Keeping animals/birds cool can be a challenge.
1. Are you able to feed a gruel made out of their feed by mixing cold water into a small portion of feed. Feed only a small amount, what they will eat up in 10 minutes, as it will spoil quickly in the heat. THis isn't always practical.
2. Frozen berries, if you like feeding fresh foods (vs. commercial crumbles).
3. Frozen 2 liter soda bottles: Fill 3/4 full and freeze. Even if they don't lie near it, it will help reduce the temps. I add 2 T vinegar to keep the water from spoiling; that way I feel comfortable reusing the bottle, just wash the outside and put it back into the freezer.
4. Do go after one more of the windows to open it up. Remove the plywood and cover with hardware cloth. THen, if you need to you can cover it. Covering can be a hinged plywood window cover, or attach a cloth curtain. A curtain is a cloth a little wider than the opening and a little longer. Tack it up across the top with 2 long strings positioned one quarter of the way in . These act as tie ups. Let it hang to block air movement or roll up cloth and tie like an old fashion western bed roll.
5. OPen all the windows and cover with hardware cloth.
6. Can you shield the roof from direct sun in any way? Or change the color to a lighter color or reflective material. Perhaps throwing an old white bed sheet over the roof. I've thought about growing pole beans on a tall trellis, for example. Cucumbers might work too.
7. Can you let them out into a run for the day to make use of natural breezes. I use old dog exercise pens. For the ones that try to fly out, I throw a tarp over or old screens and then a tarpto cover a smaller area (provides shade while catching a breeze). You might try craigs list for used items.
Hope one of these ideas might work for you, or help you think of another solution. Keeping them cool is a bigger problem then keeping them warm in the colder months.
GL