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No matter how much evidence we have that birds evolved from dinosaurs, it can still be hard to believe that the harmless little sparrow hopping about your park bench is somehow related to the mighty and ferocious Tyrannosaurus rex. But go far enough back, and there is a connection.

Now, a new finding further links modern avians and their ancient ancestors. In a paper published last week in Nature, a group of researchers from Yale University, the American Museum of Natural History, and the University of Bonn in Germany found that the coloration of bird eggshells evolved from dinosaurs, not, as previously believed, as an independent trait.

From the striking azure of an American Robin’s egg to the Jackson Pollock-esque brown on a Great Bowerbird’s, the diversity of colors and patterns in modern bird eggs derive from just two pigments: red and blue, or red-brown protoporphyrin IX and blue-green biliverdin. Birds have long been considered unique for their colored shells, but as it turns out, those same exact pigments can be found in the eggs of certain dinosaurs.

Analyzing 18 fossilized dinosaur eggshells from around the world, the researchers used Raman microspectroscopy, a nondestructive laser method, to test for the presence of the two pigments. They found them in the eggshells of Eumaniraptoran dinosaurs, a group including theropods such as velociraptors that are believed to have eventually evolved into modern birds.

The analysis also found that eggs belonging to dinosaurs that buried their eggs had no pigment at all. This, the researchers say, indicates that egg coloration might have co-evolved with the open-nesting habits of theropods.

“Nesting behavior and reproductive behaviors don't fossilize, and so egg color is the best proxy we have for certain nesting behaviors and ecologies,” says Jasmina Wiemann, a molecular paleobiologist at Yale University and the head researcher of the study. 

Luis Chiappe, an expert on early avian evolution at the Natural History Museum of Los Angeles County who was not involved in the study, says its conclusions are sound and that the new findings lay good groundwork for more research on this subject, such as analyzing the eggshells of certain sauropods that laid their eggs above ground to further test the correlation between surface nests and egg-color pigmentation.

“Those discoveries are, on the one hand, cementing the notion that birds are a branch of the much bigger dinosaur tree,” Chiappe says. “On the other hand, they’re clearly identifying some of these quintessential avian features as not being exactly that, as being essentially traits, features and functions that were inherited from their ancestors and that evolved.”

Mary Caswell Stoddard, an evolutionary biologist at Princeton University who’s studied the evolution of eggs and was also not involved with the new study, agrees the most recent findings are solid—as well as a "wonderful reminder" of the connection between birds and dinosaurs. 

According to Stoddard, the study also raises important questions and should spur further research on what the functions of diverse egg colors and patterns might have been during prehistoric times, including camouflage, temperature regulation, or, as in some parasitic birds, to mimic the appearance of host eggs.

In other words, it's possible that the origins of brood parasitism could be found to go back tens of millions of years, before the first birds even took flight. Remember that the next time you see a Brown-headed Cowbird. 

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