A systematic exposition of how birds fly. Freedman proceeds from the simpler factual information--muscle strength, feather shape and construction, skeletal design--to more demanding aerodynamic principles as he introduces the many known factors and alludes to those not yet determined. Air sacs and hollow bones account for lightweight bodies; pliant feather shafts and curved shape allow for flexible movement; and other adaptations (such as the alula) respond to changing air currents and flight plans--gliding, soaring, landing. Appropriate airplane comparisons are made and the special features of hummingbirds receive their own small section. Bjorklund's soft-focus pencil drawings are adequate for pointing up particular aspects (arm-wing bone similarities, close-up feather structure), wholly unsuited to species identification.