What causes the black spots on my duck's bill? Black spots on the bill and legs are due to melanin skin pigment.
They are normally present (not harmful) but are covered with yellow colored xanthophyll pigment absorbed from feeds containing corn, alfalfa and other feed
ingredients containing xanthophyll. In mature breeder ducks, after they have been laying eggs for awhile, more xanthophyll is absorbed by the
egg yolk than the duck can consume in her feed. The bill and legs start showing melanin skin pigment due to the bleaching or fading of the xanthophylls.