We've found a 1974 edition hardback "A Field Guide to the Birds of Britain and Europe", Peterson, Mountford, Hollom. It was in the main hide at Cors Caron, found today so likely left there over the weekend.
If you think it is yours or know who it might belong to could you contact me on andy.polkey@cyfoethnaturiolcymru.gov.uk or 01974 298697.