Speckles Make Bird Eggs Stronger, Study Finds

John Pickrell in England
for National Geographic News

October 11, 2005

For more than a hundred years scientists and birders have engaged in
a heated debate over a seemingly harmless question: Why are birds'
eggs speckled?

Many experts believed that the markings are camouflage, useful in concealing eggs from predators. But for many bird species, the facts don't quite add up.

Now British ornithologists, or bird zoologists, have shown that speckling may be a unique solution to the engineering problem of how to strengthen unusually fragile shells.

A new study shows that pigment chemicals that create the speckles may act as a kind of glue, supporting thin areas of shell and protecting them from breakage during incubation in the nest.

"Very Beautiful"

Among egg-laying animals, birds are unique, in that their eggs sport a variety of pigmentation patterns and are often covered in striking spots or speckles.

The diversity of speckling is most pronounced in passerine birds (the perching or songbirds that make up 60 percent of all bird species). Passerines include great tits, blue tits, warblers, and sparrows.

The "very beautiful" speckling patterns on bird eggs, combined with eye-catching background coloration, are one reason eggs were once so popular with collectors, said ornithologist Andrew Gosler of the University of Oxford, U.K. (Collecting bird eggs is now forbidden in many countries.)

But the purpose of speckling has remained largely unconfirmed. One theory is that spots might help a nesting bird distinguish among different eggs and thereby help parents turn and warm each egg equally.

The most popular theory, though, is that the markings help to conceal eggs from predators. Among some ground-nesting waterbirds, such as gulls and plovers, research has shown that speckling aids egg camouflage.

But perplexingly, for most species, speckling patterns are often found to do a very poor job of helping eggs to blend in.

Take the example of the white and reddish-brown speckled eggs of the great tit (Parus major): "A blind weasel could find them," Gosler said. "It's a leap of faith to believe these are in any way camouflaged."

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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