Feathers are a defining characteristic of birds, which are a type of feathered vertebrate. Birds are the only living members of the class Aves, and they have been around for over 150 million years. They are found in almost every habitat on Earth and come in a variety of sizes, shapes, and colors. They are known for their ability to fly, but some species are flightless. Birds are also known for their complex vocalizations and social behaviors.