The greys escaped from the London Zoo. Oops.

On the other hand, some idiot brought starlings to the 'States on purpose.

And English Sparrows.

Edit - Hey, that means we still owe them one more... ;)

Gila Monsters

And, who knows, maybe they'd take care of the squirrel problem. :^)

What are the English sparrows doing?

They don't really do anything at all over here. Maybe they're emboldened by living it up in the land of the free.