In this article we will discuss about Pigeons:- 1. Characteristic Features in Pigeons 2. Distribution of Pigeons 3. Habits and Habitat 4. Structure 5 . Locomotion 6. Sense Organs.
Contents:
- Characteristic Features in Pigeons
- Distribution of Pigeons
- Habits and Habitat of Pigeons
- Structure of Pigeons
- Locomotion in Pigeons
- Sense Organs in Pigeons
1. Characteristic Features in Pigeons:
The pigeons are the typical representative of flying birds and are known both as wild and domesticated forms. They exhibit most of the peculiar characters of birds and are available in large numbers.
They are scientifically known as Columba livia domestica and belong to the Order Columbiformes. The other pigeons are wood pigeons (Columba palumbus), stock dove (Columba oenas), parrot pigeons (Treron abyssinica), crowned pigeons (Coura sp.) and tooth billed pigeon (Didunculus strigirostris).
Systematic Position:
[According to J. Z. Young, 1981]
Phylum:
Chordata
Subphylum:
Vertebrata
Superclass:
Cnathostomata
Class Aves
Subclass:
Neornithes
Superorder:
Neognathae
Order:
Columbiformes
Family
Columbidae
Scientific Name Columba:
livia Gmelin
[Blue Rock Pigeon]
Vernacular Names:
Bengali—Payra, Gola payra; Hindi— Kabutar; Telegu—Cudi pavnai; Tamil—Malai pura, Mada pura; Malayalam—Kutta pravu; Marathi—Parva; Gujarati—Parevun.
2. Distribution of Pigeons:
The pigeons are seen almost throughout the world, specially in tropical and temperate zones. About ten species of pigeons are found in India. They are:
(i) Common Green Pigeon, Treron phoenicoptera (throughout India),
(ii) Orange breasted Green Pigeon, Treron bicincta (throughout India, mainly in wet, evergreen and moist deciduous forests),
(iii) Pintailed Green Pigeon, Treron apicauda (Lower Himalayas),
(iv) Wedge tailed Green Pigeon, Treron sphenura (Himalayan foot hills),
(v) Grey fronted Green Pigeon, Treron pompadora (South Western India),
(vi) Green Imperial Pigeon, Ducula aenea (N. E. India, eastern parts of Bihar and U. P., Orissa, A. P., Western Ghats, Andaman and Nicobar Islands),
(vii) Maroon backed Imperial Pigeon, Ducula badia (Tamil Nadu, Kerala, Karnataka and East Himalayan Duars),
(viii) Purple wood Pigeon, Columba punicea (West Bengal, Orissa, M.P., Bihar and N. E. India),
(ix) Nilgiri Wood Pigeon, Columba elphinstonii (Kerala to Bombay), and
(x) Blue Rock Pigeon, Columba livia (throughout India).
Blue rock pigeons are found in Pakistan, Bangladesh, Sri Lanka and Myanmar in Asia and also found in Europe and North Africa.
3. Habits and Habitat of Pigeons:
Both the wild and domesticated forms exhibit social behaviour. They fly in flocks and roost together. Usually these birds start the day with a chorus of humming notes. In the morning and afternoon, entire flock flies to search food and water.
Pigeons drink water by drawing long draughts. Such drinking is unusual in other birds, which usually sip water. It returns to its perching place during the hottest part of the day and retires at night.
Habitat:
In wild state, the blue rock pigeon lives in rocky hills and cliffs. In semi-domesticated condition they are seen to live as a commensal of man. The semi-feral stock is seen to nest in most railway stations, forts, old unused buildings, grain warehouses, temples and in their favorite haunts.
Food:
Most pigeons are grain or seed eaters (graminivorous), and some are frugivorous. Blue rock pigeons eat cereals, pulses and groundnuts
Courtship:
The pigeons are monogamous and partnership lasts for entire life. Fighting rarely occurs, though threatening posture during mating season is often displayed.
Nest building:
Nest building commences after courtship. The season of nesting varies in different species. Blue rock pigeons nest throughout the year.
