I have found that birds excercising for an hour or more at a time will pant on occasion.Not in any way through the excersion put in during the excercise. Have seen it in cocks seperated from their mates.If the cocks go away ranging and return to the loft with strangers(some of which hens),on landing some will pant-excitement/stress?? these same birds will fly for an hour the following day and land with no signs at all of this panting.So it can happen without it being an illness and its not alway related to fear of attack in my opinion. Have never seen the same behaviour in cocks settled and paired if they return from ranging with strangers.