In this article we will discuss about Birds:- 1. Characteristic Features of Birds 2. Habit and Habitat of Birds 3. Structure 4. Digestive System 5. Respiratory System and Sound Producing Organ 6. Circulatory System 7. Nervous System and Sense Organs 8. Urinogenital System.
Contents:
- Characteristic Features of Birds
- Habit and Habitat of Birds .
- Structure of Birds
- Digestive System of Birds
- Respiratory System and Sound Producing Organ in Birds
- Circulatory System of Birds
- Nervous System and Sense Organs in Birds
- Urinogenital System of Birds
1. Characteristic Features of Birds:
One of the most characteristic features of avian life is the ability of flight in most of the forms. They are noted for their great uniformity of structures. All of them have spindle-shaped streamlined body to offer least possible resistance during forward progression. The power of flight has caused greatest changes in their biological organisation.
Most of them are essentially double adapted forms, the anterior parts of the body are modified for flight while the posterior extremities have changed for movement on land. All birds are bipedal forms having well-developed legs. The pelvis and the posterior portion of the vertebral column is greatly modified for this purpose.
The union between the pelvis and the vertebral column are very strong and extensive. The fusion of the posterior vertebrae to form a rigid bony piece has added additional mechanical strength. The neck is long, mobile and supports a round head with a projecting characteristic beak.
Although the basic organisation is fairly uniform, different forms of birds exhibit a great variety of specialised features which have enabled them to adapt themselves in numerous habitats.
Besides differences in body form, behaviour, power of flight, feeding habits, modifications of the beaks, feet/ wings and many other parts have been encountered. These modifications are manifestations of adaptation in varying environment.
2. Habit and Habitat of Birds:
Birds enjoy a wide range of distribution and occupy all continents, the seas and islands, the Arctic and Antarctic regions and extend even above 6-1 km from sea level to the highest peak of mountain range. In spite of their ability to fly in air, the birds conform to the principles of animal distribution, i.e., each species of bird occupies a definite geographical range and enjoys particular habitat.
Some forms like albatrosses live on open seas except nestlings; gulls, murres and auks (extinct) lived near seacoast; woodpecker, creepers and nut-catchers make the trees their homes. Birds are also noted for their diversities in food habits and locomotory methods. Many birds capture insects as food while forms like hawks soar for hunting.
Most of the tree-dwelling birds use the crevices or holes of the trees as the nests, while others prepare nests for nesting and rest. Insect-eating birds and birds of prey usually live and hunt alone; robins, quail and juncos live in flocks, but segregate in pairs during breeding season; and birds like pigeons, blackbirds, seafowls always remain in companies.
Size of Birds:
Birds vary extensively in body size. Ostrich of Africa is about 2-1 m tall weighing up to 136-1 kg. Helena’s humming-bird of Cuba is about 6-3 cm long and weighs about 2-8 gm.
Speed of Birds:
Considerable controversies exist on the issue—how fast do birds fly? The speed capacities of birds depend on several factors, viz., the wind, the angle of bird’s flight, etc. John H. Storer, a leading worker on the aerodynamics of birds, has put forward the following data on the speed of certain species of birds.
They are:
3. Structure of Birds:
Skin:
The skin of birds is thin, loose and devoid of any skin gland. The uropygial gland is the only skin gland in birds which may be lacking in some forms. This gland is well-formed in aquatic birds. The uropygial gland is a bilobular structure situated on the dorsal side of the uropygium. It is a simple branched saccular oil gland. In chick it is composed of two lobes which open separately (Fig. 9.44). A septum separates the two lobes.
In some birds this gland produces several ducts. It has been suggested that the secretion of this gland makes the feathers waterproof or produces characteristic recognition scent or produces ergo sterol which is transformed into vitamin D by exposure to sunlight.
Nerve-endings are abundantly present in the skin. Besides the uropygial gland, certain modified oil glands are present around external ear opening in gallinaceous birds.
The feathers and the scales are the major exoskeletal structures of the birds. The beak and the claws are also specialised scale-like structures. The keratin-producing capacities of skin are mostly devoted to the production of feathers—the most important exoskeletal structures in avian morphology.
