3 options that I can think of, depending on window size.
1) There are exhaust fans made specifically for barns that can have adjustable settings and can be thermostatically controlled. If one would fit in a window, that is an option.
2) A house ceiling fab might work, they usually have 3 speed settings, and low is quite low. Would you be able to mount it to the ceiling/roof of the chicken room?
3) Again, depending on window size: there are a variety of window fans with 3 settings, low usually isn't too bad, especially if the window is high enough to not blow at 'chicken' level...though on a really hot day they might like that.
Clearly fans (such as box) at floor level wouldn't work due to bedding.Anyone else have Ideas?