Respiratory and Reproductive Systems.
Birds' respiratiory systems contain air sacs which are outpockets of the trachea and lungs (a bird can breath through a broken elbow). See a textbook explanation. They comprise, along with lungs, 20% of a bird's body volume. The airsacs allow for a one-way air flow in the lungs, which results in there being no dead space.
1) airsacs cool the bird's high metabolism
2) increase buoyancy
3) cushion gannet-like dives
1) There is a seasonal regression of gonads (1/1000 reduction in testes size)
2) No live young....rapid egg production
3) One ovary (liver moves centrally for balance); a curious exception may be in hawks, which have two ovaries.
So there you have some bird adaptations for flight. Left off is a great deal of avian physiology, a subject I hope to return to now and again