Boaz

The one that is not about heat: an open beak when it is not hot and she has not just been chased. With wheezing, gurgling or rattling, sneezing, nasal discharge, swollen or bubbly eyes, coughing, head shaking, or sitting fluffed and off her food.

Separate her and ring somebody, because respiratory disease moves through a flock. A bit of panting after being handled that stops within a minute or two is nothing at all.

You have worked it out yourself. Chickens do not sweat, they pant, hold their wings away from the body and go and lie in cool dirt. Silkie plumage does not lie flat the way normal feathers do, so shedding heat is genuinely harder for them than for your others.

One small tree is not enough shade for four of them. Tarps, shade cloth, anything that makes real shade, plus water in several places rather than one, and airflow at roof level in the coop.