It has to be hefker before it was layed.
R’ Elyashiv says that you have to physically pick up the mother and shoo her away, which is almost impossible because they have eyes on the side of their head.
All you have to do is pick up the eggs or chick 12 tfachim and put it back down. after you do it, it can be done by others again.
Its also a big mitzva to be very careful while doing it.
Star K has a small sefer online all about it, google it.