At one week, the chicks are already behaving according to their gender. The pullets pose like Mom's white porcelain casserole dish, and the cockerels dart toward one another, wings flapping, heads low, as though jousting. I suppose they have to start sometime. Sexual maturity is only three months away. Actually these birds will never live to see anything much beyond the onset of puberty. The pullets will never lay an egg, and the cockerels will never learn to crow.
Their behaviors are genetically induced, of course. Following Richard Dawkins' Selfish Gene theory, male replicators (individuals) are most likely to reproduce successfully (that is, put viable offspring on the ground to pass on genes) by competing with other males for the right to mate with the greatest number of females. Females, on the other hand, maximize their reproductive capability by nurturing two or three broods per year. She doesn't have to compete. For her, it's like being lost in the forest: if she just sits still and remains calm, somebody, some male will find her.
From the beginning, the female makes the larger investment in the potential offspring: eggs are the largest cells in the female body, and they are relatively rare, while the sperm cells are the smallest cells in the male body and they are very plentiful. Even before the fertilized egg is laid, the female has made a greater contribution to chick's viability than the male--who is busy maximizing his reproductive capability elsewhere.
Of course, the same genes that make influence sexual behavior influence the physical health of replicators. Natural selection favors individuals in the best, most robust shape of their lives at or near the onset of sexual maturity. When it comes to chickens, so do I. In the week before slaughter, a few of the cockerels' voices will have begun to change. Though there is nothing, and will be nothing, to crow about, they will have attempted a croaking, cracking crow.