EVOLUTIONARY PRESSURES IN BIRDS
Every body system of a bird shows some adaptation for flight. These adaptations are endothermy, feathers, acute senses, long, flexible necks and lightweight hones.
EXTERNAL STRUCTURE AND LOCOMOTION
Functions of feathers
The covering of feathers on a bird is called the plumage. Feathers have three primary functions:
1. They are essential for flight. They form the flight surfaces. It provides lift. This surface also helps in steering.
2. They prevent excessive heat loss. Thus birds are endothermic. They maintain of high metabolic rates.
3. Feathers also have roles in courtship, incubation and waterproofing.
Development of feathers
The development of feathers is similar to the epidermal scales of reptiles. an evidence of evolutionary ties between reptiles and birds. The inner pulp cavity of feathers contains dermal elements like blood vessels. Blood provides nutrients and pigments for the growing feather. The blood supply is cut off in mature feather. Thus the feathers become dead, keratinized epidermal structures. Feathers are embedded in epidermal invaginations of the skin called feather follicles.
Types of feathers
There are different types or feathers:
1. Contour feathers: These are most obvious feathers. They cover the body, wings and tail. Contour feathers consist of a vane. The vane has inner and outer webs and a supportive shaft. Feather barbs are the branches of the shaft. The barbules are the branches of the barbs. Barbules of adjacent barbs overlap one another. The hook like hamuli (sing.,hamulus) locks the barbules. Interlocking barbs keep contour feathers Finn and smooth.
2. Down feathers: Down feathers are present on the skin. They are insulating feathers.
3. Filoplumes: They are also called pinfeathers. They have sensory functions.
Maintenance of feathers
Birds maintain a clean plumage. It removes the parasites from the feathers and skin.
1. Preening: Preening keeps the feathers smooth, clean and in place. Preening is done by rubbing the bill over the feathers. The dislodged stimuli can be rehooked with the help of bill. Oil glands are present at the base of the tail of many birds. Secretions of these oil glands spread over the feathers during Preening. It makes the plumage water repellent and slimy. The secretions also lubricate the bill and legs. It prevents the shafing.
2. Anting: It is a maintenance behavior in some birds. It is more common in many songbirds. In this case, the bird picks up ants in the bill and rubs them over the feathers. The ants secrete formic acid. This forming acid is toxic to feather mites.
Colour patterns in feathers
There are two factors involved in coloration in birds:
1. Feather pigments: Feather pigments are deposited during feather formation. These pigments produce most colors in the plumage bird.
2. Structural colours: Structural colors are appeared due to irregularities on the surface of the leather. These irregularities diffract white light and produce colours. For example, blue feathers do not have blue pigment. A porous non-pigmented outer layer is present on a barb. This layer reflects blue wavelengths of light. The other wavelengths pass into the barb. They are absorbed bv the dark pigment melanin.
3. Iridescence: The flattening and twisting of harbules causes the diffraction of light. It causes iridescence. The interchanging colors on the neck and back of hummingbirds and grackles are examples of iridescence. Color patterns are used in cryptic coloration, species and sex recognition. and sexual attraction.
Molting of feathers
Mature feathers are damaged and become useless. Thus all birds periodically shed and replace their feathers. The process of shedding and replacement of feathers is called molting. The timing of molt periods varies in different taxa. Following is a typical molting pattern tor songbirds:
1. Juvenile molt: A chick is covered with down feathers after hatching. Juvenile feathers replace the down feather during juvenile molt.
2. Post Juvenile: A post juvenile molt occurs at the sexual maturity. It produces plumage similar to adult.
3. Prenuptial molt: Prenuptial molt occurs at sexual maturity. It occurs in late winter or early spring before the breeding season.
4. Postnuptial molt: It occurs between July and October. Flight feathers are lost in a sequence. Thus the flight continuous during molt. However, many ducks, coots and rails cannot fly during molt periods. They hide in thick marsh grasses.