Feathers:
Many ornithologists classified feathers into different types according to their structure and position.
Jollie (1962) classified feathers into two types:
(i) Plumes and
(ii) Plumules or down feathers including filo plumes.
Hyman (1942), Walter and Sayles (1949), Weichert and Persch (1977) and Kent and Miller (1997) have mentioned about three types of feathers. Hyman (1942) mentioned (i) down feathers or plumules (ii) contour feathers and (iii) hair feathers or filoplumes.
Walter and Sayles (1949) classified feathers into (i) quill, (ii) down and (iii) pin feathers. They divided quill feathers into three types – (i) tail, (ii) wing and (iii) contour feathers.
Down feathers are of two types – (i) powder down and (ii) nestling down. Kent and Miller (1997) classified feathers which are as Hyman. Pough, McFarland et at. (1990) classified into 5 types – such as (i) contour feathers (ii) filo- plume, (iii) down feathers, (iv) semiplume and (v) bristles.
In modern birds, the feathers cover the body. The feathers play the role of heat regulation, help in flight and exhibit protective colouration and sexual display. The feathers, except penguins and ratites, are arranged in definite feather-tracts (pterylae) separated by featherless areas (apteria). Many birds have their body covered by temporary feathers (nestling-down) at the time of hatching.
The nestling-down feathers assumes various forms and is replaced by permanent feathers. In some cases, the nestling-down feathers may be present in adult.
The arrangement and types of feather are important in classification. In ratites, the apteria are distinct in youngs which are obliterated in adults when the body gets a uniform covering of feathers. The barbs of the feathers are free, i.e., the barbs lack hooking apparatus. The remiges and rectrices are well-formed in flying forms. The rectrices vary greatly.
These are almost absent in birds living near the ground, being well-formed in fast-moving forms. The rectrices help in steering during progression. In penguins, the remiges are degenerated, because the wings have transformed into paddles for swimming.
The feathers are regularly shed at intervals, either at a certain stage in the life-cycle or Seasonally. New feathers are produced from the old papillae. Most of the birds moult annually after the breeding season, while others moult a second time during the year.
Colouration:
Birds are specially noted for the exhibition of different colour patterns. These colour patterns form the basis of the avian social life. The colours are used for protection, as the signs of recognition and sexual stimulation.
The colour patterns vary greatly in birds according to the habits. Cryptic (concealing) Colouration is very common which renders the birds less visible. This is aided by counter- shading (with stronger lighting the upper side becomes darker than the lower side which is whitish). In most birds the colouration is a compromise between conspicuous and concealing colourations.
Usually the females show cryptic colouration, while the males exhibit conspicuous colours. Both the sexes, in some forms, acquire nuptial plumage by the prenuptial moult and return to dull colour by a postnuptial moult. There is a general tendency to match the colour of the body with the hues of the surroundings.
Birds inhabiting the arid zones become pale-coloured, while those of humid regions are darker. The cryptic colouration in birds has a selective value. It adds one or both the sexes the garments which are used for communication between individuals of the same species for pair formation, aggression between males, selection of nest site for laying the eggs and rearing the youngs.
The colours in birds are produced partly by pigments and partly by diffraction effects. Melanins are the most common pigments which are laid down in the feathers by specialised pigment cells in the feather papillae. The processes of these cells convey the pigment to the epidermal cells.
Yellow xanthophyll in the beak of ducks and red astaxanthin of pheasant wattles are carotenoid pigments which are soluble in organic solvents. The white colour is produced by reflection. Two peculiar pigments are recorded in birds.
The turacos (plantain eaters of Africa) contain two pigments:
(i) Turacin:
A copper-containing red porphyrin and
(ii) Turacoverdin:
An iron-containing green pigment. The iridescent feathers are due to interference of light in thin surface.
Skeletal Structures of Birds:
Exoskeleton:
The exoskeletal structures are well-developed in birds. The feathers constitute the major exoskeletal structures. Besides feathers, the other exoskeletal structures are the scales covering the leg and feet, claws and beak. The construction of these structures has been discussed in the biology of Columba.
