As the air is breathed out of the lungs, some of this fresh air sac air enters the lungs. Birds have large eyes that with very good eyesight, and with many light-sensing rods and cones in the retina. 1 Birds, like reptiles, also secrete uric acid, from water-efficient metanephric kidneys. Males have paired dorsal testes and sperm is transferred from male cloaca to female cloaca. In females, usually the left ovary is developed.
Fertilization occurs high up in the oviduct and a shell gland puts on the shell afterward.
After pairing, bird engage in nest formation. Eggs are incubated, and the hatchlings have an egg tooth to free themselves. Helpless altricial young need more parental attention than precocial young. Migration is periodic round trips between breeding and nonbreeding areas. Photoperiod may be a useful clue for the time of year, and a sun compass based on UV light may be useful for navigation.