In most raptors, feathers cover all but small portions of the faces, feet, and toes of adult birds, but there are exceptions. Bald-headedness in vultures appears to be associated with their propensity for poking their heads into rotting carcasses of animals upon which they feed. Such full-feathering helps muffle an owl's flight as its flaps and glides through the still night air in search of prey. Feathered feet and toes also protect the owl against potential bites from prey. Old World fishing owls, which prey on fish in water, have unfeathered feet and toes.
Larger species tend to have greater numbers of secondaries.
The structural complexity each feather is considerable. The tail feathers of Northern Harrier, for example, are estimated to have at least 1.25 million barbules each. Scattered across their body, these somewhat matted feathers grow continually, with parts breaking off into a fine powder that apparently helps absorb the grime and debris that raptors contact. Numbers of feathers and weights of various systems in a Bald Eagle. Burton, J. A. 1973.