Aves refers to the classification of birds. Birds come in a variety of sizes and colors from two inches to ten feet. A bird's internal transport system includes a gizzard for grinding food and intestines for absorbing nutrients. Birds are endothermic and regulate their temperature through metabolism. They reproduce sexually, with fertilization occurring internally. While some birds like penguins cannot fly and swim instead, birds generally fly through powered flight. Their diet consists of various plants and small animals. Birds are important to ecosystems and some species are endangered or extinct.