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The Great Tinamou Lays the Fanciest Eggs in the Bird World

The eggs laid by the great tinamou are smooth and shiny, and now we have an idea why.
December 11, 2014, 10:00am
Great tinamou eggs. ​Image: University of Michigan Museum of Zoology

The great tinamou is a smallish turkey-like bird known for its colourful, smooth, glossy eggs. In short, the eggs are wicked cool. But exactly how great tinamous produce their positively alien-looking eggs has largely been a mystery, until now.

Researchers at the University of Akron in Ohio got their hands on a clutch of great tinamou eggs from the University of Michigan Museum of Zoology and subjected them to a litany of tests, including chemical analysis and electron microscopy, to find out how they get their glossy coating and vivid colour. The results were published today in Interface, a journal of the Royal Society B.

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The first thing the team discovered was that the tinamou eggs were iridescent; they change colour slightly, depending on what angle you're looking at them from.

"This is the first time iridescence has been reported," Matthew Shawkey, the study's senior author, told me. Rad.

The colour of the tinamou's eggs is due to the interplay between pigment coloration and what is known as nanostructure coloration. Eggs normally get their hue from a naturally-occurring chemical (pigment) that gives them a flat color. Underneath their iridescent coating, tinamou's eggs are a regular old shade of blue.

The great tinamou is kind of an uggo, unlike its eggs. Image: ​Katja Schulz/Flickr

The glossy cuticle provides structural coloration, or colour that comes from how an object's shape refracts light on a micro-scale instead of its chemical properties, Shawkey told me. It's kind of like how peacock feathers or a soap bubble get their psychedelic hues.

While the average eggshell is rough and uneven, the near-uniform smoothness of the great tinamou's egg interacts with light in a way that makes the underlying color seem to shift. Since eggs are, you know, made inside the bird, what is it about great tinamou biology that makes their eggs stand out?

"A chicken egg is rough and bumpy, but tinamou eggs look like someone's sanded it to perfection," Shawkey said. "Obviously, that's all happening in the oviduct of the bird. There may be some chemical differences between tinamou eggs and others."

The coating itself is made up of a thin cuticle made of calcium carbonate and calcium phosphate that the researchers hypothesize great tinamous produce in the final stage of egg production.

The reason why great tinamous lay such colourful and glossy eggs is a bit of a mystery, given that many of their predators are after their unhatched brood. However, Shawkey believes it may have something to do with how they mate.

"Multiple females lay their eggs in one nest, which is then incubated by a male," Shawkey explained. "So perhaps the eggs are bright to attract other females to lay eggs in the nest, because nests with more eggs can be incubated more efficiently. There is not much evidence for this, though."

According to Shawkey, the exact reasons why and how, biologically speaking, the great tinamou produces its weirdo eggs are still a mystery, and the best we can do right now is make an educated guess. Until then, Shawkey and his team are going to keep looking at trippy eggs under a microscope.