From here:
Bald Molting Birds
For most birds, molting is a process where a few feathers are lost and replaced at a timeāthe bird looks a little scruffy, but it always has a covering of feathers. In some species, however, particularly Blue Jays, cardinals, and possibly grackles, a few individuals seem to lose all the feathers on their heads at once, resulting in a very odd looking bird. |