 In many species , there is more than one focus along the wing . Here , moult begins at all foci simultaneously , but generally proceeds only in one direction . Most grouse , for example , have two wing foci : one at the wingtip , the other between feathers P1 and S1 . In this case , moult proceeds <unk> from both foci . Many large , long @-@ winged birds have multiple wing foci . 
