
The giveaway is the unique, flattened shape of the ring, perfect to fit the leg of something like.... an auk. This is exactly what it turned out to be - a Guillemot ring. It was from a chick ringed (also with BTO ring number N03971) on Skomer Island, South Wales, on 26 June, so it hadn't survived too long.
All sightings of colour-ringed birds can be reported online at www.ring.ac so keep your eyes peeled!
Thanks to Tony Murray for the finding details and Tim Birkhead for the ringing details.