1. Hollow and light bones
The bones of most birds are light weighted. But these bones are strong. Some bones like humerus have large air spaces. They also have internal strutting. It increase the strength of bone. Birds also have a reduced number of skull bones. They have a lighter, keratinized sheath called bill: It has replaced the teeth. There different adaptations for reduction of body weight in some birds. For example, some aquatic birds (e.g.. loons) have dense bones. These bones reduce buoyancy during diving.
2. Adaptation for feeding and nesting
The flightappendages cannot be used for nesting materials or feeding young. Birds use bill, flexible neck and feet for these activities. The cervical vertebrae have saddle-shaped articular surface. It provides greater freedom of movement. The first cervical vertebra (the atlas) has a single point of articulation with the occipital condyle of skull. It allows a high degree of rotational movement between the skull and the neck. This flexibility allows the bill and neck to function as a fifth appendage.
3. Adaptation in vertebrae
The birds have strong pelvic girdle, vertebral column and ribs for flight. The thoracic region of the vertebral column contains ribs. These ribs are attached to thoracic vertebrae.
Most ribs have posteriorly directed uncinate processes. These processes overlap the next rib and strengthen the ribs. Uncinate processes are also present on the ribs of most reptiles. Posterior to the thoracic region is the lumbar region. The synsacrum is forrmed by the fusion of the posterior thoracic vertebrae, all the lumbar and sacral vertebrae and the anterior caudal vertebrae. Fusion of these bones maintains the proper flight posture. It also supports the hind limbs during landing, hopping, and walking. The posterior caudal vertebrae are fused to form a pygostyle. Pvgostyle supports the tail feathers. Tail feathers are uses in steering.
4. Adaptations in sternum: The sternum of most birds develops a large median keel. Keel is used for the attachment of flight muscles. Keel is absent in some flightless hinds like ostriches. The keel is iched to the axial skeleton by ribs. Private clavicles fuse to form a farcula wishbone.
5. Adaptations in appendages and mechanism of perching
The appendages of birds are also modified. Some bones of the front appendages are lost. Or they fused to form points of attachment for flight feathers. The rear appendages are used for hopping, walking, running and perching. Perching tendons starts from the toes. These pass through the back of the ankle joint and are attached to the muscles of the lower leg.
The ankle joint is flexed during perching. Thus the tension on the perching tendons increases and the foot grips the perch. This is an automatic grasp. It can perch even during sleeping. The muscles of the lower leg can increase the tension on these tendons. Eagle grasps a fish in its talons (claw) by this mechanism.
The flight muscles are the largest and strongest muscles of birds. They are attached to the sternum and clavicles. ‘their other end is attached to the humerus. The muscles of most birds are adapted physiologically for flight. Flight muscles must contract quickly. They are fatigued very slowly. These muscles have many mitochondria and produce large quantities of ATP. ATPs provide energy for flight. Domestic fowl have massive amounts of muscle (“white meat”). The humans like it as food. These muscles contain fibers for rapid contraction. But they have few mitochondria and poor vascularization. Therefore, they are poorly adapted for flight.
The wing of birds is adapted for different kinds of flight. Birds can soar glides or flap their wings. But in all cases they use the same mechanism.
Mechanism of flight
1. Upward lift
Wings of birds form an airfoil. The upper surface of the wing is slightly convex. The lower surface is flat or slightly concave. Air moves faster overthe wing than under it. It decreases air pressure on the upper surface of the wing.Thus it lifts the bird upward. The lift of the wings must be more than the weight of the bird. The air in front of the bird creates resistance. The force of propulsion of the bird must overcome this resistance for moving forward.
2. Turbulence of air
The force of propulsion increases the angle of leading edge of the wing. As a result, the oncoming air (the angle of attack) increases lift. The bird moves upward and its angle of attack keep on increasing. The flow of air over the upper surface becomes turbulent with the increase of angle of attack. It reduces lift. Two adaptations in birds reduce this turbulence:
(a) The leading edge of wings has slots. Air flow rapidly through these slots and reduce turbulence.
