Couple of years ago I was headed to work early morning and a pigeon flew out of the hedgerow. It missed my bike completely hurrah! However I managed to head butt it on it way past me, boo! I’m not sure exactly where it impacted my helmet but it was at the front as my visor was covered in white dust where the damn thing ‘exploded’, feathers everywhere . I was amazed the visor didn’t crack, Shoei quality I guess. The only damage was a sore neck for couple of days from the whiplash effect of head-pigeon meeting at speed.
Nasty.... I'm sure one could knock you out if it got you right.... Flying rats !!