With many of us dying Easter eggs this week, I got curious about eggs that are naturally colored. We’ve raised chickens (Ameraucanas) that laid turquoise-to-olive eggs; our current flock of Black Sex-links lay in shades of tan. In fact, I usually have to buy white eggs at the store in order to achieve those pastel Easter hues.
But what about other kinds of birds? For instance, why do American Robins lay blue eggs, Burrowing Owls lay white eggs, while the American Golden Plover lays eggs that look like ovoid granite rocks, with big, black speckles on a white background? How and why do eggs come in so many colors?
As you might guess, egg color has something to do with survival of the species. Egg shells are comprised of calcium carbonate, which is naturally white. Birds who nest in cavities, such as many owls, or those who continually sit on their eggs, hiding them, tend to lay white eggs. There’s no need for camouflage.
Ground nesters, on the other hand, often leave their eggs exposed. These birds lay eggs that are typically brown, i.e., dirt colored, with speckles for further concealment. American Golden Plovers are ground nesters, which is why their eggs look like rocks. This photo shows a Killdeer’s nest, basically just a depression on the ground. That’s one of the eggs in the center of the picture. Clearly, the better hidden the nest and its contents are, the less chance of predation.
That makes sense so far, but it doesn’t explain everything. For example, why do birds like European Starlings and many thrushes (such as most bluebirds and American Robins), lay blue eggs? (The photo at left, courtesy of wikicommons, shows an American Robin’s nest.) This is especially confusing because starlings and bluebirds nest in cavities. What’s the advantage there?
An article from Australia’s Museum Victoria explains that colored eggshells are stronger than white ones. Stronger eggshells are more likely to survive until hatching.
Birds that are calcium-deficient lay thin-shelled eggs, which are more likely to break. Scientists have found that birds that have multiple clutches in a single season have more highly-coloured eggs in the second and subsequent clutches (when the mother’s calcium supplies are reduced). Patterned colouration is also more common in areas with calcium-deficient soils.
Adding pigment to eggshells may also screen the developing chicks from mutation-causing UV sunlight.
Another driving force behind the diversity of egg shapes and colors is nest parasitism. Some bird species, notably Common Cuckoos in Europe and Cowbirds here in the United States, are “obligate brood parasites.” That simply means they lay their eggs in the nests of other species, expecting to dupe the host species into raising their offspring for them. Sneaky!
Since the parasitic nestling thrives at the expense of the host species’ own offspring, it benefits the hosts to identify which egg isn’t theirs, so they can eject it from their nest. What results is an evolutionary race—the parasitic bird wants eggs that most closely resemble that of their host species, while the hosts want to identify and reject those foreign eggs. Hosts with more unusual eggs are more easily able to discern which eggs are theirs. Over time, the species’ eggs become more and more distinctive.
So, where do these colors come from? Eggs are colored with pigments added when the shell is formed inside the female bird. Unlike feathers, in which the blue color we see comes from a structural change, blue and green derives from biliverdin. (We make that pigment too—you notice it in a bruise that turns green as it fades.) Red and brown come from protoporphyrins, another pigment.
This week as you celebrate Easter by decorating eggs, remember that God beat you to it!