ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 17. CLASS BIRDS (Aves)

1. General Characteristics of the Class Birds (Aves)

Birds are a specialized branch of higher vertebrates adapted to flight. The Class of birds comprises about 9000 species.

Giants among flying birds:

✵ swan, vulture, bustard (weighing 14-16 kg, with a wingspan of up to 2.5 m).

Giants among flightless birds:

✵ African ostrich, cassowary (up to 2.7 m tall, weighing 80-100 kg).

Giants among swimming birds:

✵ emperor penguin (1.2 m tall, weighing 40 kg).

Dwarfs among birds:

✵ hummingbird (weighing 1.6-1.8 g, feeds on floral nectar, is able to hover in the air over a flower, rise vertically, and fly backward; wing beat frequency is 100 beats per second; Heart rate is 1 000 beats per minute!)

Warm-bloodedness (homeothermy), high mobility, and complex, diverse behavior have allowed birds to spread widely across the globe.

Ornithology (from Greek ornithos — bird, logos — study) is the branch of zoology that studies birds.

Progressive characteristics of birds:

1. Progressive development of The Nervous System (development of subcortical associative centers of the cerebral hemispheres, the Cerebellum, The Emergence of a thermoregulation center in the Diencephalon, with visual and auditory lobes of the Brain becoming highly developed). Complex and diverse adaptive behavior.

2. The emergence of a four-chambered heart and complete Separation of arterial and venous Blood flow due to the reduction of one of the two aortic arches.

3. The formation of spongy Lungs, which increased the efficiency of oxygen supply to Tissues and Organs and raised the metabolic rate.

4. The emergence of warm-bloodedness (homeothermy) As a result of an increased metabolic rate through the intensification of Digestion, respiration, Circulation, excretion, and the presence of insulating body coverings.

5. A constant body Temperature is maintained primarily through internal PHYSIOLOGICAL AND BIOCHEMICAL processes (endothermy).

6. Changes in Skeletal Structure (forelimbs modified into wings, modified girdles of both forelimbs and hindlimbs, bones containing numerous air cavities, a highly mobile cervical spine, and the jaw apparatus transformed into a beak).

7. Presence of dry Skin, almost devoid of glands; feathers are modified horny scales that perform an insulating function and provide body streamlining.

Structural Features of birds related to flight:

* Forelimbs have transformed into wings.

* Streamlined body shape.

* Plumage provides:

* streamlining of the body;

* the formation of lifting surfaces (wings, tail).

* Wing shape.

* Skeleton:

* strong and lightweight, with pneumatic (air-filled) bones;

* the Skull is characterized by the complete fusion of all bones into a single structure, extreme lightness, and large orbits;

* a lightweight, toothless beak;

* the clavicles fuse to form a furcula (wishbone), providing elasticity to the Pectoral Girdle;

* forelimbs modified into wings: humerus, forearm (ulna and radius), and manus (hand), where some bones fuse, and only three digits remain—the second, third, and fourth—with a reduced number of Phalanges;

* a keeled Sternum, which serves as an attachment site for the Major and minor pectoral Muscles that power wing movement;

* the Ribs are movably articulated with the sternum (The ability to change the volume of the thoracic cavity is essential for double respiration);

* all vertebrae except the cervical ones are immovably fused (ensuring the body compactness necessary for flight).

* Well-developed musculature, which enables complex movements during flight.

* A highly efficient Respiratory system (airways: glottis, Trachea, two Bronchi; spongy lungs; air sacs).

* Air sacs:

✵ prevent the bird's body from overheating during flight;

✵ lighten the bird's body during flight;

✵ reduce friction between Internal Organs.

✵ Circulatory system: only the right aortic arch is present, the left is reduced.

✵ Excretory system: the Urinary Bladder is absent.

✵ Brain: well-developed cerebral hemispheres; highly developed cerebellum (associated with complex movements requiring precise coordination).

✵ In males, the paired Testes are small in size.

✵ In females, There is a single left Ovary (the right ovary is reduced).

Features of avian life processes associated with flight:

✵ Rapid digestion of food, which provides the bird with a large amount of energy.

✵ High Metabolic Rate (due to the complete separation of venous and arterial blood, and intensive Gas Exchange in the lungs).

✵ A complex Mechanism of double respiration.

✵ Feces and excretory products do not accumulate in the body, reducing the bird's body mass.

✵ Enhanced Vision and Hearing expand spatial orientation capabilities.

Features of avian Organization

Adaptation to flight has led to a relative uniformity in the body shape of birds. The body consists of a small HEAD, a compact rounded trunk, and limbs. The head (Fig. 69) features eyes with movable eyelids (upper, lower, and nictitating membranes), nostrils, auditory openings, and a beak.

Fig. 69. Head of a pigeon:

1 — upper Mandible; 2 — lower mandible; 3 — cere; 4 — nostril; 5 — Tongue; 6 — auditory opening.

