It can be difficult keeping chickens alive, let alone cool, in a tractor in summer. Often the low ceiling is made of metal, and heat blasts through.
Although my tractor is extremely airy, and has tarp + shadecloth on top, it's in a sunny yard where it could be quite easy for the birds to overheat. To stop this from happening I simply gather some fresh tree/shrub offcuts and drape them over the unit, like so:
This isn't a cure-all against massive summer heat, in which case it's best to have all tractors parked in deep shade, but in general I've found this system works even on quite hot days. Best of all, it lets me use renewable material (tree offcuts) that release water vapour as they wilt, helping to double-cool the air. Much better than shadecloth!