Birds' eggs are unique in their diverse pigmentation. This diversity isgreatest amongst perching birds (order Passeriformes: 60% of all birdspecies), which include many familiar species including tits andwarblers. Despite intense interest, the purpose, in most species, ofthese patterns was unknown.
Most passerines lay eggs speckled with reddish protoporphyrin spotsforming a ring around the egg's blunt end, on an otherwise unpigmentedshell. Evidence in a paper by Gosler, Higham & Reynolds soon toappear in Ecology Letters now suggests that rather than giving a visualsignal, protoporphyrins strengthen the eggshell by compensating forreduced eggshell-thickness caused by calcium deficiency.
Pigmentspots on great tit eggs specifically marked thinner areas of shell,with darker spots marking yet thinner shell than paler spots, andfemales nesting on low-calcium soils, laid thinner-shelled,more-spotted eggs than those on high-calcium soils nearby. Pigmentationmay offer a way to assess eggshell quality.
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