I posted yesterday about this baby. Any advise? My biggest duckling panting everyday. In brooder, no heat lamp, ambient air about 75 degrees. Nothing in his nares has deep enough water to clean out.
Pekins are known for overheating in the first place, so when you cram several other ducklings in a pen with them, their body heat is going to cause some signs of overheating.
Sometimes you will get a "heat-prone" one like that, they're usually bigger, like in your case.
It looks like the panting is being caused by the accumulation of body heat in the corner of the pen, so if the one isn't showing other respiratory-related signs such as eye discharge, coughing, or wheezing I wouldn't worry.