The pigeon’s nest appears to be crude and disorderly arranged, but is tough enough to face the hazards of wind and rain. The nesting materials include sticks and twigs.
Eggs and Incubation:
Each clutch includes either single or two bluish-white eggs and the parents alternately sit over the eggs. Such incubation continues for seventeen days.
Nestlings:
Newly hatched nestlings are helpless, blind and without feathers. The parents provide them first with a special kind of ‘milk’ and then with partly digested seeds and grains. The feathers appear after 10th day of hatching but the young pigeons continue to stay for a month with the parents.
4. Structure of Pigeons:
External Structures:
The size varies from 20-25 cm. The appearance varies in different varieties but all of them have a compact and more or less spindle-shaped body (Fig. 9.2). The body is distinctly divisible into three regions — head, neck and trunk. Most part of the body is covered by feathers, which are integumentary derivatives.
Head:
The head is small, round and anteriorly pointed.
It contains the following structures:
(i) Mouth:
It is a wide gap at the anterior end of head and bounded by upper and lower beaks. Both the beaks are covered by a sheath, called horny sheath or rhamphotheca. The beaks are pointed, hard and suitable for its grain-eating habit.
(ii) Cere:
Near the base of the upper beak a pair of swollen, feather covered areas, called cere is present. Just anterior to the cere, a pair of external nares is present as minute slits.
(iii) Eyes:
On each lateral side of the head, a prominent eye is placed. Each eye is provided with an upper, a lower and a movable transparent third eyelid or nictitating membrane, which runs across the eyeball.
(iv) External ear:
These small paired apertures are present one on each lateral side of the head and slightly posterior to the eye. Each aperture remains covered by a special group of feathers called auricular feathers. Each aperture leads into a canal, called the external auditory meatus.
Neck:
The neck is long and flexible. It holds the head over the trunk and its flexible nature is responsible for the universal movement of head.
Trunk:
The trunk is compact, stout and immovable. All the heavier organs are placed on the central part of the trunk and immediately beneath the wings.
The trunk region contains the following structures:
(i) Forelimbs:
Paired forelimbs are connected with the anterior region of the trunk and are greatly modified as wings. At the time of rest, the wings lie on the lateral side of the trunk as a folded ‘Z’. Each wing has three divisions — upper arm, fore arm and hand. Folds of skin extend between upper arm and trunk and also between upper arm and fore arm.
The former fold is called post- patagium and the latter is known as pre-patagium. Both the patagia are greatly reduced. The portion of hand is tightly packed by a fold of skin and each has three clawless insignificant digits.
(ii) Hind limbs:
Paired hind limbs work as legs and are modified to carry the entire weight of the body during perching and walking on land. It is connected with the trunk near its posterolateral side. The thigh portion of the hind limb runs parallel to the body and during rest remains under the cover of the wings.
Each hind limb is provided with four toes of which the first one is called hallux which is directed backwards and the remaining three are pointed anteriorly. The upper part of the hind limb is feathered but the lower part is covered by scales.
All the toes are provided with sharp and pointed claws. At the time of flight, the hind limbs are kept withdrawn and are released like the wheels of an aero plane immediately before landing.
(iii) Tail:
Posterior most conical end of the trunk is known as uropygium. On the ventral side of the uropygium lies a transverse slit, called vent or cloacal aperture, which serves as the outlet of cloaca. On the mid-dorsal region of the uropygium, there is a gland, called uropygial or preen or oil gland.
This gland produces an oily secretion, which is drawn by the beaks and is sprayed all over the body to ‘dress’ the feathers. The uropygium has special series of feathers, called rectrices. These feathers in the uropygium constitute the tail of pigeon. The tail is used as steering and brake during flight and works as balancing organ during walking or perching or flying.
Skin:
The skin of pigeon is dry, thin and loose in texture. Skin glands are rare. The uropygial gland is the only integumentary gland. All over the skin numerous free nerve endings remain scattered. Near the base of the upper beak these nerve endings form specialised organ for touch in the cere.
