In reply to Adrian George:
well, there's certainly nothing here
http://www.bto.org/notices/trgworkshops_2007.htm about training to abseil of cliffs.
I'd talk to them though. Someone who supervises and 'employs' volunteers, although not bound by law, should generally treat health and safety and 'work' issues as seriously as if they were paid employees. (I know this as I run a voluntary organisation and supervise lots of volunteers). They therefore need to make sure that you have all the training and resources you need to carry out your voluntary duties safely and effectively.
Have you also thought about how you're going to get back up the rope after abseiling down and ringing birds? How are you going to reach capture and ring the birds while keeping secure?
All the best
Nikki.