THE GREAT BLACK-BACKED GULL ON SKOMER D. R. SAUNDERS Warden, Skomer Island INTRODUCTION The numbers of Great Black-backed Gulls (Larus marinus) breeding on Skomer Island have greatly increased during the last twenty years, as have other gull populations throughout the North- ern Hemisphere. Although scavenging provides much of the food of this gull, during the breeding season other sea birds are the main part of its diet. During the last three seasons control measures have been applied on Skomer in an effort to reduce the population of these large gulls and so the heavy predation on the other sea birds. POPULATION The first record of the Great Black-backed Gull on Skomer relates to two or three pairs about 1875 (Harrison and Hurrell, 1933). "A few pairs nest on the islands" (which ones are not stated) is the only reference in The Birds of Pembrokeshire by Rev. M. A. Mathew (1894). Nesting (no numbers stated) was recorded in 1897 (Drane, 1898). There are then no reports until 1926 and 1929 when 100 and 50 pairs respectively were recorded (Harrison and Hurrell, 1933). In 1945 some 40 pairs were present and the following year 60 pairs (Buxton and Lockley, 1950). By 1956 the population had risen quite phenomenally to some 220-250 pairs (Davis, 1958) whilst censuses in 1960 and 1961 showed 260 and 283 pairs respectively, but in 1962 a decrease to 235 pairs (Skomer Reports for 1960-2 (1962, in press)). BREEDING HABITS The majority of the nests are to be found on the main plateau of the island, usually on or close to the many rock ridges and out- crops. When the nest is sited on more level ground it is usually close to some convenient vantage point for the off-duty bird. Where such situations are chosen the vegetation is usually of a short nature, no nests being found in the denser bracken or in the marshy stream valleys. The nests are normally in loosely knit colonies, never as dense as those of the Lesser Black-backed Gull (Larus fuscus) or Herring Gull (Larus argentatus). Occasionally the nests may be quite isolated but this is rare. Sometimes they are in close proximity to, or within the colonies of, Lesser Black-backed or Herring Gulls. The nest is a scrape lined with dead grass, bracken stems, and so on, quite often with feathers incorporated in it. The vicinity is littered in varying degrees with pellets and food remains.