Birds of this species have a dark mark along the leading edge of the underwing, between the body and the wrist (the patagium). Most but not all color variations have a dark band across the belly. In most, adults' tails above are rusty red and juveniles' have narrow brown and pale bands. The main western population has bands on the adults' rusty tails as well and has varied plumage, organized into three main color types or morphs. (from Wikipedia)

