Color and Patterning of the Incredible Egg

Matt Monjello

From the brilliant blue eggs of the Veery to the inconspicuous clutch of the Killdeer, the great variation in color and markings we see on eggs has inspired generations of inquisitive minds.

The cause of this variation in color and markings are a result of two pigments, and their interaction with the calcium carbonate shell. Bilverdins are responsible for background colors we see on eggs. They are usually deposited first, will occur throughout the whole shell and range from blue and greenish blue in pigment. Protoporphyrins are directly related to the markings and patterns we see on eggshells. These colors can vary from yellow to brown. The greater the markings on an egg, the higher the concentration of protoporphyrins. When these two pigments combine they can create an assortment of colors that range from purple to green.

So why have birds evolved to produce such a great variety in egg appearance? There are a number of factors that seem to have played a role in answering this puzzling question. The evolution of nest-site selection, and the need for camouflage is what we will touch on in today’s blog.

Species that have evolved to nest in the open, and on the ground, generally lay eggs that are inconspicuous to visual predators. The risks associated with predation are lessened when eggs can blend in with their surrounding environment. This may be true even with blue eggs, like ones we see in the dense, dark cup nest of the American Robin. It has been hypothesized that the blue eggs act as a disruptive type of camouflage, in that they resemble sunlight spots on the dark foliage, but evidence regarding this effect and how it contributes to egg concealment is currently lacking.

White-colored eggs that have little to no markings are likely to be found in birds where some other form of concealment is at play. Two examples of this can be found in cavity nesting species (woodpeckers and kingfishers), and in birds that bury their eggs in vegetation when leaving the nest (mallard duck). The dark nest hole and the burial of the eggs both effectively hide the clutch from view. This helps the female to preserve important metabolic resources that otherwise would have gone to producing cryptic colored eggs.