You will close some of them down, relative to summer conditions; but you still need a goodly amount of air exchange going on, so you cannot shut your ventilation off.
In some ways, ventilation in chicken cage is actually more important in winter because cold air can’t hold nearly so much water vapor before it gets saturated i.e. really damp and humid and clammy, i.e. you’re trolling for frostbite and respiratory disease.
So yes, your vents will be letting in cold air, but you know what, that’s OK as long as it is not breezing down directly at your chickens. If you’re concerned about the chickens getting too cold — although most standard-sized breeds are fine down to freezing and significantly below, as long as the air is dry and relatively still and they have an appropriate-width roost and plenty of food — then insulate your chicken coop. And yes, insulation is quite useful even with vents open (for some reason this issue comes up often); would you think it pointless to wear a winter coat just ‘cuz you had no hat on? 😛 What insulation does is reduce heat loss from the chicken coop so that you can afford to admit more cold air without making the place too cold.
In a super-cold climate, and let me say that I do not consider southern Ontario Canada where I live to fall into this category, you may want to think about arranging for your vents to be taking air in from a somewhat thermally-buffered source… a predator proofed flue run along the ground a ways and covered in insulation, or a translucently-enclosed space that the sun warms, or the building’s attic, or a larger barn, or like that.