This document provides an overview of avian anatomy and physiology, describing key features of the skin, feathers, skeletal system, muscles, senses and other body systems of birds. It notes that birds have lightweight skeletons, feathers for flight and insulation, specialized digestive and respiratory systems adapted for flight, and various anatomical adaptations related to vision, hearing and other senses.

1. Avian Anatomy and Physiology

2. Learning Objectives Describe the anatomical structures of avian skin, beaks, and claws. Describe the structure and functions of feathers. List the types of feathers and the location of each type. List the unique features of the avian skeleton and musculature. Describe the shape and general characteristics of avian eyes and explain how those characteristics effect visual acuity. Describe the unique anatomical features that affect the sense of hearing in nocturnal owls. List the unique anatomical features that affect the sense of taste in birds. List the components of the avian digestive system and describe the functions of each. List the unique features of the avian circulatory and respiratory systems. Describe the composition of avian urine. List and describe the unique features of the avian male and female reproductive systems. List and describe the four classifications of newly hatched chicks.

3. Skin Epidermis : thin layer of flattened epithelial cells that produce keratin Fibrous protein necessary for the production of scales, feathers, and outer sheath of beaks and claws

4. Skin Dermis : thick layer of fibrous connective tissue Stores fat for nutrition and insulation Smooth muscles in dermis innervate feather follicles to aid in heat regulation

5. Glands Uropygial (preen) gland : On dorsal surface at upper base of the tail Preening stimulates secretion of an oily, fatty substance Bird uses beak to spread the oil throughout its feathers to clean and waterproof them

6. Beak Derivative of skin Upper and lower mandible covered with a horny keratin layer Grows continuously Variable hardness and flexibility, depending on the function

7. Claws Horny sheath derived from specialized scales at the end of each toe Grow continuously Variable types of claws, depending on perching habits and method of procuring food

8. Feathers Outgrowths of skin Made of protein Functions: Flight Protect thin skin from trauma, rain, and excessive radiation from sunlight Assist in thermoregulation and camouflage Used in communication behaviors (e.g., courtship, defense, and recognition)

9. Feather Structure Inferior umbilicus : opening at base of feather Superior umbilicus : opening on feather shaft where webbed part of the feather begins May give rise to an afterfeather : accessory feather thought to provide additional insulation Calamus : quill; round, hollow, semitransparent Extends from inferior to superior umbilicus Rachis : main feather shaft

10. Feather Structure Vane : flattened part of a feather Numerous slender, closely spaced barbs that give rise to barbules containing hooklets (hamuli) Hooklets interlock each barb with an adjacent one

11. Types of Feathers Contour Feathers Flight feathers of the wings and tail (remiges and retrices) Moved by muscles attached to the walls of the follicles Auriculars : small contour feathers around external ear openings Improve bird’s hearing ability

12. Types of Feathers Semiplume Feathers Have main rachis with barbs and no barbules or hooklets Found under contour feathers Provide insulation Provide flexibility for movement of contour feathers Help with buoyancy in water birds

13. Types of Feathers Down feathers Soft, fluffy feathers Lack a true shaft No barbules or hooklets Located next to skin under contour feathers Function in insulation

14. Types of Feathers Filoplume Feathers Bare shaft with barbs on the tip Located on the nape and upper back near contour feathers Nerve endings in follicles may play role in controlling feather movement

15. Types of Feathers Bristles Modified contour feathers Stiff rachis with few barbs at the base May play role in sense of touch May be found around the eyes, nostrils, mouth, and toes

16. Types of Feathers Powder Down Feathers Grow continuously at the base Disintegrate at their tip Create waxy powder that spreads throughout the rest of the plumage Birds without a uropygial gland have abundant powder down feathers

17. Feathers Pterylae : tracts where feathers are located Apteria : bare areas of skin between pterylae Feathers in pterylae overlap one another

18. Feather Damage Feather mites and other external parasites can chew and consume parts of the feather vanes Daily wear and tear: lighter tips of flight and tail feathers can be worn off

19. Feather Damage Fault bar (stress bar): weakened area on the feather vane where barbs lack barbules Results from stress that interrupts blood flow during feather growth Common stressor: poor diet

20. Molting Process of feather replacement Occurs once to several times a year, depending on species In most species, feather replacement is symmetrical One or two pairs of flight feathers molted at a time Many species of waterfowl molt all flight feathers at once after the breeding season

