8 Birds come in a wide range of sizes from the smallest, the bee humming bird, to the largest, the North African ostrich. The enormous variety of species can be found in habitats as diverse as polar regions and tropical rainforests. Outer surface A bird’s entire body, except its bill and feet, is usually covered with feathers. Some birds, such as vultures, have bare heads and necks. Ear coverts Eye Breast Flank Beak Secondary flight feathers Primary flight feathers Tail Mantle Toe External surface Reinforcing strut Lightweight interior Saving weight Birds need a lightweight skeleton to be able to fly. Unlike the bones of land animals, the long bones of flying birds are hollow and are reinforced with lightweight internal struts. In many flightless and diving birds, however, the bones are solid. Cross- section of a bird’s bone Digestive system As birds have no teeth, their digestive system has to carry out all the stages of food breakdown. The lower part of the stomach, known as the gizzard, grinds the food into a pulp. Stomach Small intestine Gizzard Rectum Liver Crop Gullet Birds have high body temperatures, which can go up to 43.5 °C (110°F) .