You are correct. Embryo development does not start until the hen begins the continuous brooding process. As far as I know all birds only lay one egg a day. Wild as well as domestic. It is not uncommon for chickens, ducks, and other birds to lay eggs in the same nest which will be brooded and raised by one individual female. I often saw people put up to 20 chicken eggs under a broody hen turkey. The larger turkey could brood many more eggs than a chicken hen, and she would care for the chicks as if they were her own. If birds of one species species lays its eggs in the nest of another species then this is termed nest parasitism. Like our cowbirds.