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Kleptoparasitism by the Lesser Black-backed Gull (Larus fuscus) on Skokholm Island, Pembs. M. E. Howgate I The present article is based on some observations made on Skokholm Island between 25 July and 1 August 1981 and was originally presented at the Conference on Bird Biology organised by the Edward Gray Institute of Field Ornithology in Oxford on 8 January 1982. Numerous observations have been made recording the kleptoparasitic behaviour of the Herring Gull (Larus argentatus) on the Common Puffin (Fratercula arctica), particularly on the islands of Skomer and Skokholm off the Pembrokeshire coast. So it came as some surprise that this spectacular and easily recordable habit has only recently been noted for the Lesser Black-backed-Gull (Larus fuscus) and has only been mentioned in passing. Lockley (1953) recorded the kleptoparasitism of Puffins by Herring Gulls but nowhere mentions that Lesser Black-backed Gulls took part in this activity. Mylne (1960) studying the interactions of Herring Gulls and Jackdaws (Corvus monedula) with Puffins on Skomer Island from mid-June to August 1959 also does not mention similar activity between Lesser Black-backs and Puffins. The first observation of kleptoparasitic activity involving Lesser Black-backs and Puffins was in 1964 by Harris (1965) who observed that such interactions took place 'occasionally', whereas Herring Gulls used sand-eels pirated from Puffins as a major food resource towards the end of the fledging period. (Harris noted that one in nine of Herring Gulls sampled contained sand-eels (Ammodytes spp.) but only in July). However Corkhill (1973) working on Skomer, observed that the major land-based kleptoparasite on Puffins was the Jackdaw, whereas all observed Gull attacks (both L.argentatus and L.fuscus) started in the air. But elsewhere he does refer to 'Gulls' having been observed hunting from stations within the Puffin colony. But both Richford (1978) and Ashcroft (1979) recorded only Jackdaws and Herring Gulls as kleptoparasitising the Skomer Puffins, and the latter's work extended over a five year period, from 1972 to 1977. Elsewhere only Arnason & Grant (1978) have recorded behaviour akin to kleptoparasitism involving Lesser Black-backs and Puffins. At Vik, in Iceland, Lesser Black-backs have been observed to join in the pursuit of Puffins already being chased by Arctic Skuas (Stercorarius parasiticus), but they will only glean a dropped catch and unlike the Herring Gulls and Ravens which also join in this doubly parasitic activity they never pursue their victims down their burrows. On Skokholm, however, the Lesser Black-backed Gull has become a major kleptoparasite on Puffins and has fully adopted a ground based strategy (only one aerial pursuit was seen and this was initiated only after an unsuccessful ground based attack). The fact that no Herring Gulls were seen to be involved in this activity during the week of observation is interesting but may just be a result of their slightly earlier breeding cycle. They generally nest three weeks earlier than Lesser Black-backs and thus the need for an intense exploitation of food resources associated with fledging, and which seems to trigger kleptoparasitic behaviour, may have been passed. In the study area of Crab Bay which was the only part of Skokholm Island in which kleptoparasitic behaviour of any kind was observed, the Lesser Black-backed Gulls had adopted the full repertoire of ground based attack strategies seen in the Herring Gull. Four or five adult individuals would 'stake out' the Puffin colony approximately every twenty yards across the middle of the cliff-top slope. Each individual was quite out in the open and appeared to be guarding the entrances to several Puffin burrows, staying 'on station' for anything up to an hour and a half. If a Puffin tried to land close by the Gull would either run or fly at it and peck it until it dropped its beak full of sand-eels. No Puffin without an obvious beak full of food was ever attacked, except by a juvenile Gull which would lunge at any Puff which came within range but never managed to secure a single meal. Quite often if a Puffin landed approximately equidistant between two Gulls both would attack and get a share of the spoils. The only occasion when the Lesser Black-back which came to 'help' got all the spoils was when a Puffin which had jus disappeared down its burrow was hauled out backwards by the initial attacker and thrown dov,