I have a nagging curiosity about something though.
Before mass production, with humans keeping chickens for many hundreds of years and continually increasing their egg production, how did people feed them a balanced diet economically?
The continually increasing egg production is caused by a combination of factors:
--selective breeding
--better feeding
--artificial light
--changes in management
If you were to take the chickens from a few hundred years ago, feed them modern chicken food, provide artificial light so they think it is always spring, and break each hen that goes broody so she does not spend time setting, you would get quite a few more eggs from those hens even without the selective breeding.
And people who had no incubators, and could not buy chicks from a hatchery, would probably need to have hens that went broody, even if that meant fewer eggs laid each year. So they would not have been selecting for the highest possible egg production, because they needed to select for other traits too.