I suspect this may not be a one answer question. Many people believe its temperature related. I suspect it is more closely related to the moon. Meaning length of daylight hours etc. But if the moon is the trigger, then the rut would be easy to identify beginning and end. If its temperature then theoretically you could take a herd of deer in August and move them up north to colder temperatures and trigger the doe's into season early. I assume there may be more than just 2 (temp/moon) factors involved. ????