The beak consists of an upper part—the upper mandible—and a lower part—the lower mandible, both covered with horny sheaths. At the Base of the upper mandible in some birds (Falconiformes, pigeons, Psittaciformes, Strigiformes), there is an area of thickened skin called the cere, which contains the external openings of the nostrils. The cere facilitates the movement of the upper mandible. Usually, the cere is featherless (feathered in parrots, bearded vultures, and owls) and brightly colored. In many birds of prey, the color of the cere changes with age: yellow in older individuals and blue in young ones.

The trunk of birds is streamlined. The plumage also contributes to this streamlined shape. The forelimbs are wings, which are folded and pressed against the sides of the body when at rest. The hind limbs consist of a bare part covered with horny scales and movable toes (ranging from two to four) that end in claws.

Birds have a long and highly flexible neck (all of them?), allowing them to turn their head 180° or more (for example, an owl can turn its head 270°).

The skin of birds is thin, dry, and almost entirely devoid of glands. Only at the base of the tail is the uropygial gland located, which secretes oil that birds use to preen and lubricate their plumage with their beak. Thanks to this secretion, their feathers retain elasticity and remain Water-resistant. Underneath the feathers is down, which stays dry, trapping a layer of air around the body. This insulating layer keeps the bird warm even in cold water and helps it stay afloat. Under METABOLISM/18.html">The Influence of sunlight, the fatty secretions of the uropygial gland are converted into vitamin D (the growth vitamin), which birds ingest while preening. The uropygial gland is most developed in waterfowl, whereas in cormorants and herons, it is poorly developed. In birds that inhabit arid climates, the uropygial gland is often absent (e.g., in bustards, ostriches, and some parrots).

The superficial layers of Cells in the epidermal layer of avian skin undergo keratinization. The proliferation of this keratinized epidermal layer forms the horny Sheath of the beak—the rhamphotheca. Horny scales cover the bare part of the hind limbs (the tarsometatarsus) and the movable toes, the terminal phalanges of which end in horny claws. In males of some birds (such as Phasianidae), a bony outgrowth covered with a sharp horny sheath—a spur—develops on the tarsometatarsus. The plumage, unique to birds, is also a keratinized derivative of the epidermal layer of the skin. Contour feathers, downy feathers, and down are distinguished (Fig. 70).

The primary type of feather is the contour feather: it has a strong and elastic horny shaft; the part of the shaft to which the two webs (vanes) are attached is called the rachis (which is tetrahedral in shape), while the part devoid of vanes is called the calamus (which is rounded); the feather is anchored to the skin by the base of the calamus; the vane consists of numerous thin horny plates—barbs, from which branch barbules with tiny hooks on them; these hooks interlock the barbs with each other and

form a flexible vane plate, light and almost impermeable to air. Contour feathers, covering the entire body of the bird, are anchored in the skin in specific tracts called pterylae, which are separated by apteria—areas of skin where feathers do not grow. Only in penguins are feathers uniformly distributed over the entire surface of the skin.

Fig. 70. STRUCTURE AND TYPES of feathers (after Naumov, Kartashev, 1979):

A — contour feather: 1 — calamus; 2 — rachis; 3 — vane.

B — STRUCTURE OF THE contour feather vane: 2 — rachis; 3 — barb; 4 — barbule.

Bi — barbule with hooks. C — down. D — downy feather

Depending on their Location and Functions, contour feathers are divided into:

✵ flight feathers (remiges) — long, exceptionally strong; attached to the wings;

✵ tail feathers (rectrices) — long and strong feathers of the tail;

✵ coverts — feathers that cover the bird's body; typically, the lower part of the vane consists of slender barbs without hooklets; this is the downy part of the vane, which functions to trap a layer of air near the skin, increasing the feather's thermal insulation properties.

Beneath the contour feathers are:

✵ down feathers — have a slender shaft, and the barbs lack hooklets, preventing the formation of a coherent vane;

✵ down — has a highly reduced shaft from which a tuft of barbs without hooklets radiates.

Down feathers and down provide thermal insulation. They either cover the entire body uniformly (Anseriformes) or are located only on the apteria (herons, owls, passerines). In many birds, bristles—feathers with a stiff shaft that has lost its barbs—are located at the corners of the Mouth. They serve a tactile function, and in some species that catch small prey in flight (swallows, swifts), they widen the gape.

A bird's plumage has a specific coloration that serves various purposes: it facilitates encounters between individuals of the same species, prevents interspecific conflicts, and often makes the bird inconspicuous in its habitat. Avian coloration is determined by the accumulation of pigments in the feather cells during development and by the microscopic structural Features of the feather. The MAIN TYPES OF pigments are:

✵ Melanins — produce black, brown, and gray coloration;

✵ lipochromes — provide red, yellow, and green coloration.

The combination of different pigments in a single area of a feather creates complex coloration. White color is due to air filling the colorless keratinous substance of the feather. The metallic sheen of feathers is produced by the Interference of light in the outer layers of the keratinized cells.

Over time, old worn feathers are replaced by new ones through a process called molting. Many bird species may undergo not just one, but two or even three molts per year. This is linked to seasonal events in their lives (breeding season, wintering). In some species (such as diurnal birds of prey), molting occurs gradually, without affecting their ability to fly. In other species (Anseriformes, cranes), it happens rapidly, causing the birds to temporarily lose their ability to fly (small ducks for about 20 days, swans for nearly 1.5 months). During this period, they remain in secluded, inaccessible areas.

