The origin of flight has attracted a great deal of research interest. If you want to explore further, you might want to visit the following web sites:

Vertebrate Flight Exhibit at UCMP
This comprehensive site examines the physics of flight, its evolution and origin, and the different solutions to flight within the vertebrates.

The Proof is in the Plumage
Read this July 2001 Natural History article illustrating a fossil from northeastern China's Liaoning Province that offers evidence of feathered dinosaurs.

Bird Flight
This site by Richard Dryden takes a look at the muscles involved in the flight stroke of birds.