Panting......Is this normal????

Iwas sat watching my girls all free range the garden today and they seemed to be panting like dogs  ???they all had their beaks open slightly if that makes sense. There is plenty of shade in the garden the could have retreated to and they all had access to water which they had now and then. Is this normal chook behavior???

they are being wormed right now (started Monday) with flubenvet. There breathings fine and as it got cooler they stopped doing it. It was definitely the heat but i wasn't sure if that was normal other than the panting the seem perfectly fine and it didnt seem to stop them scratching around and creating lots of mess   

Mine were the same yesterday, they also held their wings slightly away from their bodies. I assumed it was a cooling mechanism as, like dogs, they don't sweat.