21. Molting Feathers develop from papillae in feather tracts of the dermis Contain cells with genetic information that dictates type, size, and color of feathers Hormones stimulate molting, activate cells in papillae

22. Molting Newly developing feather pushes old feather out Feather emerges covered by periderm Periderm removed by preening Blood vessels from dermis reach into new feather When feather is fully grown, blood dries up

23. Skeletal System

24. Skeletal System Skeletal modifications allow for flight and walking: Reduction in number of bones Fusion of some bones to form plates Reduction in bone density Loss of internal bone matrix; bone becomes hollow and filled with air spaces

25. Skull Skull bones are thinner than in other animals Jaws extend into a keratinized bill Large eye sockets bordered by sclerotic ring Protective bony plates

26. Vertebral Column Cervical Vertebrae: Atlas contains a single condyle for attachment of the skull Allows greater range of head movements Greater number of cervical vertebrae than mammals Variable in different species

27. Vertebral Column Thoracic Vertebrae: Rigid and provide a strong support for the rib cage First few ribs are relatively short and incomplete Remaining ribs attach to underside of sternum and have a projection (uncinate process) that overlaps the adjoining rear rib to strengthen the rib cage

28. Vertebral Column Lumbar and Sacral Vertebrae: Synsacrum : strong bony plate created from fusion of several distal lumbar vertebrae, sacral vertebrae, and first few coccygeal vertebrae Synsacrum fuses with the pelvis, supports the legs Coccygeal vertebrae: First few are mobile Pygostyle : bony structure created from fusion of some coccygeal vertebrae

29. Sternum Large and concave Site of origin of flight muscles In some species, muscles attach to large bony ridge or keel In flightless birds sternum lacks a keel

30. Pectoral Girdle Paired coracoids, scapulas, and clavicles Glenoid cavity (triosseal canal): formed from joining of scapula and coracoid Site of wing joint attachment Clavicles (wishbone): positioned outward and forward from the body

31. Wings Joint at shoulder allows rotation in several planes Humerus : wing muscles attach at pectoral crest of humerus Humerus length varies Shorter in birds with flapping flight; longer in birds that glide and soar

32. Wings Joint at elbow only allows movement parallel to wing Ulna has a larger diameter than radius Patagium (propatagium): web of skin Ligament runs along its cranial edge Provides elasticity to the wing and assists in the aerodynamics of flight

33. Wings Alula bone : originates from the wrist and carries the alula feathers (steering feathers) Metacarpal bones join with the second and third fingers near the distal end of the wing Help support the primary flight feathers

34. Pelvic Girdle Three paired bones that join where the leg attaches to the body Ileum : broad and fused to the synsacrum Ischium and pubis : thin, long, and fused to the ileum anteriorly

35. Hip Femur (drumstick): short and wide; directed forward at the knee Allows for lower part of bird's leg to be under its center of gravity Greater and lesser trochanters on femurs: site of attachment of leg muscles Tibiotarsus and Fibula : end at the hock joint Hock : single, elongated bone ( tarsometatarsus )

36. Feet Metatarsal pad : bottom of foot; surrounded by two, three, or four toes Anisodactyl : one toe faces the rear, other three face forward Zygodactyl : second and third toes face forward, first and fourth are directed backward Digits are numbered: Digit one (rear toe) - one joint; Digit two (medial) - two joints; Digit three - three joints; Digit four (lateral) - four joints

37. Muscles Skeletal muscles - can have white or red muscle fibers or combination of both White fibers : thick in diameter, low blood supply, little myoglobin Use stores of glycogen to sustain muscle contraction Found in flight muscles of short-distance fliers Red fibers : thin, have rich supply of blood, fat, myoglobin, and mitochondria Found in flight muscles of long-distance fliers

38. Wing Muscles Raise or depress leading edge of wing Pull wing forward or backward Extend or flex wing Control movements of alula bone

39. Flight Muscles Pectoralis : large, superficial Origin: sternum Insertion: underside of humerus Depresses wing (downstroke) Supracoracoideus : small, deeper Origin: sternum Insertion: top of humerus Elevates wing (upstroke)

40. Leg Muscles Primarily located over the femur Small number located over the tibiotarsus and tarsometatarsus

41. Muscles of the Head and Neck Extent of jaw muscles vary, depending on bird's feeding habits Neck muscles allow movement in different directions Hatching muscle : used to help chick break shell open Atrophies after hatching

