Hello! I’ve lost several birds to the heat during summer. After researching the problem we tried some things that seemed to help a lot.
Clean out the coop often, it seems to be able to retain more heat when there’s lots of poo and loose dirt.
Increase ventilation so that the air can pass through easily. Still air seems to be worse than warm moving air.
Dunk chickens (below neck) in a bucket of water, and let them air dry. Evaporation keeps them cooler.
Give cool treats, such as watermelon, berries, and other fruits.
Reduce or do not feed scratch grains. It warms them up from the inside as they digest it.
Put liter bottles or milk jugs of frozen water in the run and coop. They like to lay against them to keep cool.
Put a shallow pool (kiddie pool) filled with a few inches (3-5) of water out so they can walk in it. Feet are one of the most temperature-sensitive and controlling parts of the body.
Increase roosting bar space. The more your chickens have to cram and squish into a small place the more body heat will be passed and therefore it will be warmer.
Hope this helps!!