The "Robin Egg Blue" Mystery

For children, "Robin Egg Blue" became an official Crayon color 20 years ago. But for biologists, blue-colored bird eggs were a biological mystery for more than a century. A couple of recent studies have offered evidence for an interesting theory that just might explain such an unexpected color.

One American Robin nest, from eggs to juveniles

You can think of this as the "motivated father and stressed mother" theory.

Birds evolved from reptilian ancestors about 180 million years ago, and early bird species probably all laid white eggs, just like today's reptiles still do. Over time, predation pressures left more survivors among those bird species whose eggs were harder to find. This was accomplished by birds adapting to  conceal and camouflage their eggs.

Most cavity nesters, like woodpeckers, still lay white eggs. These eggs are concealed from view, deep inside a tree hollow or some similar nest cavity. Some duck species and other ground-nesting birds still lay white eggs, as well. But the parent covers them with vegetation or downy feathers when they leave the nest to feed. A few bird species even bury their eggs, just like modern-day alligators.

On the other hand, many songbirds have open-to-the-sky nests, and most of these species lay camouflaged eggs. Many of us (as a species that likes to eat some bird eggs) have fallen for the common Killdeer's broken-wing ruse, which leads us just far enough from her nest that we almost never spot the four brown-speckled eggs laying uncovered and in plain view in a brown-speckled, gravel nest.

The majority of camouflaged eggshells feature brown or black splotches on an Earth-toned green or brown background. They blend in well with their surroundings, especially to the color-blind mamalian predators. These are the dull colors that you would expect for well-hidden bird eggs.

And then there's a small handful of species - mostly in the Thrush family - that lay blue eggs. The American Robin (Turdus migratorius) is the most familiar of these. Bright blue Robin eggs, unconcealed and uncamouflaged, laying in plain view in an open nest. How can bright blue eggs lead to large and widespread populations of perhaps our most familiar bird? If blue Robin eggs are easier for predators to see, why doesn't this lead to the species' extinction?

What's going on here? Part of the answer may lie in subtle chemical cues between the Robin parents.

In two recent studies, male Robins were shown to favor offspring that hatched from bright blue eggs, while discriminating against chicks that hatched from eggs that were a duller blue. The Robin dads provided almost twice as much food to chicks that hatched from brighter eggs.

A second study found similar results, though for a shorter period. By switching out eggs of different brightnesses with unrelated nestlings, researchers found that male Robins indeed provided more food for chicks that appeared (to the male Robins anyway) to hatch from brighter eggs, but only for the first several days. By day six, all surviving offspring were receiving the same amount of fatherly care.

Why would a male Robin feed some of his own offspring less often, just because they hatched from a duller egg? Shouldn't he feed all of his chicks equally? And why don't all females just lay bright blue eggs?

The blue eggshell color is produced by biliverdin, which is an antioxidant that the body uses to prevent cellular damage.  So it turns out that the level of biliverdin in the female Robin is a good indicator of her overall health. In other words, the healthiest females have the highest levels of biliverdin and produce the brightest blue eggs.

American Robin father

A male Robin appears to use egg color as an indicator of how healthy each chick is, based on the mother's health, and how likely it is to survive and carry on dad's genes. A chick from a duller egg may be more likely to perish early on, and not push the father's genes into the future. So a chick from a brighter egg is, genetically-speaking, worth more of the father's energy. But if a chick from a duller blue egg survives the most difficult early days, then it will also earn an equal amount of dad's attention.

A female Robin, on the other hand, appears to risk her own health in order to convince the male that her offspring (i.e. her genetics) are worthy of extra care. She somehow diverts biliverdin from her body to her eggs, which stresses her own health. Any female that is physically stressed prior to nesting will have less biliverdin available to divert to her eggs, and she will lay duller eggs. This would also have implications for female Robins that attempt multiple clutches in a single breeding season.

This "motivated father and stressed mother" behavior would have to produce healthier, stronger offspring to offset the highly-visible eggs that are lost to predation. And this is apparently what happens when Robin parents devote most of their energies to their healthiest offspring, their bluest eggs. This theory would also help to explain why a few cavity nesters produce colorful eggs.

Human mothers everywhere might appreciate this subtle way of getting dads to pay attention and help raise their children. And the difficulties faced by generations of mother Robins have also given our own children one of their favorite Crayons - Robin Egg Blue.