Re: Bird belt?
I picked up an Avery strap vest years ago for duck and goose. A few years ago I got tired, like you, of sweating my brains out. I picked up a couple long sleaved orange shirts that were a size too large so I could wear a sweater or the like under it. Toss the strap vest on and I can walk all day without working up too much of a sweat. The vest has enough pockets and d-rings I have yet to find anything it can't hold. Yes, it does have a game bag that comes up your back, but it doesn't have that "bloodproof" junk that never works anyway. So it breathes, as long as you don't have the waist and chest adjustment cinched down.
I even got to the point that unless it gets down below freezing that's what I wear. Duck, archery, upland, rifle... My license goes on it about now and never comes off.
Remember, shoot straight and shoot often.
Good... Bad... I'm the guy with the gun.