CHARACTERISTICS OF CLASS AVES

Characteristics of Class Aves

1. Body usually spindle shaped, with four divisions: head, neck, trunk, and tail; neck disproportionately long for balancing and food gathering.

2. Limbs paired; forelimbs usually modified for flying; posterior pair variously adapted for perching, walking, and swimming; foot with four toes (two or three toes in some)

3. Epidermal covering of feathers and leg scales; thin integument of epidermis and dermis; no sweat glands; oil or preen gland at base of tail; pinna of ear rudimentary

4. Fully ossified skeleton with air cavities; skull bones fused with one occipital condyle; each jaw covered with a keratinized sheath, forming a beak; no teeth; ribs with strengthening, uncinate processes; posterior caudal vertebrae reduced and fused as the pygostyle; pelvic girdle a synsacrum; aerythrocytes sternum usually well developed with keel; single bone in middle ear

5. Nervous system well developed, with 12 pairs of cranial nerves and brain with large cerebellum and optic lobes

6. Circulatory system consists of four-chambered heart with two atria and two ventricles; completely separate pulmonary and systematic circuits; right aortic arch persisting; nucleated erythrocytes

7. Endodermic

8. Respiration by slightly expansible lungs, with thin air sacs among the visceral organs and skeleton; syrinx (voice box) near junction of trachea and bronchi

9. Excretory system includes metanephric kidneys; ureters open into cloaca; no bladder; semisolid urine; uric acid main nitrogenous waste

10. Sexes separate; testes paired, with the vas deferens opening into the cloaca; females have left ovary and oviduct only; copulatory organ (penis) only in ducks, geese, paleognathids and a few athers

11. Fertilization internal; amniotic eggs with much yolk and hard, calcareous shells; embryonic membranes in egg during development; incubation external; young active at hatching (precocial) or helpless and naked (altricial); sex determined by females (females heterogametic)