It is represented by a patch of naked swollen skin. In spite of its thin appearance, the skin has great ability to produce keratin, which is used largely in the formation of various integumentary derivatives.
Skeletal Structures:
Pigeon possesses well-formed exoskeletal and endo-skeletal structures.
Exoskeleton:
Following integumentary structures are seen in birds which perform different functions.
(i) Beaks:
These are horny exoskeletal derivatives which cover the upper and lower jaws. It is used during ingestion, preening of feathers and fighting.
(ii) Claws:
These are pointed, sharp and horny exoskeletal organs which are present at the extremities of the toes. The claws are typically reptilian in construction and help during perching and walking.
(iii) Scales:
These reptile-like exoskeletal structures are present on the exposed parts of the hind limbs. These scales originate from the epidermal part of the skin and remain arranged in tile-like fashion.
(iv) Feathers:
These exoskeletal structures are the unique features of birds and are not seen in any other animal. These distinctive epidermal structures furnish a flexible, light-weight and resistant body covering of pigeon and are supposed to be modified reptilian scales. They cover the most parts of the body and form the wings and tail.
Development of Feather in Pigeon:
The germ of a developing feather begins as a dermal papilla of the skin composed of epidermis and dermis (Fig. 9.3 A, B). At the initial stages, the dermal cells push the overlying epidermis up ahead of it. The base of the outgrowth gradually sinks into the skin.
The outgrowth includes dermis and epidermis. The epidermis becomes hard and cornified and is pushed out from the papilla region. The dermal plug gradually withdraws leaving a hollow quill inserted in the skin. The embryonic feather is, thus, a tube of cornified epidermis set in a pit of dermis.
One side of tubular embryonic feather becomes considerably thickened to transform into the shaft (Fig. 9.3 C). From the shaft slanting barbs are developed on either side. The wall of the tube opposite to the shaft becomes very thin. Rupture of this thin portion causes the spreading out of the rolled-up feather to assume its usual form.
Pterylosis:
Each feather originates from the epidermal layer of the skin and are arranged in definite tracts called pterylae, which are separated from the featherless tracts, called apteria (Fig. 9.4). The arrangement of feathers on the body of pigeon is called pterylosis.
Plumage:
The assemblage of feathers at one time is called the plumage. Feathers may be white (due to the presence of air spaces) or coloured (due to the presence of pigments). The colour of the feathers is due to chemical pigments and the physical arrangement of the elements composing the feathers. Feathers are periodically replaced by a process, called moulting, which occurs sporadically throughout the body.
Kinds of feathers:
Pough, Heiser and McFarland (1996) divided the feathers of pigeon into five types.
a. Contour feathers (Pennae) including body covering feathers and the flight feathers (remiges and rectrices),
b. Down feathers,
c. Filo-plumes or hair feathers
d. Semi-plumes and
e. Bristles.
a. Contour feathers form the majority of the covering feathers and give the bird its external shape. They provide airfoils for the flight. Contour feathers differ from the other body feathers such as down feathers and semi plumes; in that they bear two vanes along a shaft that have interlocking barbs which give strength and structure to the feather shape.
Structure of a typical contour feather:
The feathers differ in their size, shape and function, but are built up in same general plan.
A typical contour feather has the following two parts:
(i) Shaft and
(ii) Vane or Vexillum.
The horny shaft portion is divisible into two parts, calamus (Gk. kalamos, quill) and rachis (Gk. rakhis, spine).
(i) Calamus or Quill:
The calamus is the bar- bless, hollow cylindrical stalk by which the feather remains attached with the skin. At the proximal end, there is an aperture, called inferior umbilicus into which fits the mesodermal papilla of the skin.
Through this aperture the feather receives musculature, blood vessels and nerves. At the distal end, there is another aperture, called superior umbilicus. At the time of hatching nutrition is supplied through the aperture to the developing feathers. Later this aperture becomes nonfunctional.
(ii) Rachis:
The rachis is solid pith-like in appearance and is continuous with the calamus. The distal end of the rachis is tapered. The ventral part of the rachis bears a groove, called umbilical groove. A small tuft of soft feathers, called the after- shaft, develops near the superior umbilicus.
