It’s a tricky one this though isn’t it? They’re not ‘robbing’, they’re just adapting to their environment, where you have placed a new food source. I am conflicted on this one. One the one hand, I put food out for the birds, so I am already influencing how natural selection might work, and I like red squirrels more than grey for arbitrary reasons (possibly relating to early exposure to Tufty). On the other, who am I to say one species is better than another, why not let evolution run its course?