Frequent turning of the eggs during incubation is important during the first part of incubation. If the eggs are not turned in this period, the embryo will attach (stick) to the membrane and both hatchability and chick quality will be significantly reduced. If the eggs are not turned at all during incubation, hatchabilty will drop to 20%-30% maximum.

Although eggs in a commercial incubator are normally turned until the moment they are transferred (18 days), this is not absolutely necessary. Research has shown that turning is extremely important during the first days of incubation, but that stopping to turn after approximately 10-12 days has no significant effect on hatchability anymore.

In the field, turning is stopped sometimes after 14 days to improve the air velocity over the eggs and with that improve the heat transfer and control the embryo temperature.

In a commercial incubator, eggs are normally turned once every hour. The minimum amount of turning should be approximately once every 4 hours, so 6 times a day. If eggs are turned less frequently, hatchability will be impaired. Also here, the earlier in the incubation process the more critical it is. Eggs should not be turned continuously, as this can rupture the yolk sac and result in embryonic mortality.

Normally eggs are placed vertically with the blunt end up and turned over their long axis. This will orient the embryo with the head towards the air-cell. Birds in nature place the eggs horizontally and turned over their short axis. Some researchers have shown that this has a slightly beneficial effect on hatchability, but handling the eggs in this way is more difficult. In this position, the embryo probably orients itself to the air-cell because the shape of the egg and the air-cell lift up the blunt end of the egg slightly above the sharp end.

Eggs should be turned over 45o angles to both sides. If eggs are turned over for instance only 30^o angles, hatchability is reported to be down with 5%-10%. Steeper angles of turning require more space between the setter trays. To save space, especially for bigger eggs, turning angles are sometimes put on less than 45^o.