In addition to climate parameters such as air composition and temperature, turning is an important third parameter that needs to be controlled during incubation.

In commercial incubators, hatching eggs are placed in setter trays with the air cell up and turned regularly through angles of 90° or 45° on either side of eggs’ long axis. Historically, arguments for the need to turn eggs frequently were (1) poor temperature distribution in the albumen and yolk and/or (2) the risk of the embryo and extra-embryonic membranes adhering to the inner shell membrane.

From recent research, however, we now understand that turning hatching eggs during incubation is essential for the development of extra-embryonic membranes, including the amnion and the chorion- allantois respectively. Simultaneously, extra-embryonic compartments are filled with sub-embryonic fluid, amniotic and allantoic fluids. Both membranes and fluids are essential for the optimum growth and development of the embryo (reviewed by Deeming 2002 in: Avian Incubation behaviour, environment and evolution); Baggott et al., 2002).

The formation of extra-embryonic membranes and compartments is fundamental for the transfer of nutrients from the albumen and yolk and, last but not least, from the shell to the developing embryo. It is essential that embryonic development keeps pace with the development of the extra-embryonic tissues, so that when the day 12 embryo starts to grow, the yolk lipids are prepared for uptake by blood veins grown into a well developed yolk sac.

If development of the yolk sac membranes and vascularisation lags behind that of the embryo, embryonic growth is limited. Lipids transported by the blood vessels from the yolk need a well developed yolk sac to be metabolized.

Turning is therefore essential during days 0-7, when the early extra-embryonic yolk sac membrane (area vasculosa and vitelline membrane)...