The Effects of Culling Herring Gulls (Larus argentatus) on Recruitment and Population Dynamics

Duncan, Neil

(1) In 1972, the then Nature Conservancy undertook to control the number of breeding herring gulls Larus argentatus Pont. on the Isle of May (Scotland). Culling has since continued. (2) Prior to this, herring gulls were increasing at 13% p.a. with an annual recruitment rate of 17.3%. The breeding population was estimated in 1972 at 16 700 pairs. By 1977 the population had been reduced to 3670 pairs. The number of recruits also dropped in each year well below predicted levels. (3) Over 38 000 gulls were killed between 1972 and 1976, but it seems that large numbers of recruits have been deterred from breeding as well. These are presumably breeding elsewhere. (4) The average age of recruitment was 5.0 years. The age composition (which was stable from year to year before 1972) has been greatly changed, with 52% of the population breeding for the first time in 1977. Few old birds have been left. (5) Nest density is important in regulating recruitment rate. Areas of the colony which were most attractive to new recruits had between two and ten pairs of gulls per 100 m$^2$. At higher densities the number of new recruits was sufficient to balance natural annual adult mortality. No recruits were attracted to areas with a density less than two pairs per 100 m$^2$. (6) The gull cull has lowered the pre-1972 average breeding density (estimated at 11.1 pairs per 100 m$^2$) to 2.3 pairs per 100 m$^2$. (7) In the event of culling being stopped, it is possible that the existing density retains sufficient attraction for a high recruitment rate to be maintained, with a consequent increase in the breeding population.

The Effects of Culling Herring Gulls (Larus argentatus) on Recruitment and Population Dynamics

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