Balding heads of birds

Over the past few weeks we have had several birds in our garden with bald/balding heads - a robin, chaffinch and now a dunnock.   The chaffinch had a balding chest too, and sadly we have not seen him/her (fairly young) again for a week.  Same with the parent robin, but he may have left after his parenting duties around the robin nest in our garden finished, we just don't know.

Is there anything we can do to prevent whatever is happening, or treat it? Is it mites?  Can we put something to help them in the bird bath to drink, or do anything else?