Dog With Eggs

However some have suggested that the protoporphyrin markings on passerine eggs actually act to reduce brittleness by acting as a solid state lubricant. If there is insufficient calcium available in the local soil, the egg shell may be thin, especially in a circle around the broad end. Protoporphyrin speckling compensates for this, and increases inversely to the amount of calcium in the soil.
For the same reason, later eggs in a clutch are more spotted than early ones as the female's store of calcium is depleted.
The color of individual eggs is also genetically influenced, and appears to be inherited through the mother only, suggesting that the gene responsible for pigmentation is on the sex determining W chromosome (female birds are WZ, males ZZ).
It used to be thought that color was applied to the shell immediately before laying, but this research shows that coloration is an integral part of the development of the shell, with the same protein responsible for depositing calcium carbonate, or protoporphyrins when there is a lack of that mineral.