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That sounds very strange to me. In the six hatches we go through, the birds that are hatched LATER show more aggressive tendencies. About July 1st we see more aggresiveness and have to keep the lights dimmed down some to compensate--it's not the end of the world but something to think about. We will also throw whatever weeds, etc. grow on the outside of the pens in for them to redirect some of their energy. It saves a small amount of oh-so-precious feed too.

Different strain of bird perhaps? What are the differences in living conditions? Stress will make them pick at each other regardless of time of the year.

I think it is just peeking order issues,...they are proably close to that pens ideal density, they would be 28 weeks old now full adults and be at breeding age. Reduce as much stress as possible give them other things to peak at hay, etc, and make sure they have places to get away from each other shelters, roost spots etc.