The vane or vexillum represents the broad part of the feather. It is formed by delicate closely set structures, called barbs or hypo- rachis, which extend from each side of the rachis. Each barb bears oblique rows of processes, called barbules.
The distal edge of each barb has two sets of barbules—proximal and distal. The proximal barbule from one barb crosses the distal barbule of the next. The tips of the distal barbules are provided with booklets or barbicles or hamuli and the tips of proximal barbules have flanges.
The booklets and flanges interlock to render a compact surface of the vane which resists the air. In the flight feather, the shaft is long and vanes are broad. Among the covering feathers, there are short shaft but extended vane portion.
Types of contour feathers:
(i) Flight Feathers or Quills:
These feathers are associated with flight and may again be subdivided into wing feathers or remiges and tail feathers or rectrices. The remiges are present in the wings and each wing has 23 feathers, which are again subdivided into primaries, secondaries and tertiaries.
The remiges attached to the distal part (carpometacarpus) of the wing are primaries, those on the middle (radius and ulna) portion are secondary’s and those attached to the proximal part (humerus) of the wing are called tertiaries. The rectrices are twelve- in number and are arranged semi-circularly in the tail region.
A small tuft of feathers is attached to the first digit of the wing, called alula or ala spuria or bastard wing.
(ii) Coverts:
These are smaller forms of contour feathers found at the bases of wings and tail filling the space between them and the covering acts as an thermal insulating layer.
b. Down feathers or Plumulae are short, soft, woolly feathers and without shaft, also present in between the contour feathers. The down feathers consist of a short quill or stem. The barbs are arranged in a circle at the top of the stem. Barbs are long and flexible with poorly developed barbules and without hooklets.
There are several types of down feathers:
(i) Nestling down or natal down,
(ii) Definitive down,
(iii) Powder down and
(iv) Uropygeal gland downs.
(i) Nestling down or natal down:
In the newly hatched birds there are short, soft, wooly and transitory feathers, called nestling- down or natal down. It has a short stalk with barbs at the tip and without interlocking arrangement. The nestling down provides thermal insulation very effectively. These down feathers remain attached to the region of apteria and develop before contour and semi plume.
(ii) Definitive down:
Definitive down feathers appear with the other permanent feathers.
(iii) Powder down:
The powder down feathers is difficult to classify structurally but the tops of the barbs continuously break down as a white waxy powder that is non-wettable.
(iv) Uropygial gland down:
Uropygial gland downs occur at the base of the uropygium in most birds. These are modified brush-like down feathers.
c. Filo plumes (hair feathers or pin feathers) are small, delicate and hair-like feathers. Each filo plume has a long shaft carrying a few degenerated barbs and barbules at the tip and such barbs are without interlocking apparatus.
The filo plumes are seen scattered on the body of the bird when it has been plucked. The feathers are probably sensory in function. Each feather is provided with a muscle at its base, which controls the position of feather.
d. Semi plumes are lying beneath the contour feathers and are intermediate in structure in between contour and down feathers. They possess short calamus and long rachis with plumulaceous vanes (Fig. 9.6A). They provide warmth and fill up the space within the contour feathers.
e. Bristles are stiff rachis with partial or complete deletion of the vane (Fig. 9.6B). They occur mostly on the head, at the base of beak, and even on the toes of some birds. They may screen out foreign particles from the nostrils (hawks and black birds), increase the gape of the mouth to catch the insects (fly catchers).
These are the modified filo plumes and also may be called rictal bristles when it has a bristly hair-like appearance with a small calamus and a slender rachis with a few barbs at the base of the rachis.
Uses of feathers:
(i) The nerve fibres present at the base also help the feather to act as a sensory structure.
(ii) The feathers produce a compact surface to resist air-current.
(iii) It retains heat and thus helps the bird from the temperature hazards.
(iv) It is light and, therefore, helps in flight.
(v) In some birds, a few feathers act as secondary sexual characters.
