How the Hen Makes an Egg

The egg is so familiar that we seldom remember to marvel at its making. All animals work hard at the business of reproduction, but the hen does more than most. Her “reproductive effort,” defined as the fraction of body weight that an animal deposits daily in her potential offspring, is 100 times greater than a human’s. Each egg is about 3% of the hen’s weight, so in a year of laying, she converts about eight times her body weight into eggs. A quarter of her daily energy expenditure goes toward egg-making; a duck puts in half.