I would open up the gable ends (the triangles at the top under the roof. Cover them in h/w cloth of course. That way you can get flow high up above the chickens when they roost.
I also have open soffits, but if you read the definitive @3KillerBs article you need to think about flow and the soffits will ventilate better if you can also open up the roof ridge.
I also have open soffits, but if you read the definitive @3KillerBs article you need to think about flow and the soffits will ventilate better if you can also open up the roof ridge.