What is the temperature there? Are you sure these deaths aren't from heat stroke? The first symptoms before a chicken collapses is stumbling and imbalance. An overheated chicken can die very quickly following onset of symptoms unless cooled down and given a solution of electrolytes and extra sugar.
To cool an overheated chicken down quickly, place cold compresses under the wing pits. But under no circumstances should you dunk a chicken in cold water. Do this treatment and give one teaspoon sugar in one cup cool water with a pinch of salt and baking soda. Dip the beak or tube it into the chicken, but get it into the chicken as quickly as possible.
After exploring all the possible causes of death from the most simple to the most exotic, it may be from the most obvious - heat.