crush seeds and other hard materials. • Birds may swallow sand and other hard objects to help digestion. • Most digestion occurs in the small intestine. NUTRITION AND DIGESTIVE SYSTEM • Birds usually eliminate undigested food through the cloaca. • Owls, however, form pellets of fur, bone, and feathers that are ejected from the gizzard through the mouth. NUTRITION AND DIGESTIVE SYSTEM • Birds are often grouped by feeding habits. • These groupings aren’t always true because birds may eat different kinds of food at different stages of their lives. • Robins mostly feed on worms when available. • But in winter, they may feed on berries. NUTRITION AND DIGESTIVE SYSTEM • Birds may directly conflict with human interests. • Bird damage to orchard and grain crops costs millions of dollars each year. • Flocking and roosting habits of some birds, like the European starlings and redwing blackbirds, put millions of birds in one location, where they will devastate fields of grain. CIRCULATION, GAS EXCHANGE, TEMPERATURE • The circulatory system of birds is similar to reptiles, except that the heart has completely separated atria and ventricles. • This separation prevents any mixing of highly oxygenated blood with less oxygenated blood. CIRCULATION, GAS EXCHANGE, TEMPERATURE • The bird heart is relatively large (about 2.4% of body weight), and beats rapidly. • Rates over 1000 beats per minute have been observed in hummingbirds under stress. • Larger birds have smaller hearts and slower heart rates. • An ostrich heart beats between 38-176 beats per minute. Gas Exchange • The respiratory system of birds is very complex and efficient. • It consists of external nares which lead to nasal passageways and the pharynx. • Bone and cartilage support the trachea. • A special voice box, the syrinx, produces bird vocalizations. • The lungs of birds are made of small air tubes called parabronchi. Gas Exchange • It takes 2 breathing cycles to move air through the respiratory system. • During the first breath taken in, air moves into the abdominal sacs. • At the same time, air already in the lungs moves through parabronchi into thoracic air sacs. • When breathing out, air in the thoracic sacs move out of the respiratory system and the air in the abdominal sacs moves into the parabronchi. • For the 2nd breath, air moves into the thoracic air sacs, and is expelled during the next breath out. Gas Exchange • Because of high metabolism associated with flight, birds consume more oxygen than any other vertebrate. • Bird lungs have almost a continual movement of oxygen-rich air over their respiratory surfaces during inhaling and exhaling. Thermoregulation • Birds maintain body temperatures between 100.4113 degrees. • Lethal extremes are lower than 89.6 and higher than 116.6. • On a cold day, a resting bird will fluff its feathers to increase insulation. • It may also tuck its bill into the feathers to reduce heat loss from respiratory structures. Thermoregulation • The most exposed parts of a bird are the feet and tarsi, which have neither fleshy muscles or a rich blood supply. • Temperatures in these extremities are allowed to drop to near freezing to prevent heat loss. • Shivering also generates heat in extreme cold. • An increase in metabolism during the winter months requires extra food. Thermoregulation • Some birds will allow their body temperatures to drop on cool nights. • For example, whip-poor-wills allow their body temperature to drop from about 104 down to 60.8 and respiration becomes very slow. • Muscular activity during flight produces large amounts of heat, which birds can get rid of by panting. NERVOUS AND SENSORY SYSTEMS • Birds have many sensory adaptations. • The forebrain of birds is much larger than that of reptiles. • The cerebral hemispheres are larger, including a region of gray matter, the corpus striatum. • The corpus striatum helps in visual learning, feeding, courtship, and nesting. • The midbrain also receives sensory input from the eyes. • The hindbrain includes the cerebellum and the medulla oblongata, which help motor activities and regulate heart and respiratory rates. NERVOUS AND SENSORY SYSTEMS • Vision is a very important sense for birds. • The structures of bird eyes are similar to other vertebrates, but bird eyes are much larger relative to their body size. • The eyes are usually flattened in the back, but birds of prey have eyes that protrude in the front. NERVOUS AND SENSORY SYSTEMS • Birds have a unique double focus mechanism. • This allows an osprey or other bird of prey to remain focused on a fish while rapidly descending from the air. NERVOUS AND SENSORY SYSTEMS • The retina of a bird’s eye is thick and contains both rods and cones. • Rods help in low light intensity and cones are active under high light intensity. • Cones are especially thick in an area called the fovea. • Some birds have 2 fovea per eye. • One is the “search” fovea because it gives them a wide angle of vision. • The other is the “pursuit” fovea and allows for depth perception. NERVOUS AND SENSORY SYSTEMS • The position of the eyes on the head also affects their vision. • Pigeons have eyes located on the sides of the head, giving them almost 360 degree wide view, but narrow depth perception. • They don’t have to pursue their food (grain) but can stay alert for predators while on the ground. • Hawks and owls have eyes more forward on the head. • This helps their depth perception. NERVOUS AND SENSORY SYSTEMS • Birds also have a nictitating membrane that helps to cleanse and protect the eye. • Smell is not very important to most birds. • Exceptions are turkey vultures who use their sense of smell to locate dead prey. • Most birds have well-developed hearing. • Loose, delicate feathers called auriculars cover the external ear opening. • Middle and inner ear structures are similar to reptiles. • Birds hear about the same as humans. EXCRETION AND WATER REGULATION • Birds excrete uric acid, which is temporarily stored in the cloaca. • Water is also reabsorbed in the cloaca. • Some birds have salt glands that drain excess salt through the nasal openings to outside the body. • These are very important to marine birds that drink saltwater and feed on invertebrates that have large amounts of salt in their tissues.