Nothing , as long as your run is predator-proof.
In a hot climate you might even want to consider converting your coop and run to an Open Air coop -- which is, essentially, a roofed wire box with a weather shelter at one end.
Even in cold climates ventilation is still critical, it just has to be provided in a draft-free manner so that the chickens never have their feathers ruffled by a breeze when they're sitting on their roosts. The 1 square foot per hen is considered a minimum.
In my hot climate I aim for at least...
Top-hinged windows are good because they are their own awning to keep the weather out.
Put the vent as high up on both the high and low side of the roof as possible.
Or cut a permanent opening, cover it with hardware cloth, and add an awning (mine are fairly redneck, but they can be made a...
I don't see any ventilation at all in that coop.
It's very common for chickens, who need a great abundance of fresh air for good health (recommended 1 square foot of ventilation per adult, standard-sized hen), to refuse to go into a coop that is dark, hot, stuffy, and heavy with ammonia. :(...