I'd agree that it's just a strong, natural prey drive. Our female mix Kobie has killed 4 bluejays since spring 2000--killed and then left in the middle of the lawn, without eating or mauling the carcasses. I was freaked out the first time it happened too, but people say that most dogs will do this kind of thing whenever they're able to. It's challenging, fun, and totally normal for them! Squicky for us people, though.