I’m not an expert, but my chickens have killed a mouse and they are fine, so I wouldn’t be too worried. Obviously, if they start showing symptoms or their behavior is different, a vet visit and/or some research is ideal. I don’t believe there’s too much you can do to prevent this from happening again. Keeping your birds locked up in a pen made of something like chain link with chicken wire or something like chicken wire could prevent a bird from getting close to the birds. In the little over two years I’ve had my chickens and ducks, only one animal has been attacked and killed by my chickens. Good luck with your chickens. You may experience lots of ups and downs, but your birds give you so much in return and make the downs easier to handle.