A fertile egg is one that will develop into a baby bird given the right conditions.
The time during a month that a woman can become pregnant. It is usually a period of eight days during her menstrual cycle. Up to five days before ovulation (because sperm can live this long inside the body), the day ovulation happens, and two days after (the life-span of an egg).
Fertile eggs are those which contain a live embryo. Matural female crocodiles will, like many reptiles, produce infertile eggs in the absence of a male.