A key feature of the avian skeleton is its strength and lightness, which is achieved by the highly pronounced pneumaticity of the bones (the presence of cavities filled partly with air and partly with Bone Marrow).

The avian skeleton is divided into the Axial Skeleton and the associated rib cage, the skull, and the Skeleton of the limbs (forelimbs and hindlimbs) and their girdles.

The axial skeleton, formed by the vertebrae, consists of five regions:

1) cervical — 11-25 vertebrae; the first vertebra, the atlas, and the second, the axis, have the same Structure and function as in reptiles; all other vertebrae are of the heterocoelous type, where the long vertebral body has a saddle-shaped surface at both the front and back; the articulation of these vertebrae provides their

high flexibility and mobility: birds can freely turn their heads 180°, and some (owls, parrots) up to 270°;

2) thoracic — 3-10 vertebrae; they fuse together to form the notarium (dorsal bone) and also fuse with the synsacrum; this structure makes the trunk region of the axial skeleton rigid, which is crucial during flight (preventing trunk oscillations from disrupting flight coordination); the ribs are movably attached to the thoracic vertebrae and to the sternum, forming the rib cage; each rib consists of two segments—dorsal and ventral—which are flexibly connected by Cartilage and form an angle pointing backward; this design allows the volume of the rib cage to change During Muscle contraction, which is essential for respiration; the strength of the rib cage is enhanced by the uncinate processes of the dorsal rib segments, which overlap the adjacent rib; the sternum is a thin, broad, and long plate, which in all birds (except ratites) features a high ridge—the keel (carina)—where the pectoralis major and minor muscles that power the wings attach;

3) lumbar — 5 vertebrae;

4) sacral — 2 vertebrae;

5) caudal — 5-9 free vertebrae and the pygostyle, formed by the fusion of the last 4-8 caudal vertebrae; the bases of the tail feathers (rectrices) attach to the pygostyle in a fan-like arrangement.

All lumbar, sacral, and some caudal vertebrae fuse together into a single bone—the synsacrum, which is rigidly fused with the BONES OF THE Pelvic Girdle. This ensures the rigidity of the trunk and provides a strong support for the hindlimbs.

The avian skull is formed by a fused braincase, large orbits, and toothless upper and lower jaws, of which only the lower jaw is movable.

The skeleton of the forelimb, modified into a wing, consists of the humerus, the forearm (ulna and radius), and the manus (hand). Some bones of the manus fuse to form the carpometacarpus, while the skeleton of the digits is highly reduced: only two phalanges of the second digit are well-developed, extending the axis of the carpometacarpus. Only a single short phalanx remains of the first and third digits. Among the digits, only the first remains mobile, with several feathers of the alula ("bastard wing") attached to it. The flight feathers (primaries) attach to the carpometacarpus and the phalanges of the second digit. The Modification of the manus (Formation of the carpometacarpus, reduction of digits, and limited joint mobility) provides a strong support for the flight feathers. The white-throated needletail achieves the highest flight speed—up to 170 km/h. Flight speeds of various bird species:

✵ sparrows — 25-40 km/h;

✵ pigeons — 30-60 km/h;

✵ falcons — 60-70 km/h;

✵ ducks and shorebirds — 65-80 km/h;

✵ swifts — 100-120 km/h.

The speed of penguins in the water Column is 36 km/h.

The pectoral girdle consists of three paired bones — narrow and long scapulae, clavicles, and massive coracoids. The clavicles fuse at their distal ends to form the furcula. The furcula acts as a Shock absorber, cushioning the impacts during wing movements.

The skeleton of the hind limb consists of a massive Femur (thigh bone), a lower leg (fused Tibia and Fibula), and a FOOT. Most of the foot bones fuse to form a single bone — the tarsometatarsus, which increases stride length. The phalanges of the toes (usually 3-4 toes, sometimes 2 in African ostriches) attach to the tarsometatarsus.

The pelvic girdle is formed by three pairs of bones (ilium, ischium, and pubis) fused with the synsacrum, providing strong support for the hind limbs and allowing for the attachment of massive muscles. For example, the running speed of the three-toed rhea and the two-toed African ostrich is over 50 km/h. The pubic and ischial bones in birds do not fuse along the midline of the body; this type of pelvis is called an open pelvis. It allows them to lay large eggs.

Compared to reptiles, the musculature of birds is characterized by greater differentiation, which is associated with more complex and diverse movements during flight. The most massive muscles (equal in weight to the rest of the musculature combined) are the paired pectoralis major muscles, attached to the sternum and its keel, which serve to lower the wings. The supracoracoideus muscles raise the wings. Well-developed Neck Muscles provide high mobility of the head, which is important both during foraging and in flight. Intercostal muscles facilitate respiratory movements. Subcutaneous muscles, which move the feathers, play an important role. In cool weather, these muscles contract, and birds fluff up their feathers. This increases the layer of air between the feathers and the body, helping to retain heat. Birds are characterized by the accumulation of Myoglobin in their muscles, which allows them to create an oxygen reserve. This oxygen is utilized during periods of intense activity. The highest concentration of myoglobin is found in the pectoralis major muscle, the muscular Stomach (gizzard), and The Heart. The Muscles of the hind limbs are also well-developed (there are over 30 of them). However, there are no muscles in the tarsometatarsus and toes of birds — only tendons. This is why birds can walk on snow without getting frostbite on their feet.

