Wouldn't low vents cause a cold air draft during winter months?
Vents need to be located in the proper locations in respect to the roosting area to prevent drafts in the roosting area...
You can have a lot of ventilation without an observable draft, consider your bathroom exhaust fan or a stove top hood, is there a 'draft' in the room when those vents are on? Or even the heat/AC vents in a house, unless you are standing directly in front or on top of one there is for all intent no draft...
As long as the ventilated air movement is not ruffling the birds feathers when they are at roost it's acceptable, the cold air is by far the lesser of the evils compared to the warm humid air and ammonia gas saturated air...