Bird Lays Shimmering Egg That Changes Color

The glossy eggshell of a rare bird is so luminous that it actually changes color when viewed from different angles.

The first known naturally iridescent eggs have just been identified and are laid by a bird known as the great tinamou, aka "mountain hen," according to a paper published in the latest issue of the Journal of the Royal Society Interface.

Great tinamou eggs are so luminous that viewers actually perceive them as changing color when they are seen from different angles.

"Tinamous have some of the most colorful and glossy eggs of all birds, and here we show that an extremely smooth eggshell cuticle produces their mirror-like sheen," Branislav Igic, lead author of the paper, and colleagues wrote. "Furthermore, we reveal the presence of iridescence on the blue eggs of the great tinamou, an optical effect that has not been previously reported for avian eggs."

Eggs Say 'Hey Mom We Match': Photos

Great tinamous, native to Central and South America, are about the size and shape of a small turkey. Their body coloration is a bit drab (with colors ranging from olive green to white), but they help the bird to stay well camouflaged in their rainforest habitat.

Charles Darwin himself treasured a tinamou egg from a different species. The egg was found after Darwin's death in his collections. At some point during his lifetime, Darwin accidentally cracked the egg, but the still-shiny specimen went on display earlier this year.

For the new study, Igic, a researcher from the University of Akron's Department of Biology and Integrated Bioscience Program, and the other researchers used chemical analysis and a barrage of high-powered magnifying devices to study tinamou eggs.

They determined that an extremely smooth cuticle produces the glossy appearance of tinamou eggshells. The cuticle is composed of calcium carbonate, calcium phosphate and, potentially, organic compounds such as proteins and pigments.

How does it change color? Not all color is pigment-based. A crystal, for example, may look rainbow colored from certain angles even though it is clear, simply because of the way it reflects light. Some insect bodies and bird feathers also exhibit radiant colors due to how their structures impact light.

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In this case, it appears that the eggs have the best of all color types. The researchers carefully removed the eggshell cuticles to see what would happen. The gloss and iridescence were lost, but the background coloration remained. This suggests that color comes from both iridescence and pigments.

But the real mystery is: Why would a prey species like these birds lay such eye-catching eggs? The scientists don't propose this idea, but I have to wonder that the optical illusion produced the eggshells might somehow make them invisible to certain predators.

The researchers instead suggest that females might want other females and males to find the eggs easily. Females sometimes lay their eggs within clutches of others, necessitating the ability to quickly find the eggs of different mothers.

"Bright eggs may also 'blackmail' males into comparatively high incubation attendance to conceal conspicuous eggs, thereby shortening their incubation time and reducing the risk of predation," the scientists wrote.

They also mention that smooth eggshell surfaces may prevent water from clogging pores in the shell that could impede gas exchange. They added, "A highly reflective eggshell surface may also help prevent damage to the embryo from solar radiation."

The developing tinamou chicks might therefore thrive in a perfectly comfortable environment with something akin to an auto heat shield (only better looking) wrapped completely around their colorful enclosures.

Photo: Tinamou eggs. Credit: Marcos Massarioli, Wikimedia Commons

Eggs come in a multitude of colors and patterns, from subtle pastels to vivid bright hues. Now a new study, published in Current Biology, supports a centuries-old theory that shell variation, at least for some birds, helps to camouflage eggs.

The masters of egg disguise turn out to be Japanese quail. This photo amazingly features eggs just from this one species. Mother quails somehow learn the patterning of their own eggs and choose laying spots to hide them best.

“We currently do not know the mechanisms by which the (mother) bird learns its own egg patterns,” lead author P. George Lovell of Abertay University and the University of St. Andrews, told Discovery News.

Nevertheless, the ground-nesting birds often perfectly match the eggs to substrate, helping to prevent the precious contents from being some hungry predator’s dinner.

Close-ups of the individual Japanese quail eggs really show how well the bird moms can match their eggs to the environment.

One of the first scientists to theorize that coloration and speckles evolved for camouflage against predators was Alfred Russel Wallace, who gained fame as Charles Darwin’s co-discoverer of the principle of natural selection.

Yet another Japanese quail egg matches its environment, but the egg is not invisible to us. “Some have asked why they can still see the eggs if they are camouflaged,” Lovell said. “It’s important to remember that the eggs are less visible, not invisible…The photographs are taken quite close up to the egg, and a predator wouldn’t necessarily be that close. It would be scanning an area, rather than staring straight at the egg.”

Japanese quail themselves are brown and speckled, like their eggs. The bodies of many birds appear to match their eggs. A parent bird could then likely better shield the egg when resting upon it.

Speckles appear to be key to camouflage, at least for eggs.

Avian expert Innes Cuthill of the University of Bristol’s School of Biological Sciences, told Discovery News, “Alfred Russel Wallace concluded that the brown and speckled colors of many eggs had evolved as camouflage against nest predators and that the ancestral color of white only persisted in species whose eggs were laid in well-protected cavities, were covered by nest material or, in birds such as ostriches, could be defended by their parents.”

Brown with speckles is just one egg color/pattern combo, as these reed warbler eggs reveal. Their greenish hue, set off by the green leaves surrounding the nest, would make them less visible to predators scanning the area from a distance.

This nest also includes a, slightly larger, common cuckoo egg. Demonstrating another bit of bird trickery, common cuckoo mothers frequently match the appearance of their eggs to those of reed warblers, which wind up caring for the hatchlings.

Sometimes avian parents enjoy the best of both worlds: camouflage for their eggs as well as fooling other birds. Cuckoo finches can be deadbeat parents, matching their eggs (seen in the inner circle) to those of the tawny-flanked prinia (outer circle).  The prinia parents are sometimes fooled into caring for the cuckoo finch eggs and later hatchlings.

A mother cowbird must have worked hard to find this perfect spot for her eggs. “Interestingly, all birds seem more concerned in minimizing the mismatch between nest and the darker speckles on their eggs than the mismatch between nest and the underlying, predominant egg color, but particularly so for birds with more dark speckling,” Cuthill said. 

Lovell and his team speculate that the dark and light markings serve as a disruptive camouflage, breaking up the outline of the otherwise revealing oval shape. That shape, for humans and countless other predators, serves as a visual signal for good eats.

This cockatiel, with its proud expression, has a right to boast. Its egg not only matches the mother bird’s coloration, but it also matches the environment. In the wild, a hunter would have a hard time finding such an egg.

By itself, the egg of a swan is just as visible as a bright white egg on a breakfast plate. Under the mother swan, however, the egg disappears.

This Adelie penguin laid a speckled egg that matches its environment. The matching phenomenon could lead to a twist to the old riddle: Which came first, the chicken or the egg? In this case, the teaser could be: Which came first, the egg color and pattern, or the choice of nesting site?

An even more compelling mystery concerns a common sight in springtime—bright blue eggs in a robin’s nest. To human eyes, they are beautifully unmistakable. They stand out from both the nest and the parent birds, so why are they bright blue?

No one yet knows for sure. Biologists do know that pigment glands in the mother bird’s body deposit the blue coloration onto the eggs, so it must have a critical function. Perhaps the dark color camouflages the eggs when the eggs are at the bottom of a dark nest, or the blue might blend in with the sky for some viewers. Some researchers have even speculated that the striking color helps mother birds to find their own eggs.

For now, however, the case of the mysteriously blue robin’s egg has yet to be cracked.