Whitetail deer seem to breed like rabbits. And that's one reason why they're increasing in numbers in the West. Oregon, Washington, Idaho, and Montana have all seen whitetail numbers increase, while mule deer numbers decrease.
A couple of other reasons why whitetails are on the rise over muleys:
1. Whitetails are more likely to have twins
2. Increasing rural expansion - suburbs taking over mule deer habitat and leaving it for the whitetails who can stand living closer to humans
3. Wildfires that destroy sagebrush land, destroys mule deer wintering land
4. Increased demand for forage between elk, whitetails, and mule deer leaves the less competitive mule deer at a disadvantage. Whitetails are very competitive, and adaptable to changing environments.
Whitetails are moving out of their traditional stomping grounds of valleys and river bottoms, and are taking over what was once prime mule deer habitat. For instance, whitetails can be seen the high Yellowstone plateaus, a place they've never, or rarely, been seen before.
The Billings Gazette article has the full analysis of what's going on in Montana, but is very applicable to Idaho's whitetail/muley dilemma.
