1.1
Anatomy and Physiology

Mariah Harrington

See Chapter 1.2. Parrots are a distinctive taxon. As a group, with some exceptions, they are diurnal, social, brightly coloured, arboreal birds found in tropical and subtropical climates. They have a distinctive, hooked bill they use for climbing and foraging. There are over 350 species in the order Psittaciformes, split into three families:

• Cacatuidae: Cockatoos, cockatiels
• Psittacidae: Parrots, parakeets, lorries, lorikeets, macaws
• Strigopidae: Kakapo, kea

The majority of birds seen in clinical practice are from the Cacatuidae and Psittacidae families (Figure 1.1.1).

Body Design

The majority of birds are designed for the unique and vigorous demands of flight, and parrots are no exception. Some of these attributes include:

• Strong, light skeleton
• Powerful muscles located close to the body with long tendons
• Highly specialised circulatory and respiratory systems
• Efficient and short digestive system
• High metabolic rate
• Well-developed visual processing centre and specialised eye structure
• Feathers which are fundamental to flight, communication and temperature regulation

Birds do not possess a diaphragm separating thoracic and abdominal cavities. Instead, they have a coelom, which is divided into 16 body cavities.

• Eight of the cavities are pneumatised and part of the respiratory system
• Eight of the cavities are not pneumatised - there are two pleural cavities, one pericardial cavity, four hepatic peritoneal cavities, one intestinal peritoneal cavity and one extraperitoneal space.

Integument

Like mammalian skin, avian skin has three layers; however, it is much thinner.

• Epidermis: The outermost layer, sebum is secreted from keratinizing basal cells.
• Dermis: The middle layer containing feather follicles, blood vessels, nerves and the smooth muscle and tendons that move the feathers.
• Subcutis: The deepest layer which contains fat and striated muscle for feather movement.

Figure 1.1.1 Species commonly seen in veterinary medicine.

Source: Courtesy of Aidan Raftery.

The skin of the distal leg and foot is scaled, and the skin attaches directly to bone. The bottom of the foot has segmented digital fat pads underneath the scaled skin to cushion the weight-bearing surface (Figure 1.1.2).

Birds do not have true glands throughout the skin, instead they possess an uropygial gland, sometimes called a preen gland.

• The uropygial gland is a bilobed holocrine gland, located on the rump above the tail. It secretes an oily substance that is important for waterproofing and maintaining the feathers.

  Figure 1.1.2 Plantar aspect of normal feet and nails.

  Source: Courtesy of Aidan Raftery.

• Some parrot species do not have an uropygial gland.

Feathers are a defining characteristic of class Aves; they are important for flight, temperature regulation and communication.

• Feathered skin is organised into two categories: pterylae and apterylae. Pterylae, or feather tracts, are the places where the feathers grow. Apterylae are the places where no feathers grow.
• The parts of a feather are:
  • The rachis, or shaft, which gives the feather structural integrity.
  • The vane, which is made up of filamentous barbs with interlocking barbules that allow the feather to maintain its shape during flight.
  • The calamus or quill where the feather attaches to the body.
• Feather colouration occurs as a result of dietary pigments and light refraction.
• There are six different feather types, and not all birds have all six types.
  • Contour feathers: These are the main feather type and are found all over the body; they include the remiges (primary flight feathers), the retrices (tail feathers) and coverts.
  • Down feathers: Small, fluffy feathers for warmth.
  • Powder down: Small and break down into a chalky powder that helps to maintain contour feather structure.
  • Filoplumes.
  • Semiplumes.
  • Bristle feathers.
• Different species have different numbers of remiges, but the majority of parrots have 8-10 primaries and 10-20 secondaries.

Moult is the process of losing and replacing the feathers and happens one to two times a year, usually following breeding. It is extremely metabolically demanding.

• Moult is regulated by both intrinsic factors (such as hormones) and extrinsic factors (such as photoperiod). It occurs gradually so the bird never loses the ability to fly.
• The pattern of moult is species-specific, but typically the primaries and retrices are lost and replaced in mirrored pairs.

Feather development occurs in a series of steps; the feather itself is a modified keratin structure.

• The new feather begins developing in the follicle, and as it grows, it displaces the existing feather. At this stage, the feather has a large, central blood supply.
• New feathers are encased in a fragile, waxy cuticle that protects them as they grow; these are often called pin feathers. The casing comes off as the feather matures, allowing the vane to take its shape.
• Once the feather is fully mature, the blood supply recedes, leaving a hollow quill.
• If the feather breaks while it still has a blood supply, significant haemorrhage will occur.
• Some feathers are rooted in the periosteum; the secondary flight feathers are anchored to the caudal border of the ulna in the wing.

Musculoskeletal System

Birds possess a lightweight, strong skeleton. Some bones are pneumatised; their central cavity is full of air and reinforced with trabeculae that act as internal scaffolding. The pneumatised central cavities arise from air sac diverticula and are extensions of the respiratory tract.

Osteomyelosclerosis occurs in reproductively active female birds where medullary bone is synthesised to act as a calcium reservoir during egg formation. It can be visualised radiographically as increased radiodensity in the medullary cavity of non-pneumatised bones.

The axial skeleton consists of the skull, the vertebral column and the sternum.

• The two most distinctive features of the avian skull are the large eye orbits and the bill.
  • The bill has an upper maxilla and a fused, lower mandible.
  • The rhamphotheca is the sheath of keratinised epithelium that makes up the outermost layer of the bill.
  • The bill grows continuously, and alignment is maintained by normal wear. The dorsal portion grows more quickly creating the hooked shape of the bill.

    Figure 1.1.3 Lateral view of a macaw skull showing maxillary hinge.

    Source: Courtesy of John Chitty.

  • Some species possess a sense organ called the bill tip organ.
  • Larger birds, such as macaws, have a synovial hinge joint in the maxilla that allows for hyperextension of the upper bill (see Figure 1.1.3).
  • The avian skull is lightweight and there is a large, complex infraorbital sinus which drains dorsally into the nasal cavity, not all of it is encased in bone and infections can cause soft swellings of the face (see Figure 1.1.4a and b).
• The avian vertebral column has free portions and fused portions. In some species, the vertebral bodies are pneumatised.
  • The cervical segment consists of free vertebrae with synovial joints between each vertebra, allowing for a long, flexible neck. The number of cervical vertebrae depends on the species.
  • The thoracic vertebrae are not fused, but they are inflexible and referred to collectively as the notarium.
  • The synsacrum is a fused segment of the caudal thoracic vertebrae, lumbar vertebrae and sacrum. It is fused with the pelvis to provide rigid stability during flight.
  • The caudal vertebrae are not fused and end with the pygostyle; their movement allows fine control of the tail feathers.
• The sternum is made up of the ribs and a well-developed ventral keel that provides an attachment site for the flight muscles.

Figure 1.1.4 (a and b) Macaw skull lateral and rostro-caudal views showing extent of sinus system.

Source: Courtesy of John Chitty.

The appendicular skeleton consists of the wing and the leg. In general, the muscle bellies are located close to the core of the body and have long tendons extending distally. This allows for a more streamlined silhouette for flight and better temperature regulation.

The wing is a defining feature of avian anatomy. It is a highly specialised structure that acts as a sail to both generate lift and catch air currents during flight.

• The bones and muscles that make up the thoracic girdle are what give the wing its power, particularly on the downstroke; they also prevent collapse of the thorax during wing movement.
  • The three major bones that make up the thoracic girdle are the coracoid, scapula and clavicle.
    • The coracoid is a rod that maintains the wing's position relative to the body during the downstroke.
    • The scapula lies parallel...