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Introduction
The need for egg turning during incubation is one of the most dramatic differences between the incubation requirements of birds and reptiles. Almost all avian eggs need to be turned throughout much of incubation (Poulsen, 1953; Drent, 1975) and although there are many studies of the egg turning behaviour in incubating birds (Drent, 1975), these are not discussed here. By stark contrast, reptilian embryos are usually killed by turning during incubation (Ferguson, 1985). Egg turning in birds is thought to prevent deleterious adhesions between the embryo and the shell membranes (New, 1957) but in reptiles such adhession is normal (Ewert, 1985; Ferguson, 1985). This simplistic view of the phenomenon of egg turning is widely accepted yet if the requirement and effects of turning are examined further the situation is not so clear cut. In this chapter, the effects of egg turning on avian and reptilian development are described and some suggestions are made for the physiological basis of turning in birds. The possible evolutionary relationships between tgg turning behaviour in birds and its absence in reptiles, are discussed.
The effects of egg turning on hatchability in birds and reptiles
Avian eggs
The majority of studies of egg turning on avian embryos are concerned with effects on hatchability in poultry (Landauer, 1967; Lundy, 1969). Eycleshymer (1907) first showed that turning rate affected hatchability, although by modern standards the results were poor; 58% of fertile eggs of the fowl (Gallus gallus) hatched after being turned five times a day compared with 45% of eggs turned twice a day and only 15% hatchability of unturned eggs.
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