The message
from "David W.E. Roberts" contains these words:

(snip)
Very strange - the bird was bald on the back of its head and also had some
damage at the base of the tail on the top.
The knees were abraded and bleeding slightly.
The bird could flutter but not fly.
I have never seen a bald bird before and am puzzled over the reason.

Perhaps it's been attacked by another bird worried about the jackdaw
hanging around near its own nest. Even hens, if they are put under
stress, will feather-peck other birds to baldness and bleeding...you
should see how bald some battery hens are :-(

Janet