The warm-bloodedness and high mobility of birds are sustained by the consumption of large amounts of food. The diet of birds is much more diverse compared to reptiles. Most birds are carnivorous. Many species feed on both various invertebrates and plant food, especially berries and seeds. There are few herbivorous species (geese, some galliformes), but even they consume animal food.

The Digestive System of birds begins with a toothless beak. The variety of bird beak shapes (Fig. 71) reflects their feeding habits:

✵ a sharp hook at the tip (birds of prey, owls, cormorants) or sharp cutting edges of the beak (herons, cranes) help to hold large, active prey;

✵ a thin, forceps-like beak allows pulling small invertebrates from their habitats (passerines, shorebirds, hoopoe);

✵ a wide, flattened beak with transverse horny lamellae inside forms a unique water filtration apparatus and enables feeding on small planktonic animals and Algae (anseriformes, flamingos);

✵ a straight, chisel-like beak, with which birds not only obtain food from wood but also excavate cavities in tree trunks where they live (woodpeckers).

Fig. 71. Structure of the beak and limbs of birds depending on the method of feeding and locomotion.

The Oral Cavity of most birds contains a conical tongue with well-developed spines that help hold prey. In birds with a filtration apparatus, a fleshy tongue expels water and sediment from the Oral Cavity and helps push the strained food into the Pharynx. The tongue of seed-eating birds holds seeds at the edge of the beak while they are being crushed. In nectar-feeding birds (hummingbirds), the tongue rolls into a tube for sucking. Woodpeckers have a very long, pointed tongue with barbs that help extract insects from narrow and deep passages in wood.

The ducts of the Salivary Glands open into the oral cavity. Saliva not only moistens food and facilitates its passage to the Esophagus, but also participates in digestion, as it contains certain digestive Enzymes, such as amylase, which provides partial digestion of CARBOHYDRATES. In many swallows and swiftlets, sticky saliva hardens in the air and is therefore used by these bird species to build nests.

The oral cavity leads into a long esophagus (Fig. 72). In some birds (galliformes, pigeons, birds of prey, parrots), there is an expansion in the lower part of the esophagus — the crop, which serves as a temporary reservoir for food (while The Stomach is full). In pigeons during the breeding season, cells of special epithelial glands divide rapidly, undergo fatty degeneration, and slough off into the crop cavity, where, together with Lymph, they form so-called 'crop milk' — a frothy mass containing over 10% protein and 12-15% fat. This is what the birds feed to their chicks.

Fig. 72. Digestive system of a pigeon (after Naumov, Kartashev, 1979):

1 — esophagus; 2 — crop; 3 — glandular stomach; 4 — muscular stomach; 5 — duodenum; 6 — Liver;

7 — Bile ducts; 8 — Pancreas; 9 — pancreatic ducts; 10 — Small Intestine; 11 — ceca; 12 — rectum; 13 — cloaca; 14 — intestinal Peritoneum (only a part is shown); 15 — Kidneys; 16 — Spleen.

The stomach of birds consists of two compartments:

✵ glandular, in which gastric juice is secreted by numerous glands;

✵ muscular, which has thick muscular walls, and its inner surface is lined with a cornified, rough membrane.

Food, saturated with digestive enzymes, passes from the proventriculus (glandular stomach) into the gizzard (muscular stomach). There, it is ground up by the rhythmic contractions of its walls (up to 30 per 1 s). This grinding process is aided by swallowed stones (gastroliths), which act as millstones and compensate for the lack of Teeth in the oral cavity. Finely ground food enters the intestine for final digestion and nutrient absorption, while undigested and unground remains (Hair, feathers, bones, Chitin) in many birds form a dense pellet—a cast—and are regurgitated through the esophagus and oral cavity. The ducts of the pancreas and the Gallbladder (absent in pigeons) of the bilobed liver empty into the intestine (duodenum). At the junction of the small intestine and rectum, there are small paired outgrowths—ceca, where food is digested under the Influence of the bird's own enzymes and specific microflora. The intestine is generally longer in herbivorous species, exceeding body length by 10 or more times (20 times in the African ostrich). In insectivorous species, the intestine is relatively short, exceeding body length by only 4-6 times. The intestine ends in the cloaca. Excrement does not accumulate in the intestine but is immediately voided, which reduces the bird's body mass. In chicks, a thick-walled blind sac on the DORSAL SIDE OF the cloaca—the bursa of Fabricius—is well-developed, where white Blood Cells (leukocytes) are formed; in adult birds, it is reduced.

