Answer
Birds are adapted to fly based on their having: 1) The weight-saving lack of the bladder which normally holds urine and the female’s second ovary; 3) Honeycombed, air-filled bones; 3) Feathers to help trap air when wings beat; and 4) Wings designed to beat against the air and produce lift.
Work Step by Step
This is just a summary of pp.732-733. Group most of the weight-saving features together as one of the four points in the list.