Endoskeleton:
Pigeon possesses well-developed endoskeleton. The entire endo-skeletal frame may be grouped into Axial and Appendicular skeletons. Both these groups have undergone considerable modifications, specially fusion of different parts, to meet the requirements of the bird’s peculiar way of life.
Axial Skeleton:
The axial skeleton includes the skull, vertebral column and sternum.
Skull:
The skull of pigeon is light and fragile. It is more or less round in shape. The skull bones are paper-like thin. The sutures of the skull bones are obliterated. The opening of the cranium, i.e., foramen magnum, is directed ventrally and the skull articulates with the vertebral column by a single round occipital condyle on the anterior border of the foramen magnum.
The cranial part of the skull is formed dorsally by a pair of parietals and a pair of frontals (Fig. 9.7 A). The ventral side of the cranium is formed posteriorly by basioccipital and anteriorly by basisphenoid. The basisphenoid is drawn anteriorly as a narrow rostrum. Ventrally, the basisphenoid gives rise to two membranous basitemporals which fuse to form the posterior part of the presphenoid.
The anterior part of the presphenoid is represented by the rostrum. The supraoccipital and the exoccipital form the posterior and lateral sides of the cranium (Fig. 9.7 B). On each side of the cranium lies the tympanic cavity. It is formed by the squamosal, prootic and epiotic bones.
Two orbits are separated by inter-orbital septum, which is formed by the union of presphenoid and mesethmoid bones. The nasals are forked in appearance and form the inner and outer margin of the nasal cavity.
A large lachrymal bone is present on each side between the orbit and the facial part of the skull. The facial part of the skull is united with the cranial part in the following way: Maxilla units with the jugal and quadratojugal and ultimately with the quadrate. Quadrate is a three-rayed bone which articulates with the tympanic cavity by two processes and sends an orbital process.
It bears condyle for the articulation of the mandible. The palatine articulates with the pterygoids and rostrum posteriorly. The pterygoids articulate with the quadrate and also with the basipterygoid process of the rostrum.
The vomer is absent in pigeon. The maxillo-palatines remain separate and do not unite medially. This particular type of the skull is called Schizognathous type of skull. The mandible consists of dentary supra-angular, angular, splenial and articular (Fig. 9.7 C).
Hyoid apparatus:
The hyoid apparatus (Fig. 9.7 D) consists of an arrow-shaped basihyal from which two short cornua originate. It is continued as slender basi-branchial from which originates a pair of posterior cornua. Each cornu contains ceratobranchial and epibranchial segments.
Dorsally the hyoid apparatus is represented by a slender bone, called columella, which remains united with the stapes of the middle ear by one end and the other end has three cartilages, extra- stapedial, infrastapedial and supra-stapedial, to fit into the tympanic membrane.
Vertebral Column:
The vertebral column is an elongated structure. It shows flexibility at the anterior part, compactness at the trunk region and shortening near the tail region. Entire column is divisible into three regions, cervical, thoracic and synsacral. Each region includes several vertebrae.
(a) Cervical vertebrae:
Altogether 14 cervical vertebrae support the neck region of pigeon and are constructed in such a way that while supporting the head they permit free movement of the neck.
A typical cervical vertebra (Fig. 9.8 B, C, D) has the following features:
(i) Centrum is heterocoelous (anteriorly concave from side to side but convex from above downwards and posteriorly concave from above downwards but convex from side to side). The heterocoelous centra of the neck vertebrae give greater mobility of neck and allow the head to move to an angle of 180°.
(ii) Articulating surface of the centrum contains synovial capsules, cartilaginous plates, meniscus and suspensory ligament,
(iii) Double headed vestigial ribs are directed posteriorly,
(iv) The transverse processes are perforated to allow the passage of vertebral artery, this passage is called the vertebrarterial or transverse foramen. The first two cervical vertebrae differ from a typical cervical vertebra in structure. In both, the different parts are much reduced. The first vertebra, which is also known as atlas, is a small ring-like structure which remains fused with the second vertebra, called axis. (Fig. 9.8A, B).
