The Joint Nature Conservation Committee's Seabird Monitoring Programme, which looks as if it is part of DEFRA in the UK, will have the answers about population numbers and trends. Their website shows a gradual decrease in numbers in the UK (Steve G has given some details since I started this email).
Of course, what is perceived as an increase is a change of behaviour. For herring gulls this has resulted in the increased numbers breeding and feeding in towns and cities, as well as the increased aggression of some individual gulls.
DEFRA seems to be the problem as they place their reliance on information fed to them by the RSPB who have admitted that Seabird 2000 is the most recent study they have on the Herring Gull.
Since my OP, the MP for Cheltenham (a city experiencing a massive growth in gull numbers) has agreed to look into taking the matter to the Backbench Committee. This topic has been debated in the House 3 times since 2008 with MPs drawing attention to the public nuisance and threat to health and safety that exists due to the population increase in urban roof-nesting gulls.
Most bird societies have been against a cull because they do not feel it is necessary given Seabird 2000 and the alleged decline in Herring Gull numbers. However, as there is evidence from scholarly research that these gulls are a threat to sympatric and other species, I believe it is time for an expert like Peter Rock to take a look at the problem. A major obstacle in the way is DEFRA's refusal to believe there is an increase in numbers (Seabird 2000) and their feeling that Rock has been discredited (not sure upon what basis this charge is laid). A comprehensive survey of the UK would run, I am told, over £400,000.
My question to this forum: does anyone have any information that relates to the threat the herring (and similar) gull presents to other birds? I have cited a few articles in my PDF from the Baltic Region, the US, Greece, Rome where predation has been considered to be a threat. There is very little information of this nature in the UK apart from one or two articles referring to threats to Puffins.
Second, is there evidence of a change in behaviour as the gulls increase and/or move inland and away from natural habitats and predators? The number of attacks against humans is on the increase which begs the question as to whether attacks against other species is also on the rise. If you Google the gull attacks against whales off Argentina, there are some suggestions of a behaviour change accompanying large increases in gull populations as competition for food becomes more intense. The gulls are already well established in our cities so any steps to reduce their food supply such as dissuading feeding by issuing ASBOs against the feeders, covering tips or using hessian style rubbish bags may only add to the agression. The dearth of material within the UK on behavioural changes may be due to denial or lack of interest or simply a rigid belief that DEFRA and the JNCC are right in claiming a declining population as their 12 year old Seabird 2000 establishes this as fact.
I live in Peacehaven, East Sussex and life is made miserable for the human population due to the noise and attacks. There has been a few incidents of vigilantism with large numbers of gulls being shot. A trial is coming up in Hastings for one such shooter and it will be interesting to see if "self-defence" will be convincing to the jury. Now that Scotland has declared noise and attacks as being a public health risk the door seems to be open for people to retaliate regardless of what Natural England say (no touch, no scare, no disturb, no swear at etc.). Public nuisance (noise, pollution, attacks, swooping etc) is still a criminal offense in England and a refusal to abate it when under a statutory duty to protect health and safety exists may render DEFRA liable for malfeasance.