Eggs within a clutch may be larger or smaller than one another, but once laid, an individual egg does not change in size.
The marginal size variation often seen within a clutch may be linked to the order in which eggs are laid, and to the availability of calcium and other resources required for egg formation.
Eggs tend to be smaller when they are part of a larger clutch and when formed during cooler weather. The eggshell protecting the developing embryo enables the exchange of respiratory gases with the environment, something that also results in the loss of water from the egg. The combination of a fixed egg size and water loss can thus be used by researchers to predict how far through incubation an egg is. In turn, they can calculate the likely date of hatching.
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