At least 150 million years ago, theropod dinosaurs with feathers on their arms, legs, and tails started to appear. There is still debate about whether the first feathers were ornamental or functional but Archaeopteryx (below) which means Ancient Wing, is generally regarded as the first flying bird.
Flapping flight evolved separately at least four times in the history of the Earth: first in insects, next in pterosaurs, then in birds, and finally in bats. This independent emergence of a characteristic is known as convergent evolution.
There are several theories of how early fliers gained enough speed to create lift with their wings: they may have climbed trees or cliffs and launched from a height—perhaps to pounce on prey; they may have been able to run fast enough to become airborne; or they may have first used their wings to assist with tree climbing as some young birds do today.
LEARN MORE about the evolution of flight
KIDS QUESTION: In the museum exhibit, we asked: What animals are the ancient ancestors of birds?
The answer is: Dinosaurs
The theropod oviraptorosaurs from approximately 125 million years ago are believed to have the closest links to birds. Many species of carnivorous dinosaurs have been found to have feathers. More recently, feathers have also been found in the fossil remains of dinosaurs that were herbivores.
More information on the Thermopolis Archaeopteryx:
The discovery of the Thermopolis Archaeopteryx is described in the following article:
Mayr G., Pohl B., Peters D. S.. A well-preserved Archaeopteryx specimen with theropod features. Science. 2005 Dec. 2;310(5753):1483-6.
An important feature of this specimen is that the second toe is clearly not reversed as it is in perching birds.
A visit to Stanford University’s synchrotron by Thermopolis Archaeopteryx is described here:
Bergmann, R. W. Morton, P. L. Manning, W. I. Sellerse, S. Farrar, K. G. Huntley, R. A. Wogeliusc and P. Larsonc Archaeopteryx feathers and bone chemistry fully revealed via synchrotron imaging. Proceedings of the National Academy of Sciences. vol. 107 no. 20 . 9060–9065. 2010
The Wikipedia page on Archaeopteryx is here
More Information on the Evolution of Flight:
A wonderful narrative account of the long trail and multiple controversies that finally led to the establishment of a linkage between dinosaurs and bird flight can be found in the following book:
Pat Shipman. Taking Wing: Archaeopteryx and the Evolution of Bird Flight. Simon and Schuster. NY. 1999. ISBN 9780684849652
The book is also contains excellent additional material on the origins of flight.
A good discussion of the competing theories of how the first fliers actually took off can be found on Pages 54-56 of:
Colin Tudge. The Bird. Three Rivers Press. New York. 2008. ISBN 978-0-307-34205-8