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Scientists continue trying to unravel the mystery of why birds lay different color eggs — from the bright blue of Robins' eggs to the pure white of Eastern Bluebirds'.

The latest: A team of researchers conducted the largest global study yet on this topic, and while it examined less than 10% of bird species, the research published Monday suggests a link between pigmentation and thermoregulation that keeps the eggs at a stable temperature.

"Eggshell coloration has puzzled naturalists since Darwin and Wallace. Why are eggshell colors so diverse?  The importance of this study is its geographical scale, and the broad diversity of egg colors (i.e. bird species) included.  Nothing as broad as this has been done before."
— David Westmoreland, biology professor at U.S. Air Force Academy, to Axios

Background: In the process of making eggs, some birds deposit pigment to eggshells during the last two to four hours of the 34-to-48 hour process of making the eggs, Mark Hauber, animal biology professor at the University of Illinois' School of Integrative Biology, tells Axios.

  • There are two main pigments, protoporphyrin and biliverdin, that scientists know play a role in the range of brown to blue-green colors, says Hauber, who was not part of the study.
  • However, research continues delving into whether there's an evolutionary reason behind the colors, and whether the colors play a role in embryo survival.

What they did: In the study published in Nature Ecology & Evolution, the researchers looked at the global patterns of eggshell colors by measuring the brightness and color of eggs from 634 difference species.

  • They mapped the patterns onto the geographical breeding range.
  • The authors also exposed some eggs of varying colors and brightness to solar radiation to see how the eggs' temperatures adjusted. Most types of eggs need to be kept in the 30°C to 40°C range throughout incubation, study co-author Phillip Wisocki tells Axios.

What they found: The researchers discovered "clear patterns" that the average egg coloration is significantly darker when temperature and solar radiation levels are low, says Wisocki, of Long Island University (LIU).

  • Pigmentation also tend to be darker when nests are formed in an open area, like on the ground, and lighter in dark areas, like caves, according to Wisocki.
  • The researchers also found there is a lot more variation of color in hot climates, which was a surprise because "typically thermoregulatory pressures on eggs are thought to make them lighter in hot climates," Wisocki adds.
  • Daniel Hanley, another study co-author who is also of LIU, points out that the experiment " illustrated how darker eggs tended to heat more quickly than lighter eggs" and also cooled more slowly.

What they're saying: Experts who were not part of this study tell Axios that this offers an important first look at the larger global average trends.

  • Princeton University biologist Mary Caswell Stoddard says: "Egg color can be for camouflage, to signal quality to a mate, to mimic the eggs of unrelated birds, for bacterial defense or structural support — or for thermoregulation. Of these explanations, thermoregulation has received relatively little research attention, especially at a broad taxonomic scale. That’s part of what makes this study ... so timely and exciting."
  • "This is a super exciting study," Hauber agrees. He adds that continued research is needed to look at some of the exceptions found in the hot and cold regions, as well as the many species not yet examined.
  • "It's 634 species, but also 36/40 orders of birds. A far broader sample than other studies," Westmoreland says. "The question of why birds have such diverse egg coloration is still open, as is the question of why any particular species has the color it does."

What's next: "What we still need to determine are the rates of heating and cooling of variably colored (viable) eggs as parents are incubating and leaving," Hanley says. "Replicating this portion of the study in a breeding colony of a species that produces variably colored egg would give us a clearer understanding of the biological effect of this differential heating."

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