American Scientist

In 1930s Great Britain, two children grab baskets after returning home from school and run outside. Once there, one darts toward the trees at the edge of the yard, while the other walks carefully in a nearby meadow, with eyes fixed on the ground.

“Look, I found blue eggs with reddish speckles,” one child calls out.

A few minutes later, the other child shouts, “Hey, I see pink eggs with brown dots. I don’t have any that look like these.”

Prior to the 1950s in the United Kingdom, a scene like this might have been common; children and adults enjoyed collecting bird eggs. Although the hobby was a fun way to get outdoors and learn about nature, it led to a significant decline in bird populations. In an effort to increase the number of native birds, the U.K. government passed the Protection of Birds Act in 1954, making it illegal to take or destroy the eggs of wild birds (although video documenting wild bird eggs in nests remains popular).

Nowadays, if anyone finds one of those old egg collections, perhaps in their grandparents’ attic, they must give it to the police. The police pass it on to the U.K.’s Natural History Museum in Tring, where staff either add it to the museum’s collection—if they have enough information on where it was found—or offer it to biologist Steven Portugal of the University of Oxford.

Portugal, his postdoctoral fellow Marie Attard, and their team have now imaged the surfaces of thousands of different bird eggshells—from the vast collections of the Natural History Museum and the Western Foundation of Vertebrate Zoology in California—to determine how they have adapted to withstand their environments. Portugal hopes that deciphering the conditions that drive variation in eggshell structures could help improve captive bird breeding programs, which he says often have poor hatching rates.

As Portugal and his colleagues describe in the Journal of the Royal Society Interface, they chose 453 avian species that represented a range of habitats globally, then divided the eggs based on maculation. Maculate eggs have colors, spots, patterns, and intricate designs, whereas immaculate eggs are all white. The researchers further categorized the eggs by nest type: exposed, semiexposed, and enclosed.

Materials engineer James Bowen of the Open University in the United Kingdom then applied his imaging expertise to visualizing eggshell structures on the nanometer scale. Using a profilometer, which is essentially a 3D laser microscope, he scanned small samples of the eggshell surfaces, identifying every peak, valley, divot, and pore.

“In science fiction movies, a machine scans someone’s face, and a computer recreates an image of the face on a screen,” Portugal says. “The profilometer is effectively doing the same thing, but it’s doing it through a microscope.”

The data collected corresponded to three related measurements of the eggshell surface. Roughness relays the number of peaks, whereas skewness establishes where the peaks are and how they are distributed in a peak-to-valley ratio. Kurtosis measures the geometry of these features, delineating peak heights and valley depths.

The team found that species that lay maculate eggs and build exposed nests typically have rougher eggshell surfaces. The results were similar to what had been found previously for lotus plants (Nelumbo nucifera), which live in muddy bogs in Asia, yet their leaves are never dirty. Profilometry has shown that lotus leaves have lots of peaks and are very rough, so dirty water droplets that fall on the leaf maintain their bubble shape and quickly roll off.

“We think that eggshells that have this roughness are nested in a more exposed environment and are probably using it for the same reason,” Portugal explains. “It helps keep the eggs clean and stops the pores from being completely saturated when it rains.”

Eggshell roughness, Portugal says, also helps to control heat, which is important in hot, dry climates. And pigment has an important role to play, as well. Protoporphyrin, the reddish-brown color in maculate eggshells, acts as a bacterial repellent, sunblock, and camouflage, and it can potentially make the shell a bit stronger by increasing its roughness. The research group speculates that these advantages are why eggs in dirtier or more exposed nests tend to have more pigment.

Portugal points out that two hummingbird species, the ruby-throated hummingbird and the hoary puffleg, had the highest eggshell roughness. He theorizes it’s because of the small size of the eggs and the wet climate these birds live in. When a drop of rain is almost as big as an egg, the shell would need to be rougher to prevent the developing embryo from drowning.

On the opposite end of the spectrum, bee-eaters and ostriches have the smoothest eggshells, which are also immaculate—perhaps because these birds don’t contend with the elements as much. Bee-eaters nest in enclosed burrows, so their eggs don’t need to be camouflaged. The male ostrich is so large that he can cover approximately 30 eggs from different females, keeping them protected from dangers and the Sun.

“Without successful eggs, birds will cease to exist,” Portugal says. Birds have spent millions of years developing the best possible eggshell structure to maximize the likelihood of surviving in their specialized habitats, he adds, and dramatic environmental changes can cause these adapted species to struggle. But understanding the factors to which eggs have adapted could help in preserving habitats where eggs can hatch successfully. “Unlike almost every other living thing, eggs can’t move,” Portugal says, “but they have to cope with so much.”