The base layer consists of a single, T-shirt, or shirt with long sleeves that fit tightly around your body. It should be made of elastic material that fits well without restricting your freedom of movement. Actually like a second skin.
Enjoying the outdoors is greatly enhanced if you are comfortable. This is almost always related to feeling cool or warm enough. While hunting whitetail, turkey, moose, or any other game, a modern layering system can really benefit you in that domain.