All modern birds have bills and no teeth. The shape of a bird's bill says a lot about what it eats, for example whether it specializes in seeds (stout , cracking bill), fish (pointy, spearing bill), or plants (wide, serrated bill). Birds swallow their food without chewing, so it travels to the stomach whole or in large pieces. Bird digestive tracts have some special features for digesting chunky food. A pouch in their throat (the crop), is used to store food to be digested later, or regurgitated to feed the young. An extra, muscular stomach (the gizzard) grinds food up. Birds are endotherms, using heat they make internally to keep warm. While a few species allow their body temperature to drop at night (torpor), a nearly constant body temperature is maintained by most birds most of the time. Continuously making heat requires fuel to burn, in the form of food. So, birds spend a lot of time eating.

The origin of birds was long debated but now there is general agreement that birds evolved from a group of small, ground-dwelling dinosaurs called theropods. The poster child for bird origins has been Archaeopteryx¸ a crow-sized, feathered creature found fossilized in Germany in the late Jurassic deposits (147 million years ago). Archaeopteryx has feathers and looks like a bird, but it also has many features such as three clawed toes linking its ancestry to theropod dinosaurs. The 2012 discovery of a small, feathered dinosaur fossil (Eosinopteryx brevipenna) in China provided more clues about bird origins; unlike Archeopteryx, Eosinopteryx had a small wingspan and scanty plumage on its legs and tail, suggesting that it lived on the ground, running more than flying. Both of these feathered animals lived in the late Jurassic, and comparing them with each other and with earlier theropod dinosaurs can provide scientists with new evidence about how and when flight evolved in this dinosaur group.