Short-fiber reinforced composites are made from keratin fibers obtained from poultry feathers and polyethylenes of varying crystallinity. The chemical nature of the polymer and fiber is kept constant and the molecular architecture of the polymer is varied. It is found that low crystallinity polyethylenes are reinforced by keratin fibers but high crystallinity polyethylenes are not. The keratin fibers inhibit crystallinity in low crystallinity polyethylenes but enhance crystallinity in high crystallinity polyethylenes. Microscopy shows increased adhesion between the fibers and the polymer for the more amorphous polyethylenes. A model is presented that describes composite properties as a function of fiber properties and matrix crystallinity.