In reply to knighty:
What happens depends a lot on the species. Variously:
- Vomit - Fulmar vomit is absolutely godawful stuff and besides the unpleasantness on the day, you have to wash the clothes involved in the incident again and again and again and... until the smell is faint enough that people don't cross the street to avoid you. Also a friend recently got a good face full you might get dual eye infections which will be extremely unpleasant, not to mention painful (not had this personally but a mate on Facebook recently posted some photos and they look grim as hell).
- Peck your eyes out - I learned about this in a documentary by a chap called Alfred, always be on the lookout.
- Noise - Lots of noise, nothing to worry about really.
- Sh*ting / dive bombing - Parents do this to dissuade people and protect chicks, the upside is it's unlikely to go in your eye, the downside is you get covered in bird shi*. Some will also bang you on the head when they dive-bomb, even with a helmet on it's going to be quite offputting.
All in all basically just all the worst parts of having a newborn baby.
Personally I try to avoid climbing through birds for all the above reasons but if it turns out one is in the way that I didn't see, I just get on with it. There is quite an art to fulmars in particular who can be persuaded to disgorge all their loads before you have to pass them if you are patient and agile. Waving a hand over a ledge before pulling your head over is a good idea too, hands are more resilient than faces.