Linking in from Dave's post about unfertilised chicken eggs and caviar, can someone please explain to me how and why chickens lay unfertilised eggs? (There was a recent thing in the British press about a young girl hatching two ducklings from organic eggs).

Why do they go all the way through the energy requirements for producing an egg and coating it with a shell (using the calcium and phosphate resources from their own body to do so) when there is no end product?? To do so is deleterious to the chicken with no gain.

Is there something ridiculously fundamental I'm overlooking??


maybe in the wild (ie before hens were domesticated) the majority of the eggs did get fertilised but many were eaten by predators, hence the high volume of eggs laid.

I hasten to add that is mere speculation on my part and is backed up by no evidence at all!


Why do female mammals menstrate? I think it's pretty much the same question. Ova are released in a regular cycle, a developmental environment is provided for the developing young just in case an ovuum gets fertilzed. If fertilized, mammals have a hormonal feedback to prevent menstration since gestation is internal, but in birds the egg is simply laid regardless.

Obviously this works for tropical animals which don't have a single defined breeding season, hence the domestication of jungle fowl (chickens) I expect.

IIRC when chickens mate the female is capable of retaining viable sperm for a considerable period of time (around 10 days I think).  So in a laying cycle a chicken would use this stock of sperm to fertilise the egg laid each day.  Wild or less intensively selected chicken breeds would start a laying cycle and once finished they would "go broody" and incubate the eggs to hatching prior to starting a new laying cycle.  I suspect that with wild chickens the chance of a laying cycle going unfertilised would be pretty low and probably the odd unfertilised cycle would be a small price to pay for the sheer number of fertilised eggs over a chickens life time.

Modern, intensively bred chickens can have a broody, non-laying period as short as 1 day and can lay about 300 unfertilised eggs a year but this says more about our intensive breeding programs and us taking advantage of the fact that chickens produce eggs regardless of being fertilised.

A quick look on the internet reveals that the forerunners of domestic chickens would probably only have prodcued about 50 eggs a year.  This probably isn't a great cost to the wild chicken, especially so if they are all fertilised.