Does Egg Destruction Control Gull Numbers?
Bird On! News
26th March 1997
From the early 1960s, the large gulls, Herring and Lesser Black-backed, increased hugely in Britain and over other parts of Europe. The increase was fuelled by changes in the behaviour of the birds and the provision of enormous quantities of food from rubbish tips and from fishing discards. The expansion of colonies brought the birds into conflict with Man both because of conflict with Human activities (roof nesting, nesting near airports) and with nature conservation. On the Isle of May (Firth of Forth, Scotland) huge areas of the island were covered by the nesting gulls and the other nesting birds, notably terns, were excluded.
Between 1984 and 1988 lots of hard work was put into the repeated destruction of the clutches laid by the gulls and this severely restricted the production of young birds. Annual censuses of the gulls showed that Herring Gulls declined by 6% per year until 1992 and then dramatically increased by 41% in 1993. The Lesser Black-backs decreased rapidly from 1984-87 and then increased gradually with sharp rise in 1993 (68%). Chick production was restricted to 150-390 Herrings and 15-20 Lessers whilst control was in progress but was 1917-2709 Herrings and 331-1018 Lessers in the absence of control.
In the Journal of Applied Ecology (33, 1420-1432) Sarah Wanless, Mike Harris, John Calladine and Peter Rothery were able to show a significant difference between the adult survival rates 88% for the Herrings and 91% for the Lessers (pp<0.1). However the major difference between the two species was demonstrated by a demographic model. Culling of adults is not as successful as might be expected because the effect of reducing density in the colony is to make it very attractive for new recruits! So continuous suppression of breeding output would seem to be a good way of controlling Herring but, because of recruitment from elsewhere not so good for Lesser Black-backed Gulls. In the end the maim message is that one does need to know about the demographic factors affecting your gulls before (during and after) culling.