From a Pomeranian kennel owner I know, I understand that birds of prey will attack and carry off small dog breed puppies. However, when even a small breed dog reaches adulthood, they are too large for this to be a real danger.

We have small dogs (Pomeranian and Shih Tzu) and have fenced the backyard with fencing to contain them. As foxes in our area are larger, they can't get through the fence. And, as it's vertical bar fencing, it can't be climbed.

So, IMO, birds of prey should pose almost zero threat, if your dog is an adult. And if you have a properly fenced area for your dog, foxes shouldn't be able to enter it.

Hope this helps,
Richard :) :) :)