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Because obviously red squirrels are still squirrels, but there's just far less of them. Don't think people would mind the occasional squirrel raid as long as they knew there was only going to be one of them for the entire neighbourhood. Alas.
Instead we dip into the UK's defense budget to engineer elaborste cages which leave karger birds hungry just to give a big middle finger to the grey bouncy buggers.
Bonus picture:
Don't know if/how this one got out again, it just came up when i searched for visual examples. Goes to show that we're never really safe. They can even get into our homes!
Well, that is not the reason but more of a result of their differences. Euroasian red squirrels are mostly solitary creatures (outside of mating season) with large territories for each individual, while American grey squirrels have no problems with communal living and doesn't have territories in the same way, so they tend to naturally have higher concentrated populations.
Red squirrels are also usually quite shy and timid and take a long time to adapt to nearby humans compared to the grey squirrels, which are almost domesticated in the way they are able to co-exist with humans.