42. Sense Organs Location of control centers of brain are similar to mammals Control centers for vision and hearing are relatively large Control centers for taste, touch, and smell are relatively small

43. Vision Highly developed Optic lobes make up majority of midbrain Eye shape determined by the orbits Diurnal birds usually have round or flat eyes Nocturnal species have tubular eyes Position of eyes varies

44. Anatomy of the Eye Fibrous tunic : sclera, cornea Sclera contains ring of bones: sclerotic ring Uveal tunic : choroid, iris, ciliary muscles Nictitating membrane may have clear center Neural tunic : retina Pectin: vascular structure attached to retina

45. Photoreception Rods and cones similar to mammals Nocturnal birds have more rods than cones Rods contain rhodopsin - night vision pigment Retina : lightly vascularized and contains numerous photoreceptor cells Some species have a second temporal fovea rather than a single central fovea Birds can see wide spectrum of wavelengths

46. Hearing and Equilibrium Ears are located behind and slightly below the eyes Columella : single bone in middle ear External ear is often bordered with auricular feathers

47. Hearing in Nocturnal Owls Operculum : fleshy flap of skin at each external ear opening Helps funnel sound into the ears Ear openings are asymmetrical Helps with vertical location of sound

48. Hearing in Nocturnal Owls Large eardrums, columellae, cochleae, and well-developed acoustic center in the hindbrain Very large number of auditory neurons

49. Taste & Touch Taste buds: few in number; scattered on sides of tongue and soft palate Nerve endings for touch Grandry's corpuscle : in the tongue and palate of species that dig for food Herbst corpuscle : sensitive to feather movement

50. Smell Highly variable In some species, well developed and needed for locating food In other species, less developed but adequate

51. Endocrine System Seven major endocrine glands Function similar to mammals

52. Digestive System

53. Digestive System Beaks - vary with diet and foraging habits Seedeaters - thick, crushing beak Woodpeckers - heavy, blunt, chiseling beak Raptors - sharp-edged, tearing, hooked beak Shorebirds - long, delicate, probing beak

54. Digestive System Mouth: consists of hard upper palate, soft lower palate, distinctive tongue, salivary glands, taste buds Soft palate may be enlarged into a pouch used to temporarily store food Tongue: has few muscles in most species Crop: expansion of esophagus over the furcula in some species Storage pouch for food

55. Stomach Two compartments: glandular stomach, muscular stomach Proventriculus : anterior glandular stomach; site where chemical digestion begins Gizzard : muscular stomach; striated muscles that grind food Many birds ingest small pieces of grit to aid in grinding food

56. Liver and Pancreas Liver Bilobed, right lobe larger than left Pancreas Relatively large; rests in the loop of the duodenum Endocrine portion of pancreas occupies more tissue mass than in mammals

57. Digestive System Duodenum Main digestive organ; varies in length and thickness depending on diet Ceca Paired sacs at junction of small and large intestines in some species Play role in water reabsorption and bacterial fermentation of cellulose Contents excreted independent of fecal material

58. Cloaca End of digesive tract Coprodeum : anterior section; receives excrement from the intestine Urodeum : receives discharge from the kidneys and genital ducts Proctodeum : posterior section; stores the excrement Vent : muscular anus

59. Feeding Habits Diet is determined by: Physical characteristics (type of bill, feet, plumage, development of senses) Daily activity cycle (diurnal vs. nocturnal) Flight style and endurance Habitat, time of year, geographical location Sex Age

60. Heart Four-chambered heart In cranial portion of the thoracoabdominal space Enclosed by thin, fibrous fluid-filled pericardial sac Adherent to several internal surfaces

61. Blood Vessels Pectoral and brachial arteries Large; provide blood to flight muscles and wings

62. Blood Vessels Renal portal system

63. Blood Vessels Countercurrent heat exchange system In lower extremities of some species Network of arteries and veins placed close together Heat from arterial blood transferred to cooler venous blood

64. Blood Erythrocytes Oval, nucleated Percentage of red blood cells in a healthy adult bird ~ 35% - 55% Heterophils Equivalent to neutrophils Bilobed nucleus Rod-shaped red-orange granules

65. Respiratory System

66. Oral Cavity Glottis : opening of trachea at back of tongue Choanae : internal nares Open from nasal chambers into roof of mouth Larynx : does not function in production of sound as in mammals

