My cats pretty much kill all small animals that get in their yard..... Or the neighbors.I honestly think at my house It's because we are so over populated with squirrels. They are cute. . but nasty little heathens that eat your house and stash their nuts all over the place. I think they bring fleas. And fleas can lay dormant for a long time so once they lay eggs in your house somewhere. . .watch out. And we don't have the deep freezes you have.
Which is one way I guess they could get fleas.... But I've never had a problem with them.
I wonder if our hard Winters help keep the population down.....I think a lot of parastites don't thrive around here.
A friend of mine had a flea issue last summer..... But that is the only person I can remember in a long time to have that issue.
My birds have all been really healthy too...... Knock on wood......