patch or clump of feathers on a bird
• (n.) One of the definite areas of the skin of a bird on which feathers grow; -- contrasted with apteria.
(from the article `feather`) Unlike the hair of most mammals, feathers do not cover the entire skin surface of birds but are arranged in symmetrical tracts (pterylae) with areas ... ...basal portion may be downy and thus act as insulation. The major contour feathers of the wing (remiges) and tail (rectrices) and their coverts ... In ...
Pte·ry'la noun ; plural Pterylæ . [ New Latin , from Greek ... feather + ... wood, forest.] (Zoology) One of the definite areas of the skin of a bird on which feathers grow; -- contrasted with apteria . 
feather tract
one of the feathered areas on the skin of a bird. Also calledCf.