67. Oral Cavity Trachea : long and coiled (some species) Syrinx : enlargement of trachea above lungs Muscles, air sacs, and vibrating membranes Vocalization complexity depends on number of muscles present in syrinx

68. Bronchi Mesobronchi - in lungs No cartilaginous rings Divide into secondary bronchi (ventrobronchi) Ventrobronchi - divide into parabronchi Parabronchi - connect to air capillaries Site of gas exchange

69. Air Sacs Thin walled, lightly vascularized membranes Paired - cranial thoracic, caudal thoracic, cervical, and abdominal air sacs Unpaired - interclavicular air sac Reservoirs for air Provide warmth and moisture to facilitate diffusion of air through capillaries Aid in thermoregulation Help provide buoyancy

70. Lungs Small, inelastic Attached to thoracic vertebrae and ribs Air capillaries are at right angles to the blood capillaries Carbon dioxide continually removed from blood and oxygen continually added

71. Air Flow Inspiration: expansion of thoracoabdominal space Creates pressure gradient that brings air into posterior air sacs Expiration pushes air into the lungs

72. Air Flow Second inspiration moves air out of the lungs and into the anterior air sacs Second expiration moves air out through the trachea

73. Respiratory Rate Varies with species, activity level, age, sex, time of day, and outdoor temperature Smaller birds usually breathe faster than larger birds Birds in flight have a higher respiratory rate than nonflying birds

74. Thermoregulation Air in the lungs picks up heat from body tissues and blood and removes it from the body during the breathing process Increased air flow over mouth, pharynx, bronchi, and air sacs increases cooling Achieved through panting or gular fluttering (rapid vibrations of upper throat patch)

75. Thermoregulation Birds bathe or reduce activity level during warmest parts of the day Some species defecate on their legs for evaporative cooling Can adjust the positions of body feathers and posture to promote both heat loss and heat retention Countercurrent heat exchange system limits heat loss through their bare legs

76. Urogenital System

77. Kidneys Located dorsally in slight depression formed at level of synsacrum Elongated with three divisions; no renal pelvis Three divisions subdivide into lobules containing nephrons

78. Ureters Surrounded by smooth muscle that helps squeeze urates from the kidney or inhibit their flow into the cloaca Urine passes through ureters to urodeum of the cloaca Either moved upward to colon and ceca for further reabsorption of water or propelled outward through the vent

79. Urine Composition Primary nitrogenous waste component is uric acid Waste is eliminated as a paste with a small quantity of fecal material Excreted product is called a mute

80. Reproductive System Reproduction is periodic Increasing daylight stimulates release of hormones that directly affect the reproductive process Reproductive organs larger on the left side of the body In females the right ovary is rudimentary and not even functional During nonbreeding season, gonads are relatively small

81. Copulation Copulation is achieved in one of two ways: Grooved erectile penis is attached to the wall of the cloaca to help transfer sperm into the female's vagina Sperm transfer occurs when the male and female bring their cloacae into close proximity

82. Oviduct Infundibulum : grabs ovum as it comes out of ovary Magnum : secretes layers of albumin around egg Isthmus : deposits keratin shell membrane

83. Oviduct Uterus : deposits watery albumin, a hard external shell, and pigmentation Large and muscular Also called the shell gland Vagina : secretes mucus to assist in egg laying; can store sperm for several hours to several days

84. Clutch Number of eggs that a female lays and incubates Varies among species Many species lay one egg per day Some species lay eggs every other day Some larger species may lay at 4- to 5-day intervals

85. Clutch Total number of eggs that a female can lay varies with the species Determinate layers : specific number of follicles that develop Indeterminate layers : can produce more eggs than their normal clutch size Continue to lay eggs if their eggs disappear

86. Incubation Average incubation temperature for many species is 35° C Prolactin promotes broodiness (incubation behavior) Hormones also stimulate development of a “brood patch” Area of skin on the lower abdomen where heat is transferred to the egg Incubation time varies with the species

87. Hatching Involves powerful neck muscles and specialized egg tooth on the outside of a chick's bill After hatching, neck muscles atrophy and egg tooth disappears

88. Chicks Newly hatched chicks differ in amount of feather cover, status of eyes, and degree of mobility Altricial: hatched with eyes closed and skin bare Semialtricial: covered with down, are immobile, eyes may be open or closed Precocial: covered with downy feathers, eyes open, are mobile, leave the nest quickly Semiprecocial: downy covering, eyes open, limited movement from the nest