Hi Garry, and welcome to the Forum.
Sorry towards about your chickens - there are several possible causes for chickens showing the symptoms you describe. The easiest to deal with is dust - if the bedding in their coop is dusty, or maybe you use hay in the nestboxes, they may be getting irritated by the spores. In that case, a change to dust free alternatives such as Aubiose or woodshavings may help. This hot weather isn't very good for chickens respiratory systems, any more than it is for ours.
Its possible they've developed a respiratory infection such as Mycoplasma or Infectious Bronchitis. Have you introduced any new birds recently who could have brought this in? If you look on the main Poultrykeeper website (see link at be bottom of the page) you can find out more. For Mycoplasma see http://poultrykeeper.com/respiratory-problems/mycoplasma I hope its not anything like this, but if you think it might be, then they will need antibiotics from your vet.
Do let us know how you get on.