It just dawned on me. Do you know the chicken variety that you have?? I only see one picture and it looks like it is JUNGLE FOWL. I had a hen of that species and it looked like your bird. Both hens and roosters are similar colors. with roos having larger fancier tails. That explains why...
Unusual for chicks not to return into coop. Some questions... Is this first time out? Did they walk out on their own? How far is free range allow them to roam at present from coop? Are these bantams that love trees? Is rooster trying to violate the hens and they are running away?