If you find a bird with a ring, you should report it to the BTO. Details of exactly what to record and who to contact are on their website.
If the bird has a coloured ring only then it is probably an escaped cage or aviary bird. These rings are solely for the owner’s use and it is therefore not possible to trace the origins of such birds. Lost racing pigeons should be reported to the Royal Pigeon Racing Association.
Please remember, if you see a healthy wild bird wearing a ring, feeding on your bird table for example, you must not try to catch it. In these situations you may be able to read the ring number through a telescope.