What visual feature is most visible on an adult Mississippi Kite when it is soaring?

Answer

A sharp, contrasting white or pale patch near the base of the primary flight feathers

Adults are slate-gray, but a defining characteristic is the sharp, contrasting white or pale patch near the base of the primary flight feathers, which becomes most visible when the bird is soaring.

What visual feature is most visible on an adult Mississippi Kite when it is soaring?
birdKiteMississippi kiterareness