What are you feeding them? Treats of any kind?
If your not seeing any other symptoms it maybe this one is just too warm. Let’s see if we can get some members who have Pekins to weigh in. @ruthhope @CoriM
It's good not to feed treats they need the nutrients from their feed to grow strong and healthy treats should always be minimal and always offer grit so they can process it since they don't have teeth to chew, in a separate container from their feed. Chick grit to start and reg grit as they are more adult size unless they are out where they can pick up their grit off the ground.