Most passerines lay eggs speckled with reddish protoporphyrin spots forming a ring around the egg's blunt end, on an otherwise unpigmented shell. Evidence in a paper by Gosler, Higham & Reynolds soon to appear in Ecology Letters now suggests that rather than giving a visual signal, protoporphyrins strengthen the eggshell by compensating for reduced eggshell-thickness caused by calcium deficiency.
Pigment spots on great tit eggs specifically marked thinner areas of shell, with darker spots marking yet thinner shell than paler spots, and females nesting on low-calcium soils, laid thinner-shelled, more-spotted eggs than those on high-calcium soils nearby. Pigmentation may offer a way to assess eggshell quality.
Ecology Letters