Hydrobates pelagicus
Mathew knew only of a small colony "in the chinks of an old wall on top of the cliff" on Skomer. there are still some on Skomer (about 30 pairs in 1946), but the main colony is on Skokholm, estimated in 1939 at 600 pairs. It is preyed upon extensively by Little Owls on these islands. Also breeds on islets in St Brides Bay, but not on Grassholm (1934). Occurs in winter inland occasionally after gales, eg one Haverfordwest 12 Dec 1939.
R.M.Lockley, G.C.S.Ingram, H.M.Salmon, 1949, The Birds of Pembrokeshire, The West Wales Field Society