This is a Common Myna, not a Dwarf Black Egyptian Vulture, but a Common Myna. I've noticed more of these recently around University City, and having done some research I've discovered that no one is really sure why it happens. Some say it happens in juveniles when they first moult, with the head feathers come later. However, it is also a common occurrence in adult birds, causes being attributed to lice, skin infection, fighting and pollution, but no one is sure. If you have come across the answer, we'd like to know.