Ornithology is the branch of zoology that deals with the study of birds. Birds are any organisms in the class Aves. They are warm-blooded (or homoiothermic) vertebrates that have feathers covering their body; forelimbs modified into wings; stouter hindlimbs used for walking, swimming, or perching; scaly legs and feet; jaws reduced to a toothless beak; and a four-chambered heart. Birds lay hard-shelled eggs from which their young hatch. Major subject areas in ornithology include: anatomy, physiology, behavior, ecology, evolution, and classification and systematics.

Birds evolved from a group of reptiles known as the dinosaurs (order Dinosauria), which first appeared during the late Triassic period (which ended about 210 million years ago). The earliest bird known in the fossil record is Archaeopteryx), from the mid-Jurassic period about 160 million years ago. These extremely early birds had many typical avian characteristics, such as feathers and a horny beak, and were very likely...