Welsh Journals

HERRING GULL POPULATION AND MAN J. W. F. DAVIS Harris (1970) discussed the increase in British gull populations over the last forty years, in particular that of the Herring Gull Larus argentatus on Skokholm, Pembrokeshire. This population has been increasing at a rate of 10% annually since about 1950, and he considered the possibility that the increase in the population was due to increased food availability in the form of man's waste. Skokholm Herring Gulls feed at Milford Haven fish docks and local rubbish tips. However, as Harris showed, there is little evidence that the amount of food available at these sites has increased in recent years, and at the fish docks there has been a marked decline in the tonnage of fish handled during the time of most rapid increase of gull populations. In this paper, evidence is given that suggests that these waste foods have nevertheless contributed to the increase in this gull population. GULLS AT FEEDING SITES In 1969 and 1970, 514 adult Herring Gulls were individually colour- ringed at the breeding colony on Skokholm. By May 1971, 167 (32%) of these had been seen on the local mainland. At Milford Haven rubbish tip 104 (20%) were seen, nearly all on more than one occasion. Sixty-six (13%) were seen at the fish docks; 16 of these were also seen at the tip. An ad- ditional 13 were seen elsewhere, three of these at other rubbish tips. Individual adults were often very consistent in their feeding site (for details see Davis 1973). For example, 57 of the birds ringed in 1969 were seen at the fish docks. Of these 27 were seen at least five times (maximum 44) and only one of these was recorded elsewhere. Of the 30 seen less than five times, 18 were seen elsewhere. In addition, of the latter 30 only five were ever seen feeding at the docks although the majority of sightings were of birds feeding. By contrast, all of the 27 seen more than five times were seen feeding. Many of those seen less than five times were perhaps birds feeding at the rubbish tip (about 1 km. away) that came down to the docks to bathe or roost. The birds seen more than ten times at the docks were never seen elsewhere. Though it is impossible to say that they never fed in other areas, it is assumed that the fish docks provided the bulk of their food. This consistency of feeding location was apparently maintained throughout the breeding season. From observations of colour-ringed birds and the examination of 318 regurgitations from adults and chicks at the breeding colony, it was possible to estimate the relative importance of different food sources to the Skokholm population. The important sources were: a) Milford Haven fish docks. These were visited regularly by about 8% of all colour-ringed adults and fish-waste was identified in 6% of all regurgitations. The latter figure is minimal since fish-waste cannot always be identified as such. b) Garbage and waste, except fish, attributable to man was found in 38% of all regurgitations and 20% of colour-ringed adults were seen outside the