Summary
Birds are endothermic, meaning they produce their own body heat and regulate their internal temperature independently of the external temperature. Feathers not only act as insulation but also allow for flight, providing lift with secondary feathers and thrust with primary feathers. Pneumatic bones are bones that are hollow rather than filled with tissue, containing air spaces that are sometimes connected to air sacs. Airflow through bird lungs travels in one direction, creating a cross-current exchange with the blood. Birds are diapsids and belong to a group called the archosaurs. Birds are thought to have evolved from theropod dinosaurs. The oldest known fossil of a bird is that of Archaeopteryx, which is from the Jurassic period. Modern birds are now classified into two groups, Paleognathae and Neognathae.
Glossary
Archaeopteryx
transition species from dinosaur to bird from the Jurassic period
contour feather
feather that creates an aerodynamic surface for efficient flight
down feather
feather specialized for insulation
Enantiornithes
dominant bird group during the Cretaceous period
flight feather
feather specialized for flight
furcula
wishbone formed by the fusing of the clavicles
Neognathae
birds other than the Paleognathae
Neornithes
modern birds
Paleognathae
ratites; flightless birds, including ostriches and emus
pneumatic bone
air-filled bone
primary feather
feather located at the tip of the wing that provides thrust
secondary feather
feather located at the base of the wing that provides lift
theropod
dinosaur group ancestral to birds