by Michael Sun, Science Team Columnist
Easter is the time of year when eye-catching plastic eggs filled with chocolate or candies invade the shelves of stores. You might think that humans are the only ones who produce such colorful eggs. But as it turns out, colorful bird eggs are quite common in nature.
The colors of a bird’s eggs are determined by only two pigments. Biliverdin produces a bluish-green tone you might find on a robin’s egg, while protoporphyrin creates a ruby red tone best exemplified by the eggs of the Cetti’s warbler. In addition, many eggs have unique patterns of spots and squiggles.
Pigmentation begins in the bird’s oviduct, a long tube connected to the ovary where the egg is formed, like an assembly line. At precise, genetically controlled times, certain cells lining the oviduct fire the pigments onto the developing egg, a bit like a paintball gun. This creates a wide variety of hues and patterns.
There are many different reasons to explain the wide range of egg colors. Camouflage is one; spotting and other patterns can help eggs blend in against their surroundings. These are found in birds that nest on the ground — cavity-dwelling birds don’t need camouflage; thus, their eggs are plain and white.
Another reason for differing egg colors is to defend against brood parasites, which rely on others to raise their young. Brood parasitism can be seen in cuckoos, who lay their eggs in other bird’s nests. By laying eggs with a distinct pattern or coloring, a mother bird is best able to recognize and reject any imposters in her nest.
Colors may also serve as an indicator of a bird’s fitness, as female birds that are sick or have a poor diet produce pale colored eggs. This signals to some male birds how much energy they should expend on ensuring the offspring’s survival: if the eggs are healthy, he may invest more time caring for them.
The pigmentation of bird eggs could also act as a natural sunscreen, as it can block harmful ultraviolet rays coming from the sun. Too much pigmentation may cause the egg to overheat, however, so birds have evolved to carefully balance the two extremes, depending on the local environment.
Recently, scientists have found evidence that even some dinosaurs — the ancestors of birds — laid colorful eggs. Biliverdin and protoporphyrin have been found on fossilized dinosaur eggs dating back 70 million years. More detailed analysis (done by using lasers to analyze chemical contents) shows that some dinosaurs laid eggs in a variety of colors and patterns. Scientists can thus speculate these dinosaurs built open nests, and those whose eggs did not have any pigment buried their eggs.