The modifications of these structures in various birds are discussed below:
Modifications of beak:
The beak’ is essentially a structure to obtain food, to preen feathers, to collect nest materials, to build the nests and also to act as the organ of defence. Because of the functional diversities, the beaks have undergone extensive range of modifications in different birds.
Fig. 9.45 shows a few varieties of beak in birds. The modifications of beak are essentially adaptive in nature and the form of beak indicates the food habits.
Types of beaks:
i. Tearing and piercing beak:
The beak is strong and powerful in carnivorous birds, being slender in others. In Egyptian vulture (Fig. 9.45A), eagles and hawks the beaks are hooked to tear flesh from carcasses.
ii. Filter-feeding beak:
The beak with marginal hooks and leaflets in Flamingo forms a filter-feeding apparatus to sieve small materials from water (Fig. 9.45B).
iii. Nectar collecting beak:
In Pacific Passerine the beak is modified to collect nectar from the flower (Fig. 9.45C).
iv. Seed breaking beak:
The beak of Passerine Crossbill (Fig. 9.45D) has obliquely crossed mandibles to facilitate the breaking of seed.
v. Wood-chiseling beak:
The beak of Woodpecker (Fig. 9.45E) is strongly built and acts as a pick-axe for excavating the wood to get insects.
vi. Fish-catching beak:
The beak of Albatross (Fig. 9.45F) is large and tube-like. The beak of Pelican (Fig. 9.45G) is highly modified to catch fishes. The terminal hooks and the beak pouch are specialised devices for the purpose. The beak of European Goosander (Fig. 9.45H) possesses fish-holding denticulations. The serrations in beak of Catbird (Fig. 9.45I) act as a saw through the petioles of leaves to decorate the display-ground with the green leaves.
vii. Insect-probing beak:
The Galapagos Woodpecker-finch has a short beak to trench the tree (Fig. 9.43J) with the help of a cactus spine. The cactus spine is held by the beak to probe the barks for insects. This bird is remarkable for tool-making.
viii. Insect-catching beak:
The Frogmouth (Fig. 9.45K) has a wide gap between the upper and lower beaks to facilitate insect catching. A tuft of filamentous feathers surmounting the beak also helps in the process.
ix. Water and mud-straining beak:
The beak of ducks and geese is flat and broad. It has lateral straining devices.
Modifications of wing:
The forelimbs of all birds have modified into the wings. Essentially the wings are built on the same structural plan. As a rule, the wings are extremely reduced in ratites and secondarily flightless forms. Like that of Columba, all the three digits of the wings are clawless. But the Chauna (Crested Screamer) and common fowl and goose (abnormal occurrence) claw may be present on the first digit.
In the youngs of Hoatzin two clawed digits are present in each wing. But the ratites have detained claws in the digits. The ostriches have claws in all the digits, the rheas possess claws on the first and rarely on the second and third ones. The cassowaries, kiwis and emu have’ claw only on the second digit (see Fig. 9.50B).
Modifications of feet:
The hind limbs are principally used for movement upon the surface of the earth. The hind limbs exhibit great diversity in structure according to their mode of action. Broadly, the hind limbs are distinguished into two categories: walking feet and wading feet. The walking feet vary considerably and four toes are usually present.
The first toe is directed backward. The walking feet are completely feathered at least up to the tarsal articulation, while the wading feet have partly or completely un-feathered tibial region. The wading feet are found in aquatic birds.
The toes are usually webbed to help swimming. The walking feet become specialised for various purposes. The toes are modified for grasping the prey, for perching, for climbing and for offence.
The wading feet show different gradations as regards structure:
i) Swimming feet—having three anteriorly directed toes connected by undivided web as we see in Anas.
ii) Half-swimming feet—having the web extended up to the middle of the toes as observed in Recurvirostra.
iii) Split-swimming feet—the toes have a cutaneous border as seen in Podicipes. The border of the toes is lobed in some cases.
Fig. 9.46 shows some examples of pedal specialisation in flying birds. In Cormorant the feet are modified for swimming and defence. In Megapode, the feet are heavily built and adapted for food-getting, running and defence. The claws are sharp.
