The significant propatagium

http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1097-010X(19961001)276:2%3C112::AID-JEZ4%3E3.0.CO;2-R/abstract

Through flight experiments with live birds and computer modeling we define the aerodynamic contributions of the propatagium in avian flight. From flight trials we found that in House Sparrows, with all flight feathers removed except for the distal six primaries, the loss of approximately 50% of the propatagium's projected area and its cambered profile produced a significant reduction in the distance a bird was able to cover in flight. Removal of the secondary feathers, leaving six distal primaries and an intact propatagium, did not have a noticeable affect upon flight. From the computer model which is representative of the bird wing's mid-antebrachial chord (cambered propatagium, symmetrical musculoskeletal elements, and flat secondary flight feathers), we found that the propatagium: (1) produced the majority of the lift; (2) had a higher (relative to secondary feathers) production of lift in relation to its angle of attack, i.e., steeper lift-curve slope; and (3) produced more lift with a chord only 1/5 that of the feather subsection. We conclude that the cambered propatagium is the major lift generating component of the wing proximal to the wrist.

Cambered:
http://www.thefreedictionary.com/cambered

a. A slightly arched surface, as of a road, a ship's deck, an airfoil, or a snow ski.b. The condition of having an arched surface.