(b) Thoracic vertebrae:
Four to five in number, of which the last one is free and the rest are united. The last thoracic vertebra is fused with the synsacrum. In each thoracic vertebra, the centrum is ventrally compressed into a plate, called hypophysis (Fig. 9.8E).
A pair of ribs remain attached with the vertebra by two heads—one head is united with the centrum and the other head remains united with the transverse process. This vertebral portion of the rib is attached with a bony counterpart from the sternum and bony interconnections — called uncinate processes — unite these vertebral parts.
(c) Synsacrum:
It is a compact more or less triangular skeletal mass formed by the fusion of vertebrae (Fig. 9.9B). Together with pelvic girdle, it renders rigidity to the body.
Following vertebrae constitute the synsacrum:
(i) Posterior thoracic—only one with ribs not connected with the sternum,
(ii) Lumbar—Five to six in number. Ribs are absent. Transverse processes are placed high on the neural arch and their uniting ligament is ossified to form a dorsal continuous bone,
(iii) Sacral—Two in number. Transverse processes of each sacral originate near the neural arch and the centrum is provided with a ventral outgrowth which lies pressed against the pelvic girdle,
(iv) Anterior caudal—Five in number, and all are fused with the other parts of the sacrum.
(d) Posterior caudals:
Six in number. Each one is free and distinct. Each vertebra is with a short and prominent centrum.
(e) Pygostyle or Ploughshare bone:
It is formed by the fusion of last four vertebrae. The pygostyle is a compressed skeletal structure and is turned upwards. The sternum is extremely modified due to the flying habit. It is modified ventrally into a boat-shaped bony skeletal structure, called keel or carina.
The anterior part is provided with a pair of grooves for the attachment of the coracoid bones of the pectoral girdle. The posterior part possesses two pairs of notches.
Appendicular Skeleton:
It consists of skeletal elements which constitute limbs and girdles. Among the limbs, the forelimbs are modified as wings and the hind limbs are modified as legs. The bony components of the forelimbs become greatly altered due to their transformation into wings.
The skeleton of the hind limbs has retained the typical reptilian plan. The other skeletal parts are also modified accordingly. The pectoral girdle is built up for the attachment of wings and the pelvic girdle holds the hind limbs to support the body during standing.
Pectoral girdle and wings:
The appearance of the pectoral girdle is altogether different from that of other vertebrates.
The pectoral girdle of each side contains following parts:
(i) Coracoid:
It is a massive, rod-like strongly built bone. Each coracoid is placed on the dorsoventral axis and is attached to the broad end of the keel. Anterior end contains a process, called acrocoracoid process,
(ii) Scapula:
It is a sabre-shaped bone which extends anteroposteriorly over the ribs (Fig. 9.9A). At its anterior end, it is connected with the coracoid by a ligament. The coracoscapular angle is less than 90°. This is a characteristic feature of the flying birds,
(iii) Furcula or wish bone or Merry thought bone:
It represents the clavicles and interclavicles of pectoral girdle of other vertebrates. It forms a ‘V’-shaped structure and hangs along the dorsoventral axis in front of the sternum. A small aperture (foramen triosseum) is left at the junction of the coracoid, scapular and furcula. Through this aperture, the tendon from the wing muscles passes for its attachment with the humerus,
(iv) Glenoid cavity:
It is the place where the head of the forelimb fits in. It is formed by both coracoid and scapula, the latter forms acromion process within it.
Each wing contains the following skeletal pieces:
(i) Humerus:
It is a strongly built bone with broad head. The head bears a number of prominent ridges and a pneumatic foramen.
(ii) Radius and Ulna:
Both the bones are separated in the middle but remain closely approxed at the two ends. Radius is a slender, straight skeletal piece, while ulna is curved and slightly bent,
(iii) Proximal carpals:
Two large skeletal pieces, one called radiale and the other called ulnare —remain free,
(iv) Carpometacarpus:
This small but compact bone includes two skeletal pieces, which are fused at both the ends but remain separated in the middle. One piece is stout and straight, while the other piece is slender and curved. It is formed by the union of two distal carpals and three metacarpals,
(v) Phalanges:
Following phalanges are present in each limb, all of which are devoid of claws.