In Grebe, the feet belong to half-swimming type, where the toes are laterally lobed. The feet of Jungle-fowl of India are modified for running and scratching the soil for search of food. The spur is used for aggression and defence. The grasping feet are exemplified by the Hawk where the claws are recurved and the toes have rough raised pads to grasp the prey.
The feet are also modified for perching and defence. Similar condition is also observed in Kingfisher. Typical Passerine feet are modified for perching, hopping and defence. In Weavers and some other Passerine birds, the feet are employed to weave the nestling materials. The feet of Sand-grouse are peculiar. They help in thermo-regulation in addition to the normal functions of running and defence.
The feet of Swift are modified to cling to the vertical surface of the trees or walls by curved claws. Similar modifications are observed in the feet of Woodpecker. In Jacanas, the modification of the toes is remarkable. The toes are un-webbed, but extremely elongated and clawed to ski over unstable floating leaves of water-lilies.
In some birds, e.g., owls, nightjars and some aquatic forms like bitterns, cormorants, gannets, one of the claws in each foot becomes pectinated. This comb-like feather-cleansing structure is highly developed in bitterns.
In moth-eating nightjars, these pectinated claws help to clear moth scales from the tactile feathers round the mouth. The pectinated toilet claws are adaptive features and are encountered in unrelated forms. It occurs in species that feed on mucus- coated aquatic or semi-aquatic organisms.
Endoskeleton:
The endo-skeletal framework is greatly modified in birds for carrying the weight of the body in two distinct ways—during flight on the wings and during rest on the legs. For this purpose, the axial as well as the appendicular skeletons have become extremely altered.
Skull:
This axis of bird is strikingly shorter than any other vertebrate. But the neck remains long in most cases. Avian skull is essentially uniform throughout the class.
The following features characterize the avian skull:
(i) The bones of the skull are fused together resulting in the obliteration of the sutures. These sutures are present in the skull of most ratites.
(ii) The facial portion is prolonged into a beak composed mainly of large triradiate premaxillae.
(iii) The occipital condyle is single and round. The condyle is formed by the basioccipital.
(iv) A spacious round brain-case,
(v) The orbits are large except in Apteryx. The orbits are separated by a thin inter-orbital septum formed anteriorly by the mesethmoid which is continued anteriorly with the cartilaginous internasal septum,
(vi) Rod-like jugal and quadratojugal reach the quadrate posteriorly,
(vii) The quadrate is freely articulated,
(viii) The infra-orbital arcade is complete, while the supra-temporal arcade is mostly incomplete,
(ix) A large parasphenoidal rostrum is present,
(x) Pre-frontals and post- frontals are absent and the orbit is not closed from the temporal fossa. But in parrots, a backwardly directed process of the lacrymal meets the postorbital process of the frontal beneath the orbit,
(xi) The squamosal is closely united with the skull,
(xii) Secondary palate is absent.
These are some of the characteristics which are almost universal in most of the forms. But a detailed study reveals many differences, specially of the arrangements of the palatal bones. Four distinct broad categories of avian skull are observed.
These are:
a. Dromaeognathous skull:
This type of skull is seen in ratites and Tinamus. The basipterygoid processes are large which spring from the basis phenoid to articulate with the posterior ends of the pterygoids. The vomer is large and broad (Fig. 9.47 A). The palatines do not articulate with the rostrum. The maxillopalatine processes are small and do not unite with one another.
b. Schizognathous skull:
This type of skull is found in gulls, penguins, fowls, pigeon, etc. The vomer is small and pointed in front. But in pigeons, the vomer is absent. The basipterygoid processes may be absent or spring from the base of rostrum. The maxillopalatines do not unite with one another and the vomer is short, pointed or absent. Fig. 9.48 shows the details of a typical schizognathous type of skull.
c. Aegithognathous skull:
In passerines and swifts, the skull is almost similar to the schizognathous type, but the vomer is broad and truncated in front. The skull of crow shows the typical condition.
d. Desmognathous skull:
The vomer is small and the maxillopalatines are large and spongy (Fig. 9.49). These bones unite with one another in the middle line ventral to the vomer. This type of skull is found in storks, birds of prey, ducks, geese, parrots, etc. In parrots (Fig. 9.50) the skull is peculiar by having movable upper beak. A true cranifacial hinge is present between the skull and upper beak.