The rate of digestion in birds is extremely high: dry seeds are digested in 3-4 hours, beetles in about 1 hour, caterpillars in 15-20 minutes, and juicy berries in half an hour. To maintain the high metabolic rate required to keep a constant high body temperature (up to 42°C) and to sustain flight, birds feed frequently and spend most of their time searching for food. Due to greater heat loss compared to larger species, small bird species require relatively more food. The daily mass of food consumed by small passerines is 50-80% of their body weight, while in larger species it is 15-40%. The daily food intake also depends on the caloric value of the food and the ambient temperature: colder weather increases food requirements. Small birds can starve to death in just 15-30 hours, pigeons in 7-9 days, while large eagles and owls can survive up to 1 month.

The excretory system of birds consists of paired pelvic kidneys and Ureters that open into the cloaca. A distinctive feature of avian nephrons is that, unlike in reptiles, they possess a U-shaped segment—the Loop of Henle, which is densely surrounded by capillaries. The high permeability of the loop of Henle's walls allows for the passive reabsorption of water from the primary urine. This mechanism ensures the formation of concentrated urine. Additional water absorption occurs in the cloaca. All of this allows metabolic waste to be excreted with minimal water loss. Birds do not have a urinary bladder. Highly concentrated urine is excreted along with feces (droppings). Bird droppings are a mixture of fecal matter and urine.

In addition, most birds possess nasal glands located on the frontal bones above the eye orbits. These are particularly well-developed in marine birds and some desert birds that are forced to drink saltwater. In the secretion of the nasal glands, which is excreted as droplets from the nostrils, the concentration of common salt (NaCl) is 4-5 times higher than in the blood and twice as high as in seawater.

The respiratory system (Fig. 73) of birds consists of:

✵ the airways: paired nostrils; Nasal cavity; Larynx; trachea (the walls of which consist of cartilaginous rings that maintain a constant lumen for air passage); two bronchi (with walls consisting of cartilaginous half-rings);

✵ spongy lungs.

Fig. 73. Diagram of the air sacs of birds from the ventral side (after Naumov, Kartashev, 1979):

I — trachea; 2 — lung; 3 — cervical sac; 4 — interclavicular sac; 5 — 8 — outgrowths of the interclavicular sac;

9 — anterior thoracic sac; 10 — posterior thoracic sac; 11 — abdominal sac.

The lower part of the trachea and the initial sections of the bronchi form the syrinx (lower larynx), a vocal organ unique to birds. Within the lungs, each bronchus branches into 15-20 secondary bronchi, most of which end blindly. The secondary bronchi are interconnected by numerous small parabronchi, from which many bronchioles arise—radially arranged alveolar outgrowths densely surrounded by pulmonary blood capillaries. This is where blood oxygenation takes place. Some of the secondary bronchi pass through the lungs and open into large, thin-walled air sacs: the unpaired interclavicular sac, and the paired cervical, anterior and posterior thoracic, and abdominal sacs, which are located between the internal organs, with their diverticula extending under the skin and into the cavities of large bones (humerus, femur). The volume of the air sacs is 10 times greater than that of the lungs.

The breathing mechanism of birds:

✵ during inhalation, the contraction of the intercostal muscles increases the volume of the rib cage, causing the elastic air sacs to expand and draw in air. At the same time, air from the lungs enters the anterior air sacs (interclavicular, cervical, anterior thoracic), while fresh air from the environment travels through the trachea, bronchi, and their branches into the lungs and posterior air sacs (posterior thoracic and abdominal);

✵ during exhalation, the volume of the rib cage decreases, and under pressure from the internal organs, air is forced out of the air sacs. Oxygen-rich air from the posterior air sacs (posterior thoracic and abdominal) enters the lungs, while air from the anterior air sacs (interclavicular, cervical, anterior thoracic), which is depleted of oxygen but rich in carbon dioxide, is pushed into the trachea and expelled.

Thus, oxygen-rich air flows through the lungs almost continuously during both inhalation and exhalation, constantly enriching the blood with oxygen (double respiration).

The respiration rate in birds varies depending on oxygen demand (see Table 12):

Table 12.

RESPIRATION RATE OF BIRDS DEPENDING ON PHYSIOLOGICAL STATE

In addition to enhancing respiration, the air sacs prevent the bird's body from overheating during flight, as they receive cool

air. The increase in intra-abdominal pressure during exhalation facilitates defecation (the elimination of excrement through the cloaca). The air in the air sacs lightens the bird's body during flight. Diving birds submerge more easily by increasing the pressure in their air sacs. By filling the spaces between internal organs, the air sacs also reduce friction between them.

The CIRCULATORY SYSTEM OF birds is characterized by a four-chambered heart (left and right atria; left and right ventricles), which prevents oxygenated (arterial) blood from mixing with deoxygenated (venous) blood (the left side of the heart contains oxygenated blood, the right side contains deoxygenated blood). A distinctive feature of birds is the relatively large size of their heart: in most species, the heart mass accounts for nearly 1% of body weight, and in birds with rapid flight, it reaches 1.5 — 2%; smaller species have relatively larger hearts than larger ones. The heart operates at high intensity: in medium-sized birds (weighing about 0.5 kg) at rest, the heart rate is 200-300 beats per minute, increasing to 400500 during flight; in small birds at rest, the heart rate is 400-600 beats per minute, increasing to 1000 or more during flight. Birds have high blood pressure — 120 — 200 mmHg (compared to 70 — 160 mmHg in mammals and 30 — 50 mmHg in reptiles). The SYSTEMIC AND PULMONARY circulations are completely isolated from each other, preventing any mixing of blood.

