Welsh Journals

JACKDAWS STEALING FISH FROM GUILLEMOTS ON SKOMER T. R. BIRKHEAD The number of Jackdaws (Corvus monedula) on Skomer Island has increased dramatically over the last 25 years. In 1946 only 20 pairs bred on the island (Buxton & Lockley 1950) but by 1972 P. Corkhill (pers. comm.) estimated that there were 250 pairs. The Handbook (Witherby et. al. 1938-41) does not mention fish in the diet of Jackdaws, but on Skomer this species regularly attacks Puffins (Fratercula arctica) during the breeding season to steal fish from them (Mylne 1960 and Corkhill in press). Since Mylne (1960) is the first to record the exploitation of Puffins by Jackdaws, it is possible that this is a recently acquired habit of the local population and may be a consequence of the increased population. Jackdaws have not previously been recorded stealing fish from other auks. The present paper describes some observations made on Skomer of Jackdaws stealing fish from Guillemots (Uria aalge.). During the summer of 1972 I spent over 60 hours observing Guillemots on the north side of Skomer near the Bull Hole, where in late June there were 175 chicks and up to 700 adults present. Jack- daws frequently patrolled the cliff ledges, usually flying within 2m. of the cliff and frequently following the same regular flight-path which coincided with all groups of breeding Guillemots. This was in contrast with Herring Gulls (Larus argentatus), Lesser Blackbacked Gulls (L. fuscus) and Greater Black-backed Gulls (L. marinus), which patrolled less frequently along random flight-paths. Of 175 Jackdaw patrols observed, 161 involved single birds, 11 involved 2 birds and single groups of 3, 4 and 5 birds each were seen on one occasion. Jackdaws were seen patrolling for most of the daylight hours. During a watch made from 1200-2200 hrs. on 28 June, Jackdaws were seen at least once every hour and altogether on 43 occasions; the last patrol was at 2100hrs. During observations from 0500-1200hrs. the next day Jackdaws were first seen at 0605hr., and then on 31 other occasions. Apart from a suggestion of a peak between 0700 and 0800hrs. no particular period of the day was especially favoured. Since the colony of Guillemots extended out of sight round the cliff, it was not possible continually to watch the same Jackdaw for long, but it appeared that at least five individuals were involved. During late June, when Guillemots were feeding chicks, virtually every food item was a Clupea sp., usually about 10cm. in length. A patrolling Jackdaw on seeing a Guillemot with a fish generally made an abrupt turn, sometimes almost a somersault, and alighted about 0.3m. from the Guillemot. The Jackdaw then intimidated the Guillemot by repeatedly edging forward. If the Guillemot, while manipulating the fish in its bill, accidentally dropped it, the Jackdaw