A cockerel is a young rooster, a male chicken which has not been castrated. The point when a chicken transitions from being a cockerel to a rooster is a bit ambiguous, but in general, a rooster is a chicken which has fully matured, and mated with hens, while a cockerel is either still growing, or inexperienced with hens. When a cockerel is castrated, it becomes a capon. Capons do not develop in the same way as uncastrated birds, and they tend to be calmer and easier to handle as well as physically different from roosters and cockerels.
The term comes from the Old English cokerel, which is a diminutive form of cok, or “rooster.” At one time, adult male chickens were referred to as cocks, but when this term acquired a new definition in slang, the word “rooster” came into being. “Rooster” dates to around the mid-1800s, and it is particularly popular in the United States, a nation where double-entendres about cocks abound around the barnyard.
Cockerels have several traits which distinguish them from hens, often at a very early age. When chicks are first born, it is usually difficult to distinguish between males and females, but within weeks, cockerels have markedly pronounced combs on their heads, and they may appear bulkier, with longer and more decorative tail feathers. As they develop, these traits will become more and more notable.
Roosters tend to be larger and heavier than hens, with muscular bodies and spurred feet used in fighting. They also have long, decorative tail feathers, and heavy wattles and combs. A cockerel often looks particularly sleek and graceful, since the bird has not yet bulked out or begun fighting to defend territory with other birds, although cockerels can and will squabble if left alone with each other. It is common for a cockerel to seem especially proud and self assured, which explains why “cockerel” is sometimes used as a slang term to describe someone with an inflated ego.
Once a cockerel matures into a rooster, he will be happiest with a flock of hens to look after. Roosters generally rely on patrolling their territory to keep their hens loyal, and they will fight viciously with roosters who attempt to steal hens from the flock. The fighting instinct of roosters is world famous, and in some regions, roosters are actually bred specifically for fighting as a blood sport.