In fact most perching birds lay eggs that are mostly white except for a ring of reddish spots around the blunt end. The spots are caused by compounds called protoporphyrins, which are known to be strong and flexible.
Gosler and his team consistently found a ring of protoporphyrin speckles on many species' eggs. This pattern suggested the pigment might have an engineering function.
Calcium Deficient
Gosler's team studied great tits living in woods near Oxford. The research shows that birds nesting in areas of the woods where the soil is known to be low in calcium produce eggs that are more heavily speckled.
Calcium carbonate is the main construction material for birds' eggs. Great tits get the nutrient from eating snails, which take calcium from the soil and use it to make their own shells.
The team carried out meticulous studies of the birds' eggshells to show that speckled areas of shells are significantly thinner than unpigmented patches. Also, heavily speckled eggs are lighter, and therefore thinner, than their less spotty counterparts laid in the same broods.
The team concludes that the rings of speckles found around the blunt ends of many eggs act to strengthen inherent weak spots in the ovoid design. Their findings are detailed in this month's issue of the science journal Ecology Letters.
Gosler speculates that spots of pigmentation are laid down within the mother bird while the eggs are forming. When cells are short of calcium at a particular part of the shell surface, pigment may be added to make up for the deficiency.
The scientist hopes to prove that speckling serves the same purpose in other species of birds.
"The opinion that speckling on birds' eggs is for camouflage has been very widespread in the ornithological literature," commented Raivo Mand, an avian-ecology expert with the University of Tartu in Estonia.
"Nevertheless, in several hole-nesting birds the eggs are covered with reddish spots, although there is no reason for camouflage," he said. "Conversely, in many species that nest on open ground exposed to predators, their eggs are not speckled at all."
Gosler's study is a "completely new and revolutionary explanation for this paradox," Mand said.
Useful Application
In addition to solving a historical debate, the finding may have a useful application for future bird conservation.
Pollution from insecticides such as DDT—which is still used to kill malaria-bearing mosquitoes in some parts of the world—can damage bird populations by reducing eggshell thickness.
The pesticide has had a severe impact on species such as eagles.
Conservationists measuring the impact of insecticides on birds could potentially now survey egg strength by measuring speckling intensity, Gosler suggests.
Speckling intensity could give a quick and dirty estimate of the relative effects of pollution on different populations of birds, he says.
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