They are:
(i) One pointed phalanx is attached with the first metacarpal,
(ii) Two phalanges remain attached with the second metacarpal, one of them is flange-like and the other is pointed.
(iii) One pointed phalanx is attached with the third metacarpal (Fig. 9.9C).
Pelvic girdle and hind limb:
The pelvic girdle is compact and ankylosed with the synsacrum.
It contains the following skeletal parts on each side:
(i) Ilium:
It is a broad and large bone, fused with the synsacrum on its inner side. It is concave anteriorly and convex posteriorly,
(ii) Ischium:
It is a broad bone, which is fused with the ilium at its posterior end. Anteriorly, an ischiatic foramen separates the ilium from ischium. No ischiatic symphysis is present,
(iii) Pubis:
It is a narrow, curved and elongated skeletal piece, which is placed parallel to the ischium and separated from it by obturator foramen. No pubic symphysis occurs in pigeon,
(iv) Acetabulum:
It is the cavity where the head of the femur fits in. Its dorsal side is formed by the ilium and the remaining part is formed by equal share from ischium and pubis. The edge of the cavity is drawn into a process, called anti-trochanter, which works against the trochanter of the femur of hind limb (Fig. 9.9B).
Each hind limb is composed of the following skeletal pieces:
(i) Femur:
It is a short and narrow skeletal piece. Its proximal end contains pointed trochanter and rounded head. The hind limb is placed parallel to the sagittal plane of the body. The distal end is drawn into
a pulley-like condyle, containing a patella as knee-joint,
(ii) Tibiotarsus:
It is a long bone formed of tibia with the fusion of two distal tarsals,
(iii) Fibula:
It is a small rudimentary bone situated on the lateral surface of the tibiotarsus.
(iv) Tarsometatarsus:
This bony segment is formed of some of the tarsals and the metatarsals of the second, third and fourth digits. A small vestige of the first metatarsus is located on the posterior side of the distal end of the tarsometatarsus. The ankle joint of bird — being mesotarsal — resembles that of reptiles.
(v) Phalanges:
Pigeon has four digits; the fifth one is lacking. Phalanges are present in the following number at different toes and the distal phalanges of all the toes are provided with sharp and pointed claws. The first toe or hallux has two phalanges and the second, third and fourth toes have three, four and five phalanges, respectively.
5 . Locomotion in Pigeons:
Like other flying birds, pigeon is specially adapted to move both in the air and on land. Flight is caused by the activity of the wings and other structures also help in the process. In addition to its ability to fly, pigeon can stroll on the ground and at the same time can perch on the branches of the-trees.
6. Sense Organs in Pigeons:
Pigeon possesses well-developed special sense organs to receive external stimuli.
Olfactory organs:
These paired organs are present near the base of the upper beak. Each olfactory chamber is covered externally by ectoethmoid which is drawn into three spiral processes, called the turbinate, to increase the surface area of the mucsus membrane.
The olfactory chambers are separated by mesethmoid. Each chamber is divisible into an anterior non-sensory part, called the vestibule and a posterior sensory part. The vestibule, has many layered epithelial lining but the posterior part is lined by only one layered epithelium with innervation from olfactory nerves. The power of smell is very poor.
Eye:
The eyes are well-developed and are built on typical vertebrate plan.
Shape and Size:
They are extremely large in size. The shape of the eye-ball is not spherical but partly biconvex. The lens and cornea bulge forward and the shape is maintained by a ring of bony sclerotic plates present in the sclera (Fig. 9.31 A).
i. Sclera:
The sclera is a tough white fibrous outer layer of the eye ball. The anterior transparent continuation of the sclera is called cornea.
ii. Choroid:
The choroid is the black pigmented layer situated just internal to the sclera. The iris is the pigmented partition pierced by the pupil at the centre. It separates the anterior and posterior chambers of the eye. The iris is the anterior continuation of the choroid and composed of striated muscles.
