Food is not chewed in the mouth but ground in the gizzard, a digestive organ near the stomach. The bird's beak, made of keratin, has proven to be easily modified during evolution, taking on a great variety of shapes suitable for different diets. Flight and Sight Flying requires great expenditure of energy from an active metabolism. The efficient lungs have tiny tubes leading to air sacs that help dissipate heat and contribute to the trimming of density. For safe flight, senses, especially vision, must be acute.
Birds have excellent eyes, perhaps the best of all vertebrates.
Reproduction With brains proportionately larger than those of reptiles and amphibians, birds generally display more complex behaviour. Avian behaviour is particularly intricate during breeding season when birds engage in elaborate ritual courtship. Since eggs are shelled when laid, fertilization must be internal. The act of copulation is somewhat awkward because the male of most bird species has no penis. He must climb atop the female's back and then twist her tail so the mates' vents, the openings to their cloacas, can come together.