(b) Some birds also have alula on the anterior margin of the wing. The alula is a group of small feathers supported by medial digit.
The slotting of the feathers and the presence of an alula reduce turbulence. The angle of attack increases during take off and landing, hovering flight. Thus alula is elevated. The angle of attack decreases during soaring and fast flight. It reduces slot.
3. Strokes of wings
The distal part of the wing generates propulsive force for flight. Distal end of wing is farther from the shoulder joint. Therefore, the distal part of the wing moves farther and faster than the proximal part of the wing. I he wings produce two types of strokes.
(a) Power stroke: The down stroke is called ilowa stroke. During power stroke, the leading edge of the distal part of the wing moves slightly downward. It creates thrust like airplane. Feathers overlap on a wing. Therefore, air presses the leathers at the wing margins together during the down stroke. The leathers do not allow air to pass between them. It increases the lift and propulsive flutes.
(b) Recovery stroke: The upstroke is called recovery stroke. During recovery stroke the distal part of the wing is oriented upward. It decreases resistance. Feathers slightly separated from each other on the upstroke. It allows air to pass between them. It reduces the resistance during recovery stroke.
Role of tail in flight
The tail of a bird acts as balancing, steering, and braking structures during flight. It also enhances the lift of wings during low-speed flight.
1. The birds spread the tail during horizontal night. It increases lift at the rear side of the bird. As a result bird dips the head for descent.
2. The closing the tail leathers has the opposite effect.
3. Tilting of tail sideways turns the bird.
4. Tail acts as an air brake. The tail moves downward during landing of bird.
5. In some species like sunbirds and widow bird, the tails of the males have special ornamentation. It attracts females and improves chance or reproduction.
Types of flight
Different birds use different kinds of flight during different times.
1. Gliding flight: The wings remain stationary during gliding night and the bird loses altitude. Water fowl use gliding flight for landing.
2. Flapping flight: Flapping flight generates the power for flight. It is the most common type of flying. Many variations are present in wing shape. Therefore, flapping patterns is different in different species.
3. Soaring flight : Soaring flight allows some birds to remain in air without spending much energy. Wings remain stationary during soaring. The birduses updrafts and air currents to gain altitude. Hawks and vultures are soaring birds. They encircle the mountain valleys. They soar downwind to pick up speed. Then the turn upwind to gain altitude. Then the bird slows down and starts losing altitude. It turns downwind again. The wings of many soaring bids are wide and slotted. It increases their activities at relatively low speeds. Oceanic soarers like frigate birds have long narrow wings. These wings provide maximum lift at high speeds. But it reduces their activities. It also makes takeoff and landing difficult.
4. Hovering flight: Hummingbirds perform hovering flight. They fan their wings beck and forth hover in still air by (50 to 80 beats per second). They remain suspended in front of a flower or feeding station during hovering.
NUTRITION AND THE DIGESTIVE SYSTEM
Birds feel great appetites. This appetite support a high metabolic lats. High
metabolic rete is necessary fr0 endotherim and flight. For example,
humming birds feed during the day. But they can’t maintain metabolism at night. Therefore, they become torpid at night. They reduce temperature and respiration rate. They again become activeand feed in the morning.
1. Bill and tongue
The bills and tongues of birds are modified for different feeding habits and food sources.
(a) The tongue of woodpecker is barbed. It is used or extracting grubs (larvae of insect) from the bark of trees.
(b) Sapsuckers dig holes in trees. It uses a brush like tongue for licking the sap from the holes.
c) Hummingbirds are nectar feeders. Their tongues roll to form a tube for extracting nectar from flowers.
Crop is present in man birds. Crop is a diverticulum of the esophagus. It is a storage structure. It allows birds to quicklv ingest large quantity of food. Then they move to save places and digest the food. The crop of pigeons produces “pigeon’s milk.”
Pigeon’s mill, is a cheesy secretion. It is formed by the proliferation and sloughing of the lining or the crop Young pigeons feed on pigeon’s milk till they are able to eat grain. Vultures and birds of prey also use their esophagus for storage. The insectivorous birds feed throughout the dav. Therefore, crop is less developed in them.
