WHy does my dog mark bird droppings?

I can say birds vocally mark territory. Also if you listen. You can hear when birds are fighting by the sounds. One bird has entered another's territory or taking their girl. If your dog eats it, to the vet. But they are marking on it. I would say pay attention before during and after. Thet might tell you something. How the body moves or where their attention goes. I do not know why. But dogs tend to speak in different ways.