This seems like a fairly bizarre phenomena in birds but can easily be explained. In most cases, we are observing one of two things:
1) Molt: Birds have to molt their feathers, otherwise they will wear out and the bird could lose flight. Different species will have varying methods but the timing of molt and which groups or tracts of feathers are molted is often similar. As an example, Blue Jays are now going through a fall molt, known as a definitive prebasic molt, in which nearly all the feathers are replaced. The outer flight feathers (primaries) are replaced from the inside going out and delayed enough so that the bird never loses flight. In contrast, the capital-tract feathers, the tract along the bird’s head, are dropped nearly simultaneously, resulting in a bald bird. This only lasts about a week and can occur in Blue Jays of all ages. Note this molt only occurs in the fall, typically between June and November while the birds on still on their breeding ground. If you see a bald bird at another time, refer to the next answer.
2) Parasites: Most birds get parasites, especially mites. Usually, by bathing and preening, birds are able to clean themselves and rid their feathers of mites. The problem here arises with a birds inability, or difficulty, in preening their heads. Mites, which are in the class of Arachnids, can destroy the shafts of feathers as a result of their feeding and thus causes the balding we see in some birds.
A Maine native, Doug grew up in Hollis and graduated from the University of Maine in 2011. Throughout college Doug worked at Scarborough Marsh Audubon Center and was hired as Maine Audubon’s staff naturalist in the summer of 2013, a long time “dream job.” In his free time, Doug volunteers as one of Maine’s eBird reviewers, is the owner and moderator of the ‘Maine-birds’ listserv and serves as York County Audubon board member and Secretary of the Maine Bird Records Committee.