The stomach of birds has two regions. These regions are proventriculus and ventriculus.
(a) Proventriculus: It secretes gastric juices. Gastric juice initiates digestion.
(b) Ventriculus or (gizzard): It has muscular walls to crush the seeds and other hard materials. Birds swallow, sand and stone in gizzard. These stones help in digestion.
Most of digestion and absorption occurs in the small intestine. Intestine receives secretions from the pancreas and liver. These secretions help in digestion. Paired ceca are
present at the union of the large and small intestine. Ceca are blind-ending sacs. They contain bacteria. These bacteria help in the digestion of cellulose.
Cloaca is common opening for urinogenital duct and anus. Cloaca is present in most of the birds. It eliminates undigested food. However, owls form pellets of bone, fir and feathers. It ejected these pellets through the mouth.
Feeding habits in birds
Birds are grouped on the basis of their feeding habits. These groupings are are artificial. Birds may eat different kinds of food at different stages in their life history. They can change their diets according to availability of food. For example. Robins feed largely on worms and other invertebrates when these foods arc available. Robins may feed on berries ( fruits like tomato) in the winter.
The feeding habits of some birds are harmful to man. Birds damage orchard and grain crops. Millions of birds arc collected in local area due to flocking and roosting habits in some birds. They destroy fields of grain. Recent monocultural practices line increased the problem. The birds form large flocks in these areas.
Birds of prey have minimum impact on populations of poultry and game birds and on fisheries. It has been mistaken though that they are responsible for these loses. Thus human are killing them by poison or shooting them.
GAS EXCHANGE AND TEMPERATURE REGULATION
The circulatory system of birds is similar to reptiles. but the heart of birds has completely separated atria and ventricles. Thus they have separate pulmonary and systemic circuits. Therefore, oxygenated blood does not mix with less oxygenated blood. The sinus venosus has gradually decreased in size during evolution of vertebrates. It is a separate chamber in fishes,amphibians and turtles. It receives blood from the venous system. In other reptiles, sinus venous is a group of cells in the right atrium. It acts as pacemaker in their heart. In birds, the sinus venosus is also present as a patch of pacemaker tissue in the right atrium. The bird heart is relatively large. It beats rapidly. The rate of heart of humming bird is 1, 000 per minute in stress condition. Larger birds have smaller hearts and slower heart rate. For example, the heart rate of an ostrich is between 38 and I 76 beats per minute.
The birds are endotherms. Therefore, they need large quantity of blood or flight. Birds have following adaptations far supplyof blood:
1. They have a large heart.
2. They have rapid heart rate.
3.They have complete separation of highly oxygenated blood from less oxygenated blood.
The respiratory system of birds is complex and efficient. It consists of external nares, pharynx, trachea air sacs and lungs.
(a) External nares open into nasal passageways and pharynx.
(b) Pharynx opens into trachea. Bones and cartilage support the trachea. A special voice box syrinx is present in the birds. It is located where the trachea divides into bronchi. The muscles of the syrinx and bronchi and properties or the trachea produce bird vocalizations.
(c) The bronchi open into air sacs. The air sacs occupy much of the body. They extend into some of the bones of the skeletal muscles. The air sacs and bronchi connect to the lungs.
(c) The lungs of the birds are made or small air tubes called parabronchi. Parabronchi divides to form air capillaries. These air capillaries are associated with capillary beds for gas exchange.
Mechanism of respiration
Two factors are involved in inspiration and expiration:
1. Increasing and decreasing volume of the thorax
2. Alternate expansion and compression of air sacs occur during flight and other activities. The movement of the sternum and posterior ribs during breathing compresses the thoracic air sacs. The contraction of flight muscles distorts the furcula. Alternate distortion and recoiling compress and expand the air sacs between the two shafts of bones.
