I understand that the domestic chicken has been artificially selected to lay many more unfertilised eggs than it did in the wild, plus the eggs are much larger.

But for what purpose did  their ancestors, the jungle fowl ,lay eggs without any sexual union. And also do any other wild bird species lay unfertilised eggs?

I always thought that birds ( except the chicken)  only laid fertilised eggs as part of their biology following mating.

Also just viewed a video of an x battery hen laying an egg. It took over 40 minutes in which the hen appeared to be labouring. Yes it seemed a struggle for her and when the camera focused on her vaginal opening it was a tight passing.

So is this camparable to a mammalian birth?  Does the hen feel pain each time she lays and is it strong pain?   And what is an average time for the actual  delivery of the egg to the outside world?

Many thanks. Eileen

I can answer only the question why hens lay unfertilised eggs. It is comparable to non-toxic almond trees, non-woody apples, and so on: oops! But guess which birds humans would instinctively or by accident select for?

Imagine a small village that keeps fowl as a food storage method. Roosters can be very nasty to keep, they pick and kick and scratch a lot more than hens. So they keep only hens and eat the roosters outright.

Now, would you eat the bird that never lays an egg or the one that occasionally lays eggs?

Exactly - now which bird has the chance to breed if occasionally there is a rooster around (e.g., a wild one drops in, or a caught one is allowed to live for a few days)?

Exactly again - and thus we selected for the laying of unfertilised eggs.