Almost every structure in a bird's body is changed in some way that enhances flight. Another adaptation reducing the weight of birds is the absence of some organs. Females, for instance, have only one ovary.
Birds are toothless, an adaptation that trims the weight of the head.
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.