Two ventilatory cycles move air through the respiratory system or a bird. The process of respiration is divided into two cycles:
1. Cycle 1: Air moves into the abdominal air sacs during inspiration. Air is already present in the lung. This air moves into the thoracic air sacs through parabronchi. The air in the thoracic air sacs moves out of the respiratory system during expiration. Now the air in the abdominal air sacs moves into parahronchi.
2. Cycle 2: At the next inspiration, the air moves into the thoracic air sacs. It is expelled during expiration.
Comparison of respiration in birds and other tetrapods
The birds have a greater rate of oxygen consumption due to high metabolic rate. The respiratory cycle in other tetrapods is a simple back-and-forth cycle. Ventilation does not c place during expiration. Thus much “dead air– remains in the lungs. But the birds has unique system of air sacs and parabronchi.Thus, there is continuous movement ofoxygen-rich air over respiratory surfaces during both inspiration and expiration. The quantity of “dead-air“ in the lungs is much lesser than other vertebrates.
The respiratory system of birds is more efficient than other tetrapod. It supports high in metabolic rates. Similarly, the oxygen tension is low at high altitudes.
The birds live and fly at high altitudes due to this efficient gas exchange system. Thus, birds geese fly can reach the Himalayas at 9, 200 m during migrations.
||Other Tetrapods(Amphibians, Reptile and mammals)
|The birds have a greater rate of oxygen consumption due to high metabolic rate.
Birds have one way flow of gases. Ventilation does not take place during expiration.
Parabrochi are present.
Alveoli are absent.
The birds have unique system of air sacs and parabronchi. Thus there is continuous movement of oxvgen-rich air over respiratory surfaces during both inspiration and expiration. The quantity of “dead air– in the lungs is much lesser than other vertebrates.
The respiratory system of birds is more efficient than tetrapod. It supports high metabolic rates. Similarly, the oxygen tension is low at high altitudes.
|Tetrapods have comparatively lesser metabolic rate.
The respiratory cycle in other tetrapods is a simple back-and-forth cycle.
Parabrochi are absent.
Alveoli are present.
Much dead air remains in the lungs.
Their respiratory system is not much efficient.
Birds maintain body temperatures between 38o and 45oC. Lethal temperaturefor birds are lower than 32oCand higher than 47° C. There are following adaptations for thertnoregulation in birds:
1. Resting bird fluffs its feathers in a cold day. It increases their insulating properties and dead air space within them.
2. Birds also tucks its bill into its feathers. It reduces heat loss from the respiratory tract.
3. The most exposed parts of a bird are the feet and tarsi. They are not covered by flesh n muscles. They are also not supplied with rich blood supply. Temperatures in these structures can drop near freezing point. It prevents heat loss. There is a countercurrent heat exchange between the arm blood flowing to the legs and feet and the cooler blood flowing to the body from the legs and feet. It prevents excesive heat loss at the feet. Thus heat is returned to the body.
4. Shivering also generates heat in extreme cold.
5. Increases in metabolism during winter months require additional food. Some birds become torpid. Their body temperatures drop on cool nights. For example, the body temperatures of whippoorwill drop to 16 C from 40 C. Thus its respiratory rate become very slow.
6. Muscular activity during flight produces large quantities of heat. Birds can dissipate this heathy panting.
7. The vascular membranes from the floor of the mouth we fluttered. It increases the evaporative heat loss.
NERVOUS AND SENSORY SYSTEMS
There are many sensory adaptations in birds.
1. Fore brain: The forebrain of birds is much larger than reptiles. Birds have large cerebral hemispheres and corpus striatum. Corpus striatum is a region of gray matter.The corpus striatum functions in visual learning. feeding, courtship and nesting. A pineal body is present on the roof of the forebrain. It stimulates ovarian development and regulates other functions controlled by light and dark periods. The optic tectum (the roof of the midbrain) and corpus striatum integrates sensory functions.
2. Mid brain: The midbrain also receives sensory input from the eye.
3. Hind brain: The hindbrain includes the cerebellum and medulla oblongata. Medulla oblongata coordinates motor activities. It regulates heart and respiratory rates.
