However, the photographic markings on passerine eggs have been suggested to reduce brittleness by acting as a solid state lubricant. If insufficient calcium is 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. To understand the functional significance of eggshell spotting (or maculation) it is important to quantify this pigmentation For the same reason, later eggs in a clutch are more spotted than early ones, as the female's store of calcium is depleted.