Corvus mondedula spermolegus
Common resident, except at Skokholm and Grassholm, where only a spring migrat at same time as Rook. Breeds on cliffs and in ruins, etc, inland. A writer in the Field, 2 June, 1860, noted that it bred in burrows at Skomer; it still dows this - in sea-bird burrows on the edge of the cliffs.
R.M.Lockley, G.C.S.Ingram, H.M.Salmon, 1949, The Birds of Pembrokeshire, The West Wales Field Society