Eggs are fertilized when conceived. If you are asking about birds they are fertilized the same way human eggs are fertilized, through sexual intercourse. You can find more information here: http://www.health002.com/Female/how-are-...

An egg is fertilized with sperm. In the case of a chicken egg. When a male and female chicken mate the sperm fertilizes the yolk. When the chicken lays the egg it some out fertilized. You can find more information here: http://www.users.on.net/~greggles/reprod...