Birds belong to the scientific group Aves, which includes such common creatures as pigeons, sparrows, geese, chickens and crows. The various pet birds are, for the most part, parrots, although some people also keep canaries, other songbirds or even chickens as pets.
Birds are a very diverse group of creatures. Tiny hummingbirds resemble insects in their movement, while some ostriches are as tall as humans. Scientists disagree somewhat on how to classify the approximately 10,000 species of birds, but they do know that the group arose sometime in the Mesozoic from a subgroup of the dinosaurs. This technically makes birds reptiles, although most people would readily distinguish birds from slow-moving, cold-blooded reptiles like turtles or lizards.
Although there are thousands of species of birds, more than half of them are classified as passerines. These are small to medium-sized creatures which chirp and flit from tree branch to tree branch with their distinctive, four-toed feet providing grip. Birds such as sparrows, jays and wrens are passerines.
Parrots, the most common birds in the pet trade, are not classified as passerines; they have their own group, the Psittaciformes. Like passerines, parrots have four toes, but parrots have two toes facing backward and two facing forward on each foot. They also have heavy, curved beaks. Parrots are among the longest-lived and most intelligent of birds. Some common parrots include the African gray, lovebirds, cockatiels and macaws.