You'll need more vents than that. Chickens make a lot of moisture between breathing and pooping. I read somewhere that said you should have 1 square foot of vent for every chicken. I don't have quite that much. Yes, closing that window is a good idea or there will be a breeze on the birds. It's easy to add vents this time of the year but will be harder if you wait until winter.
I have a 6 by 8 foot coop, two 1' by 3' vents under the eaves, a 2' by 2' vent in the back door that gets no breeze at all, and their pop door which is open year round. The vents are up high, the roost is down low except for the pop door which opens to a run that has the sides covered in clear plastic shower curtains during the winter.