I don't think it's heat. I have raised several batches of Jumbo Cornish here in Mojave, CA. It gets 105+ in the summer.
I found some rules for those big meat birds.
1. Plenty of clean water.
2. Plenty of shade.
3. A breeze is good in the heat so don't keep them locked up in a coop.
4. Restrict food as they grow, they will eat themselves to death!
5. Segregate from other breeds.
6. Very clean coop and pen.
Hope this helps.
Michael
I found some rules for those big meat birds.
1. Plenty of clean water.
2. Plenty of shade.
3. A breeze is good in the heat so don't keep them locked up in a coop.
4. Restrict food as they grow, they will eat themselves to death!
5. Segregate from other breeds.
6. Very clean coop and pen.
Hope this helps.
Michael