All bird eggs begin to lose water from the moment they are laid because water vapour diffuses out through eggshell pores. The eggshell is a layer of calcium carbonate crystals which are laid down on to two fibrous shell membranes. The calcium layer is breached by hundreds of pores, the size, shape and number of which are fixed when the egg is laid. These pores allow oxygen to enter the egg and carbon dioxide and water vapour to leave. It is important to realise that eggs vary in their porosity. This can be due to differences in shell thickness or in the number or diameter of the pores in the eggshell. Even for eggs from the same hen, eggshell porosity will vary slightly within the clutch and different females will lay eggs with different porosities.

Therefore, for a given humidity in the setter the individual eggs will be losing different amounts of water. The frequency distribution of the different rates of water loss based around an average of 15% are shown in Figure 1. A few eggs lose insufficient weight loss whereas a few other lose too much but most of the eggs are losing between 10-20% of their initial weight. Figure 2 also illustrates the hatchability of eggs which have lost varying amounts of weight. From 10 to 20% the effect of weight loss on hatchability is minimal but at the extremes of the range hatchability drops off very quickly although this affects relatively few eggs. Embryos in eggs which lose insufficient moisture find that excessive water retention makes it hard to hatch because of subcutaneous oedema. Embryos in eggs which lose too much water die of dehydration.

Achieving the correct weight loss during incubation is critical for maximising hatchability. In Figure 2, incubating the same batch of eggs at a slightly lower humidity increases the average weight loss to 17% which shifts the curve to the right. This means that the number eggs which would lose over 20% of their mass is increase from 6 (at 15% average weight loss) to 39 and %hatchability drops from 81% to 79%. The effects of the same batch eggs achieving a range of average weight losses on hatchability are shown in Figure 2. As the average weight loss moves further from the 15% optimum the adverse effects on hatchability increase. Monitoring weight loss during artificial incubation is, therefore, very worthwhile.

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