Pulmonary Circulation: the pulmonary artery originates from the right ventricle and divides into right and left branches, through which deoxygenated (venous) blood enters the respective lungs. Gas exchange occurs there, and the oxygen-rich (arterial) blood returns to the left atrium via the right and left Pulmonary Veins.

Systemic circulation begins at the left ventricle, from which a single vessel arises — the right aortic arch (the left aortic arch is completely reduced in birds). It initially branches into two vessels, which in turn divide into smaller Arteries. Through these, oxygenated blood reaches all Organs of the body, and via capillaries, they are supplied with oxygen and nutrients. As a result of gas exchange, the deoxygenated blood (rich in carbon dioxide) returns through the veins into the short and wide left and right venae cavae, which empty into the right atrium.

All features of the AVIAN CIRCULATORY SYSTEM correspond to their high metabolic rate. This ensures continuous and intensive oxygenation and nutrient supply to all organs and tissues, as well as the removal of Metabolic waste products from them.

The avian nervous system consists of the brain, Spinal Cord, and the nerves branching from them. The brain mass of birds is significantly increased, ranging from 0.2 to 5–8% of their body mass (in reptiles, the brain mass accounts for 0.01–0.4% of body mass). While the mass of the BRAIN AND SPINAL cord is roughly equal in reptiles, in birds, the brain is larger: 1.5:1 in galliformes and 2.5:1 in pigeons. In the avian brain, the cerebral hemispheres of the Forebrain and the cerebellum are the most highly developed, which is associated with the complex nature of movements requiring perfect coordination. Birds have well-developed conditioned Reflexes.

The Sensory Organs of birds are highly developed:

✵ organs of vision — large eyes (Fig. 74), similar in structure to those of other vertebrates, yet possessing several unique features: the sclera is strengthened by a bony sclerotic ring; at the point where the Optic nerve exits, there is a pecten — a folded, highly vascular structure whose primary function is to supply oxygen to the vitreous body and retina and to remove metabolic waste; the lens is held in place by the ciliary body and ligaments; protected by three eyelids; the eyes provide a wide field of view (the monocular field of view is 150-170°, but in ducks and waders, the total field of view can reach 360°), high visual acuity, and double accommodation (focusing) — not only does the distance between the lens and the retina change, but the shape of the lens changes as well; there are 50,000 to 300,000 photoreceptors (rods and cones) per 1 mm² of the retina, and up to 500,000 to 1 million in the fovea (area of sharpest vision); they can distinguish colors; the primary Processing of visual stimuli occurs in the visual centers of the brain;

Fig. 74. Diagram of a bird's eye (after Naumov and Kartashev, 1979):

1 — sclera; 2 — choroid; 3 — retina; 4 — cornea; 5 — sclerotic ring; 6 — ligament;

7 — ciliary body; 8 — iris; 9 — ligament; 10 — lens; 11 — lens capsule;

12 — vitreous body; 13 — pecten; 14 — optic nerve.

✵ Organ of Hearing: the Inner ear (cochlea), Middle ear (a single auditory ossicle — the columella), and tympanic membrane, which is located below the skin level and is reached by a canal — the external auditory meatus; in some birds (owls), a facial disc of feathers surrounds the ear opening, which serves

the function of funneling sound waves; in owls, a skin fold appears as a precursor to the External ear; hearing is extremely acute; most species hear in the range of 30 to 20,000 Hz; some species (oilbirds, swiftlets) are capable of perceiving ultrasound up to 35-50 kHz;

✵ Organ of Balance is represented by the semicircular canals of the inner ear;

✵ Organ of Smell (it was previously believed that because the olfactory lobes of the forebrain are small, the SENSE OF SMELL is poorly developed; however, scientific experiments indicate otherwise): an enlarged nasal cavity surface covered with olfactory epithelium;

✵ ORGAN OF TASTE: taste buds are located in the mucous membrane of the oral cavity and on the tongue; they can distinguish sweet, salty, and bitter;

✵ Organ of Touch: sensory cells are located in the skin, on the beak, in the oral cavity, and on the hind limbs.

Birds are dioecious organisms. Sexual Dimorphism is pronounced in all birds: males are larger than females, more brightly colored, and have a loud voice and a complex song.

Avian reproduction begins in spring. In most bird species, males establish a specific territory where they will eventually build nests (nesting territory) and sing intensively. Each bird species is characterized by its own mating song. The white stork performs its "song" by clattering the two halves of its bill together while throwing its head back onto its spine. The common snipe performs its song using its tail feathers. Flying high into the sky, the male then dives headlong downward, spreading its tail feathers. The air passing through these feathers vibrates them, producing a sound that resembles the bleating of a lamb. The singing of males not only attracts females but also signals to other males that the territory is occupied. The singing ceases with the end of the breeding season.

