If you can locate an old Standard of perfection book,look at the feather parts.What you are seeing is the under-fluff,or under-color,the web is the surface color and the shaft is the center part.The flights and tails are stiff and quills are large. Hope this helps.In your brown Leghorn,looks to be a S.C.Dark Brown,this is part of the back/saddle section.The more saddle ,the less brake at the jucture of the tail.This make the continued concave sweep or back outline.