Vision is an important sense for most birds. Most structures of bird eyes are similar to other vertebrates. But eyes of bird are much larger relative to body size than other vertebrates. Their eyes are flattened in an anteroposterior direction. The eyes of birds of prey has e a bulging cornea. Therefore, their eyes protrude anteriorly. Birds have a unique double-focusing mechanism. Pad like structures control the curvature of the lens. The ciliary muscles change the curvature of the cornea. The bird collpse has double and instantaneous focusing mechanism.They remain focused on a fish and jump on it.
Ratina:The retina of birds is thick. It contains both rods and cones becomes active in loss light intensities and cones become active in high light intensities. Cones are especially concentrated (1,000,000/ mm) at fovea. Fovea is a focal point of retina. Some birds have two foveae per eye.
(a) Search fovea: One fovea is called search fovea. It is present at the center of the retina. It gives the bird a wide angle of monocular vision.
(b) Pursuit fovea: The other foveae is present at the posterior margin of the retina. It is called pursuit foveae. It functions with the posterior foveae of the other eye and form binocular vision. Binocular vision produces depth perception. It is necessary to capture prey. Other birds use the “search fovea” to observe the landscape below them during flight. They use pursuit fovea or depth perception. It is needed during landing on a branch of a tree.
Formation of vision: The position of the eyes on the head also influences the degree of binocular vision.
(a) Monocular vision: The eyes of the pigeons are located on the sides of their head. It gives them a nearly 360 monocular field. But it gives a narrow binocular field. They do not have to pursue their food. A wide monocular field of view keeps them alert from predators during feeding on the ground.
(b) Binocular vision: Theeyes of the hawks and is are further forward on the head. This increases their binocular field of view and decreases their monocular field of view.
Olfaction plays a minor role in the lives of most birds. The olfactory epithelium is poorly developed.
External nares open near the base of the beak. But turkey vultures have well developed sense of olfaction. It locates their dead and dying prey largely by smell.
Most birds have well-developed hearing. Loose, delicate feathers called auriculars cover the external ear opening. Middle ear and inner ear are similar to reptiles. The sensitivity of the avian ear (100 to 15,000 Hz) is similar to the human ear ( 16 to 20,000 Hz)
EXCRETION AND OSMOREGULATION
Birds and reptiles face similar excretory and osmoregulatory problems. Birds excrete uric acid. It is temporarily stored in the cloaca. Water is also reabsorbed in the cloaca. The excretion or uric acid conserves water and promotes embryo development in terrestrial environments.
Some birds have supraorhital salt glands.These glands remove excess sodium chlorlde from the body through the nasal openings. These glands are especially important in marine birds. These birds drink seawater and feed on invertebrates containing large quantities of salt in their tissues. Salt glands can secrete salt in a solution. This solution is two to three times more concentrated than other body fluids. Thus salt glands compensate for the inability of kidney to remove concentrate salts in the urine.
REPRODUCTION AND DEVELOPMENT
The sexual activities of birds include establishing territories finding mates, constructing nests, incubating eggs, and feeding young. All birds are oviparous. Their gonads are present in the dorsal abdominal region, next to the kidney.
Male reproductive organs
Testes are paired. Coiled tubules vasa differentia transfers sperm to the cloaca. The vasa differentia enlarges to form seminal vesicle. It stores sperms temporarily and maturation sperm occurs before mating. Testes are enlarged during the breeding season. Birds have no intermittent organ (copulatory organ). Males mount on female and the sperm are transferred by cloaca contact.
Female reproductive organs
Two ovaries are formed during development in females. But only the left ovary fully develops. A large funnel-shaped opening (the ostium ) or the oviduct envelops the ovary. It receives eggs after ovulation. The eggs are fertilized in the upper portions of the oviduct. The albumen is secreted by glandular regions of the oviduct. Albumen surrounds zygote. A shell gland present in the lower region or the oviduct. It secretes shell around the egg.The oviduct opens into the cloaca. •
Many birds establish territories prior to mating. The size and function of territory vary greatly among species. Territories allow birds to mate without interference. They provide nest locations and sometimes food resources for adults and offspring. Breeding birds defend their territories. They expel intruders of the same sex and species. Threats arc common. But actual fightis minimum.
