In the winter you want the vents up high. The two bad things you want to get rid of are ammonia and moisture. Ammonia is lighter than air so will rise to the top. Warm air holds more moisture than cold air and will also rise. Even in winter there is some temperature difference out there. You also do not want a breeze hitting the chickens when they are on the roost. Some are street-smart enough to get down off a roost if the wind is hitting them but not all are. Help them out.
That roof looks pretty flat. Still you can put a roof vent or even a cupola up there to help vent at the top. With snow, I would not count on a ridge vent to do much good, though in summer it will help get rid of hot air.
You don’t have much room for it, but a gable vent will move a lot of air. Maybe you could fit one or two of those in. Remember you don’t want moving air to hit them, but if you put the gable vents on one end and the roosts in the far end, like a cul de sac, you can get some good ventilation with a cross breeze without that breeze hitting them.
I don’t see any overhang. A normal way to vent and keep rain or snow out is to put an opening under the overhang and cover it with hardware cloth. With that building I don’t see that as an option for you.