There's going to be a flow of air between low and high vents, if a roost is between those two openings then you could get too strong a draft there. Great for summer, not so much in winter. That's why having closable vent covers is good as it will allow you to regulate the air flow. You won't know how strong a draft in until you get in there and 'feel' it. It's really hard to explain and harder to specify where someone vents should be.
