Joseph Carson, of Waihopai School, asks :-

What makes a hen lay an egg and why so regularly?

Pete Hodgson, a veterinarian who was a member of parliament for Dunedin North, responded.

A hen lays an egg because she wants to hatch a chicken.

All birds reproduce that way. The hen's egg is fertilised by sperm from a rooster which mates with the hen before the egg shell forms. The fertilised egg looks pretty much like an unfertilised egg until the chicken starts to develop in it. The hen sits on the egg (incubates it) to keep it at the right temperature for the chick to develop.

Why do hens lay eggs so regularly? Mainly because they are bred to. A wild hen will lay only a couple of groups of eggs (clutches) each season. When they have laid about a dozen eggs, at the rate of about one a day, they stop laying and start sitting.

But we take the eggs away from domestic hens each day so the hen just keeps on laying. We select hens which tend to lay a lot of eggs and breed from them so that we now have hens which will lay eggs all year round except in late autumn and winter, when they stop laying to moult (grow a new covering of feathers). At our place the hens are run outdoors and stop laying for two months while they take their time growing new feathers.

Hens that are bred indoors can be forced to moult at a certain time, by altering food, water and (especially) light. After only a short period, hens that have been forced to moult start laying again, so that they produce eggs nearly all year round.