Q. When the chicks have just hatched, how do chicken farmers tell future hens from future roosters?

A. Sexing hatchlings is important for the egg industry. Roosters do not lay eggs, obviously, and they are not needed in order for hens to lay eggs.

But it is surprisingly difficult to tell hatchling males from females; there are no external genitalia, and the internal equipment is only subtly different.

Though some breeds may show early differences in feather color, often it can take weeks for obvious signs of gender — like combs, wattles and behavioral traits — to emerge.

An alternative to waiting was developed in the 1930s by two Japanese researchers, Kiyoshi Masui and Juro Hashimoto. It is called venting or vent sexing.