When a bird molts out a feather, a new one is forming in the follicle. This live tissue has a central artery and vein to supply nutrients to the growing feather. There is a sheath that encloses the developing feather. Also, blood feathers are shorter than mature feathers, since they have not grown to their full lengths. Blood feather are located on different areas of a bird's body, depending on what molting stage he is in.
Birds usually molt in a particular pattern, so that in the wild they are always able to fly.
Birds may molt, one, two, or three times per year, depending on the species. The pattern of molting follows an orderly progression. In a complete molt, the sequence is usually first the inner primaries, then the secondaries and tail feathers, and lastly body feathers. Matched pairs of feathers are usually shed at the same time so that flight is still possible. Baby birds growing their first set of feathers have all their blood feathers at one time, all over their bodies.