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At the risk of contradicting Melissa, our experience is that once rats move in, they will not move on. We have a terrible rat problem in our back garden that began about two years ago with a solitary rat taking up residence in a flower bed. We now have a veritable rat city in the back garden that has steadfastly refused to move on, despite the attentions of the Council's pest control department (twice, at £40 a time) and me constantly filling in the rat holes that now lead to some kind of underground rat warren that extends over four back gardens.
We stopped feeding the birds, next door bought a cat (and she's a brilliant ratter), and we have seen at least two birds of prey in the past three months who are presumably attracted by the rats. We believe the rats moved in because of the proliferation of food in the pigeon loft two doors down, and that their numbers are so great we are fighting a losing battle.
Much as though I'm an animal lover myself, I would advise Phil to act strongly now before the situation gets out of hand. I think rat poison is a cruel way to kill the animals, but the situation will not improve unless steps are taken.
I'm not sure if this is an urban myth or not, but there's a statistic that's been bandied about for years that, in the UK, you are never more than 10ft from a rat. Yuk!
Give my regards to Norvegicus when you see her next, Phil!