But it takes a lot of extra calcium to produce thicker shells, and a penguin can't just run to the corner drugstore to pick up some calcium-rich antacid tablets. New research led by a University of Washington biologist shows that during the period when eggs are being laid, female penguins have significantly more mollusk shells, mainly clams and mussels, in their stomachs than males do. The mollusk shells gradually leach calcium used to form eggshells.
Penguins typically nest on hard surfaces near coastlines and, if soil conditions are right, they might build burrows. Because they can't fly and are made to be agile swimmers rather than to walk gracefully on land, it is rare that they can collect much soft nesting material.
Still, their eggs are rarely broken because the shells are more than 50 percent thicker than expected for their size, which is about twice that of a chicken egg, said Dee Boersma. The University of Washington biology professor has studied penguins in South America, Antarctica and various South Seas islands, and said the thick-shell pattern follows for some other birds too.
"The birds that lay eggs on rocks or those that tend to lay them from great heights, like ostriches and rheas, also tend of have thicker eggshells," she said.
Both female and male penguins typically fast for a week or more before eggs are laid. Both genders ingest mollusk shells before the fast begins, which could alleviate hunger during the fast. But females lacking other sources of calcium ingest significantly more mollusk shells, which apparently supplements calcium taken from their bones for eggshell formation.