University of California
Ucmp: Aves More on Morphology
A good discussion of the major adaptations of the skeleton of birds in order to adapt to flight.
University of California
Ucmp: Vertebrate Flight
A very solid Berkeley site on the structure and function of bird flight. Topics include skeletal and muscular adaptations for flight.
Other
Ornithology: Bird Flight
Excellent coverage of the adaptations and actions of birds in regard to flight. Contains specific information, diagrams, and pictures.
TED Talks
Ted: Ted Ed: Why Can't Some Birds Fly?
Gillian Gibb explores what makes birds give up the power of flight.