The mate is attracted to its territory during mating. Different birds have different methods attraction of females:
1. Male woodpeckers drum on trees to attract females.
2. Male ruffed grouse fan their wings on logs. Its sound can be heard from many miles.
3. Cranes have a courtship dance. This dance includes stepping, bowing, stretching and jumping displays. The female gives readiness signals and mating starts. Mating occurs quickly but repeatedly. Thus all the eggs are fertilized.
Most birds are monogamous. A single male pairs with a single female during the breeding season. Some birds (swans, geese, eagles) pair for life. Frequent mating strengthens the pair bonds. Monogamy is common when resources are widely and evenly distributed. Thus one bird cannot control all these resources.
Monogamy has also advantages. Both parents participate in nest building and care of the young. One parent incubates and protects the eggs or chicks. The other parent searches food.
Some birds are polygynous. Males mate with more Man one female. The females care for the eggs and chicks. Polygyny occurs in species whose young are less dependent at hatching. Sometimes, the resource are scattered in patch. It attracts many females to a small breeding area. Prairie chickens are polygnous. The males display groups called leks. The males in the center positions are preferred and attract the majority of females.
A few bird species are polyandrous. In this case, the females mate with more than one male. Female in spotted sandpipers birds are larger than males. The female establish her territories. She defends her territory from other females. She lays eggs for each male. The male builds a nest in the territory. If a male loses his eggs to a predator. the female replaces them. Polyandry produces more eggs than monogamous mating. It has advantages when food is plentiful but the chances of successfully rearing young are low due to predation.
The nesting behavior of birds is species specific. Some birds choose nest sites away from other members of their species. Some birds nest in large flocks. Unfortunately, nesting behaviour is the cause of extinction of some species of birds.
Nest construction begins after pair formation. The female initiates this instinctive behavior. A few birds do not make nests. Emperor penguins breed on the Snow and ice of Antarctica. Thus no nest materials are available. Their single egg is incubated on the web of the foot.The foot has a fold of abdominal skin.
Caring of eggs and hatching
The group of eggs laid and chicks produced by a female is called a clutch. Different species have different clutch size.
(a) Most birds incubate their eggs. Some birds have a featherless. vascularized incubation or brood patch. Brood patch keep the eggs at temperatures between 33 and 37″ C. Birds often turn the eggs. It prevents the attachment of egg membranes. Thus it prevents the deforming of the embryo.
(b) Some birds spray cool water on the eggs. It makes the eggs humid.
(c) The Egyptian plover carries water in the breast feathers. The incubation period lasts between 10 and 80 days.
Hatching: The young-bird develops an air sac one or two days before hatching. This air sac penetrates into the blunt end of its egg. It inflates lungs of young and it starts hatching. The young bird pecks the shell and hatching occurs.
Caring of young
There can be two types of young:
1. Artificial young
Some birds are helpless at hatching. They depend on their parents for hatching. The young are entirely dependent on their parents are called altricial. They are often naked at hatching. Endothermy is not developed at start in altricial young. Thus they are brooded constantly at first. They gross rapidly. When they leave the nest, they are nearly as large as their parents.
2. Precocial young
Precocial young are alert and lively at hatching. They are covered with down feathers. They can walk, run, swim, and feed themselves. Usually, one parent is present to lead the young to food and shelter.
