I have jumbo cornish x rocks that will be 3 weeks on Friday. How can I tell which are roos? There are some that are a lot bigger than the others. Are these the roos.
The first sign with our "babies" was behavior. The cockerels would pal around together and frequently "fight" by ruffling up their feathers and chest-butting each other. We were then getting the scythe-like tailfeathers and markedly bigger combs than the pullets. Finally, at about four months, one has started crowing. The spurs are still barely there. These are mostly RIRs and Barred Rocks. The behavior can be misleading, as one of the rowdy gang, and Ameraucana, proved to be a pullet by being the first of the bunch to lay an egg. We were pretty sure of here anyway, as she doesn't have the comb or tailfeathers.
The differences in the comb colors and the development of wattles is probably going to be your first clue (@ 4 -6 weeks). Spur development comes quite a bit later. Cockerels tend to be slower in feather development too. Look for bare "wing bows" (what I call the shoulders). Behavior is not an indicator of sex, unless you're talking about crowing, which also comes later.