There are two different varieties of angels—the naked, long-distance flier and the feathered, short-distance glider (The Four-Winged Angel). But both varieties have the following anatomical features that makes flight possible:
The bones are hollow, with criss-crossing struts or trusses for structural strength.
The human lungs make up 7% of the overall body volume. The lungs of an angel take up 15%. Gas does not get mixed during the inhalation/exhalation cycle as is the case in other mammals.
Though leg length in the proportion to the rest of the body is still human, arm length in proportion to the rest of the body is more akin to a more arboreal, more acrobatic ape—the gibbon.
The rib cage is barrel-shaped, like a Neandertal.
The sternum, or breastbone, has been modified into a keel.
The flocculus, an area of the brain dedicated to concentration, makes up 2% of the brain’s overall mass.
The flier’s wings, true to the name, are formed from skin connected by two or three of the five fingers of each hand that have each been elongated to triple or even triple-and-a-half the length of the body. The remaining two to three fingers don’t have nails but claws.
The glider has all five of its fingers on both hands at normal size, but to make up for that, all four limbs have been modified into feathered wings. Jutting from the posterior is a short but broad fan of tail feathers for better steering.
Are there any other anatomical features I might have missed that could make either angel flight-capable? Would the flier be a biped or a quadruped?