In most species, at the beginning of the breeding season, males and females form pairs: some for a single season, and others for many years (storks, swans, eagles, geese, herons). All these birds are called monogamous. In some bird species (black grouse, peacocks, waders, hummingbirds) — polygamy — pairs do not form even for a short period, and mating occurs during brief encounters between males and females. During these encounters at specific display grounds (leks), males compete in unique tournaments, demonstrating complex behavioral displays to win over females. Females prefer to mate with the winners of these tournaments. After breeding, the males stay separate from the females, who raise the chicks.

After mating, birds construct nests of various designs. In monogamous species, both partners build the nest, while in polygamous species, only the female does. Larks, gulls, and terns build nests on the ground. Sand martins, bee-eaters, and kingfishers nest in burrows they dig in cliffs. Woodpeckers and tits nest in tree hollows. In many bird species (weavers, penduline tits, tailorbirds, warblers, orioles, chaffinches), the nest resembles a basket woven from twigs. They can build such nests in trees, bushes, rock crevices, etc. Some birds (grebes, terns) build floating nests in dense aquatic vegetation. They lay their eggs in the nest. The cuckoo does not build its own nest, instead laying its eggs in the nests of other birds. Nightjars and some waders do not build nests at all, laying their eggs directly on the ground. Guillemots lay their egg on bare cliff ledges. Emperor penguins, which live in Antarctica, hold their single egg on their feet, covering it from above with a fold of abdominal skin (at an air temperature of (-5) to (-10)°C, the temperature inside the egg remains +36°C).

The Reproductive System of birds:

✵ females have only one left ovary and oviduct;

✵ males have small, paired, bean-shaped testes (during the breeding season, their volume increases nearly 300-fold), vasa deferentia, and a Seminal Vesicle (a reservoir for mature sperm) located in front of the cloaca.

Copulatory organs, in the form of an unpaired, eversible section of the cloacal wall, are present in only a few birds, such as ostriches and waterfowl (Anseriformes). In other birds, Fertilization occurs when the external opening of the male's cloaca is pressed against the female's cloaca. During mating, sperm is transferred from the male's cloaca to the female's cloaca. Fertilization takes place in the oviduct, after which the ovum is moved toward the cloaca by contractions of the oviduct walls, growing larger and becoming covered with membranes (vitelline, albumen, two shell membranes, and a calcareous shell) before entering the cloaca as a fully formed egg (Fig. 75).

Fig. 75. Structure of a bird's egg (after Naumov and Kartashev, 1979):

1 — germinal disc; 2 — different layers of yolk; 3 — vitelline membrane; 4 — albumen; 5 — chalaza;

6 — outer albumen layer; 7 — air Cell; 8 — shell.

The process takes 12-48 hours. In the smallest birds, hummingbirds (weighing 1.6-1.8 g), the egg weighs about 0.2 g, which is nearly 10% of their body mass; in waders and small passerines, the egg mass is 15-20% of their body mass; in the African ostrich and large penguins, the egg mass is about 1.5% of their body mass.

Embryonic development begins only when the egg is warmed (incubated), starting from the germinal disc (zygote) located in the yolk. The yolk is the primary store of nutrients used to form embryonic tissues and meet major energy demands. Surrounding the yolk is the albumen (egg white), which consists of two layers—a thicker and a thinner one. The albumen is the main source of water required for embryonic development and serves as an additional energy reserve. Externally, the albumen is covered by two thin, parchment-like shell membranes—outer and inner—which separate at the blunt end of the egg to form an air cell. Twisted cords of dense albumen, called chalazae, run from the inner shell membrane to the yolk. Thanks to the chalazae, the yolk is suspended so that the germinal disc always remains on top, regardless of the egg's position. The outermost layer of the egg is a strong calcareous shell (containing 92-95% calcium carbonate, small amounts of magnesium carbonate, calcium and magnesium phosphate, and 3-5% organic matter). In most birds, the shell is colored by pigments secreted by the glandular Cells of the oviduct. The shell contains numerous pores that allow oxygen to reach the embryo. As the embryo develops, some of the salts from the shell enter its bloodstream and are used to form the skeleton. Consequently, the salt content inside the egg increases 4-5 times, while the shell becomes more brittle, making it easier for the chick to hatch.

