Feather barbules are of two types, pennaceous (making up the flight surface of the feather) and plumulaceous (insulation barbule). Plumulaceous barbules are the most useful for the idendification of the type of bird from which the barbule came, but the pennaceous barbules are also useful though generally not as specific as to the type of bird. A useful recent publication is that of Carla J. Dove and Sandra L. Koch: "Microscopy of feathers: a practical guide for forensic feather identification.",THE MICROSCOPE, VOL. 59, NO. 2, PP. 51-71 (2011).