Bird Evolution

Birds evolved from reptiles and, except for their feathers and warm-blooded condition with related anatomical changes, are very similar to reptiles. A group of fossil reptiles, known as pseudosuchians, which occurred during the Triassic Period, seems to have been the stock from which birds evolved.
The first arboreal forms with feathers and feeble powers of flight but with unchanged reptilian structure, appeared in the Jurassic Period.
In the Cretaceous are found birds which still have teeth, but otherwise are like modern birds with well-developed flight.
By the beginning of the Cenozoic Era, the diversification of the more modern groups of birds began, which now comprises most of the world's bird life.

Text based on display in the Bird Hall, National Museum of Natural History: Smithsonian Institute.