a secure run with half inch hardware clothe on all sides. Roof is ten feet tall and covered. I live in Nevada, so rattlesnakes are a thing, but I’ve never seen one. No sign of injury. Can chickens have heart attacks? We have 18 other chickens, one group is 5 months, the other Is 3.Can you give some more info?
what setup do you have?.
what is in your flock?
was he penned alone or with others?
do you have snakes or other predators where you live?
is there any sign of injury on his neck, legs, etc?