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."

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