Trying to achieve maximum chick uniformity by weighing the eggs from the many different aged donor flocks or by storing eggs from a few breeder flocks, does not guarantee good final flock uniformity. On the one hand, stored eggs for over 7 days and eggs from very young breeder flocks require longer incubation time than the others. The first chicks to hatch risk dehydration, waiting for the completion of the hatch. On the other hand, experience shows that mixing of the different “donor flock” micro-organisms may compromise the sanitary level of the whole hatch.