Feeding of young
Young altricial birds have huge appetites. One or both parents continually search food for it. The young may consume a mass of food that equals their own weight each day. Adults bring food to the nest Or they regurgitate food stored in the crop or esophagus. The adults produce vocal signals or color patterns on the bills or throats. It initates feeding responses in the young. Parents feed gaping mouths. Many young have brightly colored mouth linings or spots. It attracts parent’s attention. The first-hatched young have large size. It can stretch its neck higher. Therefore, it is given food first
Life span of birds
Life is brief for birds. About 50% of young leave the nest. Remaining birds died. Most bird have life span of 10 to 20 years in captivity. But this life span is shorter in natural conditions. The average American robin lives 1.3 years. The average black-capped chickadee lives less than 1 year. Mortality is high in the first year due to preditors and adverse weather.
MIGRATION AND NAVIGATION
The periodic round trips of birds between breeding and non-breeding areas are called migration. Most migrations are annual. They have nesting areas in northern regions and wintering grounds in the south. 70% of the earth’s land is in the Northern Hemisphere. Therefore, Migration is more important for species found in the Northern Hemisphere. Migrations involve east/west movements or altitude changes. Migration allows birds to leave the climatic extremes. Thus they secure adequate food, shelter, and space throughout the year.
Stimulus for migration
Birds migrate in response to species-specific physiological conditions. Following factors stimulate the migrations
1. Innate (genetic) clocks and environmental factors influence the migration.
2. The photoperiod is an important migratory sign for many birds.
3. The changing photoperiod starts seasonal changes in development of gonads. These changes act as migratory stimuli.
4. Increasing day length in the spring promotes development of gonads. The decreasing day length in the fall initiates regression of gonads. The changing photoperiod also promote fat deposition in many birds. This fat acts as an energy reserve for migration.
5. It is believed that the anterior lobe of the pituitary gland and the pineal body are involved in photoperiod responses.
Mechanism of migration
The mechanics of migration are species specific. Some birds are long- distance migrants. ‘[hey store fat equal to 50% of their body weight. They make non-stop journeys. Other species start migration slowly. They begin their journeys early and stop frequently for feeding and resting. Many birds can fly at altitudes greater than 1,000 m. Thus do not hit the tall obstacles. Many birds have very specific migration routes.
Birds use two forms of navigation:
1. Rout based navigation
It involves following the track of landmarks during an outward journey . Those landmarks are used in a return trip.
2. Location based navigation
In this case direction of the destination is followed from the information available at site of start of the journey. It involves the use of sun compasses and other celestial . The magnetic field of the earth is also followed.
(i) Sun compass
The lenses of birds are transparent or ultraviolet light. Their photoreceptors respond to ultraviolet radiations. Thus they orient themselves according to position of sun. This orientation sign is called a sun compass. The sun movesbetween sunrise and sunset. Birds use internal clocks to sense the sun rises in the east. The sun is overhead at noon. It sets in the West.
The biological clocks of migratory birds can be changed. In an experiment birds ready for northward migration is held in a laboratory. The laboratory sunrise occur later than the natural sunrise. After that they are released to natural light conditions. They fly in a direction they sense to be north. But it is really northwest. Night migratory birds can also use the sun. They fly in the proper direction from the sunset.
Celestial cues other than the sun are also used for navigation. The North Star lines up with the axis of rotation of the earth in the Northern Hemisphere. The angle between the North Star and the horizon decreases as we move toward the equator. The latitudes of the earth can be determined by this method. Birds also use similar information to determine latitude.
(i) Magnetic compass
Some zoologists believe that the birds use magnetic compasses to detect the magnetic field the earth. They determine their direction from this magnetic field. If the magnets are strapped to the heads of pigeons, it disorient the birds. European robins and the garden warbler use the earth’s magnetic field for orientation. But magnetic receptors are not found in any of the birds. There were early reports that magnetic iron and magnetite a e present in the head and necks of pigeons. But it does not help in greater understanding of magnetic compasses. Further experiments can not prove the magnetic properties in tl ese regions. Magnetic iron is also found in bacteria and other animal tissues. But they not dead associated with a magnetic sense. Some zoologists believe that he pineal body of pigeons use the sun compass in responses to magnetic fields.
Many navigational mechanisms are present in the birds. It suggests that birds use d Ilerent sources of information in different circumstances.