Vertebral column:
The vertebral column is highly modified in birds to offer strength and rigidity. The cervical and thoracic vertebrae have heterocoelous centra for universal mobility of the neck. The number of different vertebrae is variable in different birds. The number of cervical vertebrae ranges from nine (in songbirds) to twenty-five (in swans).
The thoracic vertebrae become more or less fused to form rigid thoracic basket. The thoracic vertebrae are opisthocoelous in penguins, gulls, cormorants, stone-curlews and godwits. The posterior cervical vertebrae and anterior thoracic vertebrae bear hypophyses for the attachment of neck muscles. The posterior vertebrae become fused in all the birds to give mechanical strength.
The synsacrum is formed by the fusion of posterior thoracic, lumbar, sacral and anterior caudal vertebrae. The number of sacral vertebrae range from one to five. In all living birds, except some ratites, a laterally compressed pygostyle is present. This ploughshare-shaped bony plate is formed by the fusion of four to six caudal vertebrae.
The centra of the movable vertebrae in birds are separated by synovial cavities. Each such cavity is traversed by fibrocartilage, called meniscus which bears a pore through which passes a fibrous cord—the remnant of the embryonic notochord.
Ribs:
The thoracic vertebrae carry ribs. Each rib is double-headed. The capitulum is attached to the centrum and the tuberculum joins with the transverse process. Each rib consists of a sternal portion and a vertebral portion. These two portions are united by synovial joint. The vertebral portions of ribs possess backwardly directed uncinate processes.
Sternum:
The sternum is a broad plate covering the ventral side of thorax and part of abdomen. In cairnates, the sternum bears on its ventral side an anteroposterior keel for the attachment of the pectoral muscles.
In ratites, the keel is either absent or represented as a vestige and the sternum thus appears raft-like. In many carinates, the keel becomes reduced and small as in Notornis, Gallirallus and is almost absent in Didus, Strigops, Hesperornis, etc.
The anterior margin of the sternum produces anteroposterior processes, while the posterolateral part is usually fenestrated or shows deep notches. The development of keel in birds is correlated with the power of flight, it is well-developed in flying forms and absent or reduced in flightless members. So the absence of keel in birds is purely adaptive in nature having no phylogenetic significance.
Girdles and limbs:
Both the girdles and limbs have become greatly modified for flying and walking.
The pectoral girdle exhibits adaptive divergence in ratites and non-ratites. In ratites, the scapula and coracoid are small and fused with one another. But in non-ratites the scapula and coracoid are large and are united by ligament. In these forms the scapula bears an acromian and the coracoid has an acrocoracoid process.
Both these processes are either absent or extremely reduced in ratites. The coracoscapular angle is usually less than 90° in non-ratites, but in ratites this angle approaches two right-angles. Another peculiarity is the absence of furcula in ratites.
In most of the non-ratites the furcula (probably consisting of clavicles and interclavicles) is loosely attached with the anterior end of the sternum. The skeleton of the wing becomes greatly altered from the typical vertebrate plan. Fusion of terminal bones to limit the movable joints has increased the efficiency of the wings.
In ratites and other flightless birds the structure of the wing has become reduced. In penguins, due to the transformation of the wings into swimming paddles, the bones have become flattened and a sesamoid bone, called patella ulnaris, replaces the olecranon process. The wing bones are reduced in Hesperornis, but in Moas there is no trace of wing skeleton.
The pelvic girdle and legs have become specialised for standing, walking, perching and swimming. The pelvic girdle has well- developed expanded ilia which are attached to the sacrum. The ischia are directed backward. The pubes are slender bones. In Apteryx, the pubis and ischium are entirely free, while, in cassowaries and emu, the pubis and ischium are united with the posterior end of the ilium.
