Why do chickens lay eggs ?
The amateur evolutionary biologist tries to answer.

Asked Kathy age 6. Because they are birds replied Robert. Why don't birds give birth live like humans ? Hmmm good question.

I tried to answer. It has to have something to do with flying. Can you imagine flying when pregnant (especially hard for men to imagine but I doubt many women can imagine it either).

But wait, the egg is as heavy (a little heavier) than the chick which hatches. hmmmm.

Mammals typically give birth to many offspring at once. Lucky ducks lay one egg at a time. I think the advantage of eggs is that they can be phased. Birds have one fully developed egg, another semi developed, a third real small and one about the size of a human egg. That means that they are not weighed down by several fully developed eggs (or fetuses) all in there together. my guess is that no mutant mammal has ever managed to reliably give birth to a mature offspring while not mis-carrying an immature fetus. Bats typically have one or two offspring a season. This is a major evolutionary handicap which they have someohow managed to overcome by developing sonar.

Now even with sonar I couldn't handle one or two kids a year. Oops one or two a year is, by normal animal standards, hardly any.