 The outermost primaries of large soaring birds , particularly raptors , often show a pronounced narrowing at some variable distance along the feather edges . These <unk> are called either notches or <unk> depending on the degree of their slope . An emargination is a gradual change , and can be found on either side of the feather . A notch is an abrupt change , and is only found on the wider trailing edge of the <unk> . ( Both are visible on the primary in the photo showing the feathers ; they can be found about halfway along both sides of the left hand feather — a shallow notch on the left , and a gradual emargination on the right . ) The presence of notches and <unk> creates gaps at the wingtip ; air is forced through these gaps , increasing the generation of lift . 