The pubes and ischia remain separate in most birds but, in ostrich, the pubes are united by symphysis, while in Rhea, there is an ischiatic symphysis. The ischia, pubes and ilia participate in the formation of acetabulum which is perforated. The pubis in Apteryx is peculiar by having a forwardly directed pectinal process.
The bones of the hind limbs exhibit less diversity in different birds except penguins where the tarsometatarsus is short and broad. Three metatarsals composing each tarsometatarsus are easily distinguishable from outside. There are four digits in the leg of birds. No bird possesses the fifth digit in adult. A vestige of the metatarsal of the fifth digit is found in the embryo of some birds.
The endoskeleton has become modified mostly for aerial movement. The bones have become light due to the extension of the air-sacs in majority of the birds except some diving forms. Pneumacity of the bones shows regional specificity.
In most of the birds all the bones are pneumatic except forearm, hand, shank and foot. In penguins, Apteryx and many song-birds only the skull bones are pneumatic to make the skull light. In birds the bones have become pneumatic without reducing the mechanical strength of the bones.
4. Digestive System of Birds:
Despite great diversities in the mode of feeding, the digestive system in birds shows uniformity throughout the class. The mouth is bounded by upper and lower jaws which are covered by rhamphotheca to form the beak. The rhamphotheca is mostly simple, while in ratites and Albatross it is compound.
The beak has assumed extensive variations which are correlated to the feeding habits of the birds. Teeth are absent in all modern birds. The tongue is long and thin. It contains taste-buds and mucous glands. There are three salivary glands—one median sublingual and two angular glands which usually produce saliva to moisten the food. In graminivorous birds, the saliva contains diastatic enzyme.
The buccal cavity is usually small in birds, but in some forms, specially pelicans, the floor of the buccal cavity is dilated to form a large gular pouch to store fishes. The pharynx leads into a wide oesophagus, which produces a large receptacle (crop) in grain- eating birds.
The crop stores A large quantity of food and produces ‘crop-milk’ during breeding season. The oesophagus leads into the stomach which is divided into a tubular glandular proventriculus and a muscular gizzard. The gizzard is highly muscular in grain-eating forms, while in carnivorous forms it is simple and assumes the characters of normal stomach.
The duodenum and the coiled intestine are short in carnivorous birds, but in grain-eating varieties these are elongated. The duodenum receives the pancreatic and bile ducts. The gall-bladder is present in most birds, but in a few forms (as in Columba) the gall-bladder is totally lacking. Two small blind caeca are present at the junction of the ileum and rectum.
The caeca play two functions: digestion of vegetable fibres by bacterial and/or enzymatic action and help in the absorption of water. The elaboration of the cloacal chamber into the coprodaeum, urodaeum and proctodaeum helps in reabsorption of water. The bursa fabricii (‘cloacal thymus’) opening into the proctodaeum helps the young birds to protect against local infection. It becomes extremely reduced or atrophied in adult.
5. Respiratory System and Sound Producing Organ in Birds:
The respiratory system is remarkably modified to provide large supply of oxygen necessary for active metabolism. The lungs are proportionately small in size and slightly distensible. The functional efficiency of lungs is enhanced largely by the development of air-sacs. The air-sacs play an important role in the life of birds. These sacs become greatly reduced in ratites and other flightless forms.
The glottis is slit-like and situated posterior to the root of tongue. The glottis leads into larynx which cannot produce any sound. The sound is produced by a specialised apparatus, called syrinx or lower larynx.
This apparatus is simply constructed in many birds, but the associated muscles become very complicated in the ‘song-birds’ (order Passeriformes). Most birds can produce calls and songs. The parrots, magpies, mocking-birds, moynas and many others possess the inherent capacity of mimicry.
The avian notes are used:
(i) To convey different ‘ideas’ to their fellows;
(ii) To assemble the individuals of the gregarious species;
(iii) To attract the mates in chosen territory;
(iv) To warn at the time of danger; and
(v) For reciprocal directional calls between youngs and parents.
Birds Singing:
The physiology and acoustics of bird vocalization are unique features in animal kingdom. Controversies exist regarding the physiological processes employed by a song bird. The vocal organ of bird is the syrinx situated at the region where the two bronchi unite to form the trachea. The membranes and the associated musculature and the air-sacs help in the process.