The posterior chamber is bordered anteriorly by the iris and is filled with vitreous humour while the anterior chamber is bordered anteriorly by the cornea and posteriorly by the lens. The chamber is filled with aqueous humour. The vitreous body is a jelly-like mass and the aqueous humour is watery. The biconvex lens is a soft elliptical body which remains suspended from the ciliary body by ligaments.
Accommodation:
Accommodation is effected by changing its shape by the pressure exerted upon it by the ciliary muscles behind. The ciliary body is the enlargement of the choroid containing striated muscle which controls the lens.
The ciliary muscle is divided into anterior (Crampton) and posterior (Brucke) portions. The Brucke’s muscle draws the lens forward into the anterior chamber and the Crampton’s muscle is so oriented as to pull it on the cornea. By its action, the radius is shortened and, thus, assists in accommodation.
Retina:
The retina is a light coloured tissue internal to the choroid. It is composed of cones and rods. The retina contains a depressed area, called fovea, which is the region of acute vision in bright light. The blind spot is the point of exit of the optic nerve from the retina.
Projecting into the vitreous body from the blind spot, a characteristic structure, called pecten, is present (Fig. 9.32B). The pecten is highly variable in size and shape in different birds. In pigeon, it is composed of a thin dark pigmented plate.
This plate is folded fanwise and assumes the form of a comb. It is made up of a vascular network supported by pigmented glial cells. The pecten is an enigmatic organ in avian eyes and its physiological role is not yet ascertained. This structure is claimed to perform a number of functions in the life of a pigeon.
The functions are:
(a) As the pecten is an erectile structure, it is possible that irregular shadows cast by this organ provide many blind spots on the retina. This action enhances the on-and-off effects in the visual field and thus increases the acuity of visual perceptions.
(b) The pecten is a highly vascular structure and provides nutrient substances to vitreous body and retina.
(c) The pecten protects the retina from strong light and also
(d) serves to warm the eye. It has long been claimed that pecten is in some way concerned with the process of accommodation, it is not likely that the pecten actually helps in the process. The accommodation is actually done by the action of the ciliary bodies.
Glands:
There are glands associated with the eyes. The lacrymal gland is situated at the ventral margin of the orbit. The Harderian gland is located on the anterodorsal surface of each eye.
The movement of each eyeball is controlled by superior oblique, superior rectus, internal rectus, inferior oblique, external rectus, inferior rectus, quadratus and pyramidalis muscles. The upper and lower eyelids are present. The third eyelid, nictitating membrane, is prominent.
Ear:
The ears contain three usual divisions— the external, the middle and the internal. The external ear is represented by a tightly stretched circular skin, called tympanic membrane or tympanum. It remains hidden by feathers and the external auditory meatus is found by probing just posterior to the angle of the jaw.
The middle ear is tubular and its cavity is connected with the pharynx through an eustachian tube which equalises the air pressure on the two sides of the tympanum. Within the middle ear cavity there is a rod-like columella with its process, called extra-columella, which conveys the sound waves to the internal ear. The internal ear contains a small membranous labyrinth (Fig. 9.32C), which is enclosed within a bony labyrinth.
The footplate of the columella fits into the fenestra ovalis. Both the vestibular as well as auditory parts of the internal ear are well-developed. The vestibular part is peculiar by having connections with the cerebellum which suggests its importance in flight. The anterior semicircular canal is the largest of the three canals. The auditory part is greatly developed.
There is a distinct cochlea which encloses a basilar membrane. This membrane contains many fibres bearing hair cells. Another aggregation of hair cells together with calcareous particles is present at the apical end of the cochlea. The cochlea bears an extended lagena which is responsible for receiving the low pith sounds (Fig. 9.32 C).
Miscellaneous Sense Organs:
Besides the three pairs of special sense organs, there are many other receptors for external stimuli. The Grandy’s corpuscles present in the skin of the beak are touch receptors. The Herbst’s corpuscles present in the dermis are numerous and are vibration and heat receptors. Chemoreceptors for smell are little developed. There are few taste-buds on the tongue.