My story takes place in a post-apocalyptic world where there are no humans. On the island of Manhattan, which has been abandoned for a long time, there are communities of animals with human-level intelligence. This includes pigeons. Physically, these birds appear to be the same New York pigeons we see today eating garbage and basically making a mess of things. However, by the time the story takes place, they have developed a complex multi-party democracy that, by coincidence, runs more or less along Westminster system lines.
These are urbane and cultured pigeons, with a rich tradition of literature, philosophy, mathematics and, their specialty, diplomacy. Recognizing that their battle prowess is, frankly, pathetic, the pigeons specialize in manipulation, subterfuge and information control. They can't win a straight fight against their enemies, so they have gotten very good at getting their enemies to fight each other.
Here's the issue: Why do these pigeons stay in Manhattan? The other animals, that's easy enough to handwave, as the bridges and tunnels are destroyed and the rivers are now plagued with alligators. But that should prove no issue to the pigeons, who can just fly over them. And, indeed, this could be seen as the smartest thing for them to do. On the island they have to deal with other intelligent animal communities. Away from it, they're the smartest things around.
But I don't want that to happen. I want the story to remain in Manhattan (perhaps the greater NYC area at most). So why in the world don't the pigeons leave?
While I am prepared to use a science/tech explanation as a last resort (e.g. there's a special enzyme only found in foods there, intelligent animals have nanobots that can't operate off island, engineered retrovirus needs other animals to remain active, etc.) I'd ideally prefer to use a more social or political reason. An explanation for why colonization beyond the island just isn't practical, as reasoned by the pigeon government itself.
But then, what would this explanation be? Why do these pigeons stay in Manhattan?
EDIT: All the answers so far point to a solution that I feel silly not considering before: they simply don't want to. I mean, similar to that one answer, I'm someone who doesn't really like leaving the city that much, and as much as city living can be a pain, moving to a desolate countryside full of dangerous animals appeals not at all to me--I mean, if nothing else, where would I get my lox bagels? Why wouldn't it be the same for birds that think like humans?
And I'm someone WITH opposable thumbs. It would be even rougher for a bird, or even a flock of birds especially when, as pointed out, your only real advantage is your intelligence and communication skills. Out in the wild, there's no one to negotiate with, and whatever animal you meet is more likely to care about eating you than hammering out a multilateral trade agreement.
Maybe there were a few tries, but they all turned out like Roanoke and eventually the pigeons decided it wasn't worth it.