At the early Selection/3.html">Stages of development, the embryo passes through the same phases as all Chordates: it has a notochord, gill slits, and a tail. As development progresses, plumage and a beak appear, while the tail disappears. Using its beak, the chick pierces the inner membranes of the egg and begins to breathe with its lungs for the first time, thrusting its beak into the air cell. The chick's peeping marks the beginning of pulmonary respiration. With a tubercle on the tip of its beak (the egg tooth), the chick gradually chips through the eggshell and emerges. From pipping the shell to hatching, it takes several hours for small birds and 1-3 days for larger ones. During hatching, the adult bird sits on the nest, keeping the clutch warm. Based on the physiological maturity of the chicks at the moment of hatching, all birds can be divided into two groups (Fig. 76):

1) precocial (maturonatous, mature): chicks are covered with down, have open eyes, and can leave the nest after a short time to follow the adult bird (Fig. 76, B); this is characteristic of ostriches, bustards, geese, ducks, chickens, swans, most waders, and cranes;

2) altricial (immaturonatous, immature): chicks are naked, helpless, with eyes and ear openings still closed (opening on days 4-7); their bodies are only sparsely covered with thin down, and they are weak and unable to stand (Fig. 76, A);

Fig. 76. Chicks (after Naumov, Kartashev, 1979):

A — altricial chick; B — precocial chick.

these chicks remain in the nest for a long time (10-12 days), and both parents care for them by brooding and feeding them; During the first half of their nestling life, the chicks are poikilothermic: their body temperature depends entirely on the ambient temperature; if the adult birds leave them unbrooded for a long time, their respiration rate and heart rate drop, and they enter a state of torpor; upon the parents' return and subsequent warming, their metabolic rate and activity increase; this adaptation enables rapid growth, as the nutrients from food are not spent on thermoregulation but are used solely for body growth; with intensive feeding and brooding, chicks of many species increase their body mass by 20-50% in a single day; after fledging and learning to fly, they begin to feed independently; this is characteristic of passerines, pigeons, and woodpeckers.

The fecundity of birds is significantly lower compared to reptiles:

✵ large birds of prey, penguins, guillemots, and small hummingbird species lay 1 egg;

✵ pigeons, swifts, small penguins, and cranes have a clutch size of 2 eggs;

✵ sandgrouse and most gulls lay 3 eggs; waders lay 4 eggs;

✵ most passerines lay 5-8 eggs;

✵ ducks lay 6-14 eggs; partridges lay up to 26 eggs.

This is due to low embryonic and post-embryonic mortality resulting from Various Forms of parental care. Practically all birds incubate their clutch of eggs, warming them with their body heat. In monogamous species, both partners participate in incubation, while in polygamous species, only the female does. The duration of the incubation period depends on the size of the egg and the bird, the type of nest, and the intensity of incubation. Small passerines incubate for 11-14 days, the crow for 17 days, the raven for 19-21 days, the mallard for 26 days, and swans for 35-40 days. The longest incubation period—about two months—is found in large penguins, albatrosses, and vultures. After the chicks hatch, the adult birds feed them and clean the nest of eggshells and droppings. Often, in the presence of danger, an adult bird gives alarm calls, and the chicks freeze and hide until another signal announces that the threat has passed. In some species, when threatened, the female attempts to appear injured or unable to fly, beating her wings on the ground and crawling to lure the predator away from the chicks; once the predator is drawn to a safe distance, the bird suddenly takes flight, later returning to recall her chicks.

The only birds that do not incubate their clutches are megapodes (mound-builders), cuckoos (50 species), African weavers (10 species), and South American ducks (1 species). The heat required for The Development of megapode embryos is generated by the decomposition of organic matter in their nesting mounds (up to 3 m in diameter). Adult birds merely monitor the temperature inside the mound by inserting their bills. If the temperature inside the nest is excessively high, the birds scrape away some of the soil from the top; if the temperature is insufficient, they pile on an additional mixture of soil and plant debris. Cuckoos and the other aforementioned species lay their eggs in the nests of other birds (brood parasitism); to prevent the host from recognizing the intruder, the eggs closely mimic the color and size of the host's own eggs. The cuckoo chick typically hatches first and uses its back to eject the host's eggs from the nest. This ensures the highly voracious cuckoo chick receives undivided nourishment until it fledges (within 20 days). Usually, the young cuckoo grows to exceed its foster parents in size.

In seasonal climates, the annual Life Cycle of birds is divided into the following periods:

✵ preparation for breeding: the initiation of gonad development triggered by increasing day length; migration to breeding grounds from wintering sites (spring migration), and completion of the pre-nuptial molt;

✵ breeding period: establishment of nesting territories, pair formation, maturation of Gametes, mating, nest building, egg-laying, incubation, and rearing of chicks until they become independent;

✵ post-breeding molt;

✵ preparation for winter: intensive feeding, increased fat accumulation; in some species, autumn migration;

✵ wintering: birds of each species find locations that provide them with food and shelter; birds typically do not experience a decrease in activity or enter torpor (however, common poorwills enter true hibernation for 2-2.5 months: their body temperature drops to 18-19°C, and their heart and respiration rates decrease; nocturnal torpor is characteristic of hummingbirds).

Depending on their response to seasonal changes, all birds are divided into three main groups:

1) resident birds — birds that remain in the same area year-round (sparrows, partridges, black grouse, magpies);

2) nomadic birds — birds that, after the breeding season, travel hundreds of kilometers in search of food but do not venture far from their nesting grounds (woodpeckers, jays, tits);

3) migratory birds — birds that, after the breeding season, leave their nesting sites for wintering grounds and return to breed the following spring (white stork, common crane, barn swallow, cuckoo, corncrakes, nightingales, swifts, waders, starlings).