[QuickSilver]

First, dogs can have OCD, just like people, so it's possible there's a neurological basis. I believe drugs are available for that if necessary.

Second, animals in zoos can develop these types of behaviors from boredom and a lack of exercise. For this, things that stimulate his prey drive can be helpful: fetch, fox-on-a-stick, as well as more intellectually challenging games, like find the treat.

Third, you can probably train him out of this behavior. Start only letting him into the yard when you go with him and keep him distracted and engaged. If he returns to his vigil, take him away, either back into the house, or back to a game you're playing. Try to make it as easy as possible for him to pay attention to you - start in the farthest corner of your yard, or right outside on the steps at first.

Also, if you don't already, try taking these dogs to a new park or something else novel. Let them tire themselves out there before bringing them back to your yard. That should make it easier for them to focus on you while you train them.

Good luck! Let us know how it goes.