Avian Anatomy: Systems and Adaptations

Avian Anatomy

Learning Objectives

• Explain anatomical adaptations unique to birds related to:
• Integument
• Cardiorespiratory
• Gastrointestinal
• Musculoskeletal
• Urogenital
• Immune systems
• Understand and describe clinical applications of avian anatomy
• Discuss examples of diseases or conditions unique to birds relating to their specific anatomical adaptations
• Understand the anatomical position of the avian skeleton
• Identify relevant soft-tissue external structures assessed during physical examination
• Differentiate between different types of feathers
• Identify soft-tissue organs from the respiratory, gastrointestinal, immune, and urogenital tracts

Integument

• Ornamental features are a part of the integument
• Skin of birds is very thin and lacks sweat glands
• Avian skin has feather tracks and scales on their feet and legs
• Beaks are a derivative of skin
• Uropygial glands are at the tail base for waterproofing
• Patagium is the triangular area between the shoulder, elbow, and wrist joints
• The surface area of a wing is increased by the Patagium
• Keeping a bird's wing aerodynamic is one role of the Patagium

Integument: Feathers

• Pteryla is the tract in the skin where feathers grow
• Apteria is featherless skin between pterylae
• Contour feathers are externally visible feathers of flight
• Alula, primary, axillary, and secondary
• Down feathers have no barbules, found under contour feathers, conserve heat, and are abundant in waterfowl

Integument: Ornamental Features

• Ornamental outgrowths of the skin have a thick and vascular dermis, which makes them easily injured
• Ornamental features include:
• Comb
• Wattle
• Snood
• Ear lobes are a remnant of an ear lobe. They are sometimes white, but most birds don't have them

Musculoskeletal System

• The avian musculoskeletal system is highly adapted for flight
• Compact and strong bones that have a high content of calcium phosphate
• Includes a prominent sternum, a pelvis that is open ventrally, a forelimb modified to form a wing, and considerable fusion of vertebrae

Musculoskeletal System: Key Structures

• Notarium is a fusion along the dorsal spinous processes of most of the thoracic vertebrae
• Keel and Sternum:
• Supports internal organs
• Aids in respiration
• Contributes to body condition
• Helps with positioning for radiography
• Synsacrum is a fusion of the last 2 or 3 thoracic vertebrae, all of the lumbar, sacral, and first few caudal vertebrae
• Pygostyle is a fusion of the most caudal 3-4 vertebrae
• Lower jaw articulates with upper jaw via quadrate bone
• Upper jaw movable with respect to cranium
• Large orbits separated by interorbital septum
• Single occipital condyle
• The clavicle is modified into a wing
• Birds have a well-developed clavicle
• Right and left clavicles unite to form the furcula, which is the wishbone
• Coracoid bone extends from the head of the humerus to the keel and acts as a brace against the vigorous up and down strokes of the wing
• The ulna is larger than the radius
• The bird has two carpal bones: radial carpal bone and ulnar carpal bone Three metacarpals
• Pelvic limb allows for sole locomotion on the ground
• Hip bones; ilium, ischium, and pubis; are fused dorsally with the synsacrum
• Femur resembles mammalian bone and may be used for sampling bone marrow
• Tibiotarsus is where the tibia is fused distally with the proximal row of tarsal bones
• Tarsometatarsus is where the distal row of tarsal bones is fused with the metatarsal bone

Musculoskeletal System: Wing Muscles

• Triceps muscle (1)
• Biceps brachii (2)
• Brachial vein (3)
• Patagium (4)
• Extensor carpi radialis (5)
• Carpal joint (6)
• Flexor carpi ulnaris (8)
• Cutaneous ulnar (wing) vein (9)
• The pectoralis is superficial; its contraction produces a powerful downstroke of the wing
• The supracoracoideus is smaller and mainly used for takeoff, not flight
• Both originate on the keel and insert on the humerus and have opposing actions

Cardiorespiratory System

• Larynx is not involved in vocalization
• Has a laryngeal mound that's a landmark for endotracheal intubation
• Has no epiglottis
• The trachea runs on the right side of the neck
• It has complete cartilaginous rings
• Ends at a bifurcation and the syrinx
• The syrinx is the vocal apparatus at the tracheal bifurcation
• Bird lungs have a constant volume, unlike mammals
• Bird lungs are closely associated with the ventral aspect of ribs
• Air sacs are external to the lung
• Bird air sacs include:
• Clavicular and cervical (singular)
• Cranial thoracic, caudal thoracic, and abdominal (paired)
• Heart structure is similar to that of mammals
• The heart is not covered by the lungs
• The apex of the heart is covered by the liver

Gastrointestinal System

• The avian GI system is composed of:
• Esophagus
• Crop
• Proventriculus
• Ventriculus
• Paired ceca
• Ileum
• Jejunum
• Colon/rectum
• Duodenal loop
• Meckel's diverticulum
• Cloaca
• Vent
• Avian species lack a soft palate; instead, the hard palate contains the choana
• The choana (longitudinal fissure)connects the oral and nasal cavities, and are lined with caudally directed papilla that move food caudally
• Salivary glands and taste buds in birds are poorly developed
• Birds that eat dry diets (seeds) have more salivary glands than birds that eat wet diets (fish)
• The crop is is an out-pouching of the esophagus
• Cranial to the thoracic inlet on the right side in chickens
• Some bird species do not have a crop
• Serves as a temporary storage space so birds can eat and fly
• The Proventriculus is the glandular part of the stomach that secretes digestive enzymes
• The Ventriculus is the non-glandular part of the stomach that is thick and muscular; breaks up larger food
• In the Small Intestine: The Duodenum forms a tight U-shaped loop
• Lying between these segments is the pancreas
• In the Small Intestine: the Jejunum forms loose coils along the edge of the mesentery On its antimesenteric border Mckel's diverticulum is a remnant of the fetal yolk stalk
• In the Small Intestine the Ileum is short, and between the two caeca
• The Large Intestine: Comprises the ceca along with the colon (rectum) The Ceca are particularly well developed in the chicken The colon (rectum) is the final, straight segment that passes to the cloaca
• The terms "colon" and "rectum" can be used interchangeably
• 3 parts of the cloaca:
• Coprodeum: Dilated distal portion of the colon.
• Urodeum: Entrance of ureters and genital tract.
• Proctodeum: Cloacal bursa

Urinary System

• Kidneys and ureters are present in birds
• Ureters drain into the urodeum
• Avian species have no renal pelvis, urinary bladder, or urethra except in ostrich
• Male birds have paired testes inside the abdomen, but no scrotum Epididymides and deferent ducts are present

Female Reproductive System

• In egg production, the oviduct of the chicken hen contains:
• Infundibulum: 7 cm in length, thin coating of dense albumen, occurs in 15 minutes
• Magnum: 30 cm in length, albumen, occurs in 3 hours
• Isthmus: 8 cm in length, primarily shell membranes, in 1 hour
• Shell gland ("uterus”): 8 cm in length, shell, occurs in 20 hours
• "Vagina”: Passage of egg occurs in seconds
• Only one oviduct is present on the left side

Immune System

• Birds have a thymus that is responsible for T cell development and develops from the outpouching of the foregut
• Birds have a Bursa of Fabricius that is responsible for B cell development and develops from the outpouching of the hindgut
• No palpable lymph nodes as in mammals
• Thymus and Bursa of Fabricius regress early in life Spleen: at the junction of the proventriculus and ventriculus -clinical relevance for several diseases