When a chicken doesn't feel well, she usually stops drinking and can thus become dehydrated rapidly. A very useful way of providing additional fluids and helping to manage and prevent dehydration is giving her SQ fluids. It provides fluids into the space under the skin from where it can be slowly absorbed into the blood and body. It's something quite easy to do at home and will save you trips to the vet.
What you need is a bag of Lactated Ringer's solution, a drip set, and needles (I use 20 gauge). Note that the solution and needles require a prescription from your vet.
All must be sterile and be kept sterile. Be careful to not touch the ends of the drip line with anything. And change the needle every time.
I usually give 60 cc of fluids twice a day, but do follow the instructions of your vet.
PS: Aramis, the little red hen in the video, completely recovered from the heat exhaustion. This SQ fluids saved her life.