Feather type and distribution depend on species, season, and life stage. In most birds, feathers grow from discrete tracts (pterylae) adjacent to featherless areas (apteria). Birds such as penguins, for whom insulation is paramount, have a dense, uniform feather coat.

There are seven main feather types, the most prominent being contour feathers encompassing body and flight feathers. That latter includes remiges (primary and secondary wing feathers) and rectrices (tail feathers). Primary feathers attach to the underlying bone.

The only skin gland is the bilobed, subcutaneous uropygial gland, located near the dorsal tail base. It is not present in all species. The sebaceous secretion, spread via preening, conditions the feathers and has antibacterial properties. It is not responsible for waterproofing. Feather interdigitations impart waterproofing.

The skin is very thin on the body, becoming thicker on the legs, feet, and species or sex-dependent ornamentations (combs, caruncles, wattles, and snoods). Appendicular skin binds tightly to the bone.

Legs, feet, and beaks are often featherless. A well-vascularized feather-free ventral brood patch may develop to aid in egg incubation. The keratinous covering of the beak (rhamphotheca) and the fleshy cere should be smooth and uniform (Figure 15.1). Overgrowth or irregularities of the rhamphotheca may indicate husbandry issues, liver disease, or viral infections.

Legs and dorsal feet are often covered in keratinized scales but may be feathered in some species (owls). The soles are featherless, centering on the large metatarsal pad and smaller digital pads. Pads should be smooth and bulge slightly. Plantar dermatitis (bumblefoot) is common if birds are housed on improper substrate or have sustained foot trauma.

Musculoskeletal adaptations, including pneumatized bones, vertebral fusion, and modifications for attachment of flight muscles, provide a stable, light, and aerodynamic structure for flight. Pneumatized bones of the skull, long bones, and vertebrae have honeycombed centers filled with air. The spaces in the humerus and femur are continuations of the air sacs (see respiratory system) allowing infections to spread from the bones to viscera.

Vertebrae are tightly fused, except for cervical vertebrae, one or two bones between the synsacrum and notarium, and the tail. Cranial thoracic and sometimes the last cervical vertebrae fuse to form the notarium. Fused lumbar vertebrae form the synsacrum. Unfused tail bones terminate at the pygostyle, which provides attachment for the rectrices. The vertebrae between the synsacrum and notarium are prime spots for malformations, fractures, and deviations secondary to nutritional imbalances.

The ventromedial keel of carnate (flying) birds attaches to the sternum, providing attachment for flight muscles. The large superficial and deep pectoral muscles pull the wing down (Figure 15.2). The tendon of the deeper, smaller supracoracoideus muscle attaches to the dorsal humerus after passing through the triosseous canal, a structure formed by the bones of the pectoral girdle (scapula, furcula, and coracoid), facilitating upstroke. The ratite sternum is flat and the pectoral muscles relatively small, but thigh muscles well developed.

Muscle tissue ranges from pale tan to red depending on myoglobin content. More myoglobin is present in the pectoral muscles of strong flyers and in the limbs of terrestrial species (Figure 15.2).

The four-chambered heart lies in a midline notch between hepatic lobes (Figure 15.3). The pericardium adheres to the ventral sternum. The atria have thin walls, and the right is frequently dilated by a postmortem blood clot. The right atrioventricular valve has a muscular septal and a membranous mural leaflet, while the left atrioventricular, pulmonic, and aortic valves are completely membranous.

On transverse section, the right ventricular free wall is thin, and the left wall and septum are at least 4 times the thickness of the right. The right ventricular lumen is a small slit, while the left is a small central cavity.

Large blood vessels around the heart including the vena cava, aorta, and subclavian, along with the coronary vessels are common sites for atherosclerotic plaques (see Figure S15.6).