When a song bird undertakes singing, it closes the valve situated between the lung and the syrinx. Then the bird starts compressing the air contained in the air-sacs. Pressure in the clavicular air-sac surrounding the syrinx forces the internal tympani form membranes into the bronchial passage to close it momentarily.
Tension is then applied to syringeal musculature to withdraw the bulged membrane from the opposite bronchial passage. This act creates a passage through the bronchial tube. The song is produced when the air streaming the passage stimulates the tensed membrane to vibrate.
When one of the two voices is utilised, no tension is applied to the other membrane and the bronchial passage remains closed. When a duct is sung, both the membranes remain under tension. As a consequence, two air streams are produced and two vibrating membranes are employed to produce two simultaneous sounds.
Thus sound is produced in an air stream at the syringeal apparatus. The sound is modulated by elastic membrane vibrating in a restricted bronchial passage. The syrinx can produce two notes simultaneously. The sounds thus generated can be modulated in frequency or in amplitude or both with considerable rapidity.
6. Circulatory System of Birds:
The circulatory system is very efficient to allow birds a high rate of metabolism and a high but constant body temperature. The heart is specially noted by having efficient and powerful forwarding chambers. Complete separation of oxygenated and deoxygenated blood yields a high arteriolar pressure allowing essential materials to reach the different tissues quickly.
The size of the heart and the rate of heart beat depend on the size and activity of the birds. Usually, the larger birds have smaller heart with lower rate of heart beat and this condition is usually reverse in smaller varieties. The rate of heart beat is nearly 500 in sparrow, 300 in hen and less than 100 per minute in a turkey.
The arteries, specially supplying the wings, are enormously developed. Another characteristic feature of the circulatory system is the considerable reduction of the renal portal system.
The red blood corpuscles are oval, nucleated and carry large amount of haemoglobin. The functional efficiency of the RBC in birds is higher than that in mammals. In birds, the BMR (Basal Metabolic Rate) and body temperature are considerably higher than that of mammals.
The body temperature of birds is usually 42°C, reaching nearly to 45°C in some cases. The physiological mechanism of the regulation of constant body temperature in birds is difficult to interpret. The role of air-sacs in the temperature regulation is significant. The air-sacs can conserve heat and can also lose heat by ventilation, when necessary. Loss of heat in birds is also minimised by having avascular extremities.
7. Nervous System and Sense Organs in Birds:
The brain and the sense organs agree in all essential features with those of Columba already discussed. The eyes show great variation in size. In hunting birds, the eyes are proportionately larger in size. The retina of diurnal birds is composed largely of cones.
Nocturnal birds have mainly the rods in the retinal layer. The pit-like fovea exhibits modifications in some cases. The Kingfishers possess double foveae. During diving under water for a catch, the image is transferred from one fovea to the other.
Another peculiarity of avian eye is the presence of a pleated projection, called pecten, emerging from the blind spot. The pecten is small in nocturnal forms and largest in farsighted diurnal hunters. In spite of diverse opinions regarding the physiological role of this structure, no general agreement exists on this issue. Possibly this structure provides nutrition to the retina.
8. Urinogenital System of Birds:
The structure and function of kidneys are similar throughout the class. The glomeruli show the general trend of reduction. In sea- birds — particularly the penguins and cormorants — bilateral nasal glands are suggested as additional salt-excreting organs. These glands help in extra-renal excretion of sodium chloride form the body.
Fertilization is internal and all birds lay eggs. In ratites, anseriformes and some other birds a penis to assist in internal fertilization is present. The eggs are incubated for the growth of the embryo.
The incubation period varies in different birds. The young may be precocial or altricial. The absence of right ovary and oviduct in many birds is a peculiar occurrence in avian anatomy. Some birds (e.g., hawks) have both the ovaries functional.
Reproduction in birds is usually a seasonal phenomenon. The gonads increase in size and become functional during the breeding period which is controlled by the endocrine system. Many species exhibit courting performances during breeding season. A few land birds lay eggs on bare rocks or ground and others build nest for the eggs and youngs.