Bird eggs vary widely in shape, size and color. Often, the species can be identified using characteristics of an egg.
Egg coloration remains a topic of great interest in the ornithological community. The eggs of many birds are speckled with dark colors against a white background. It’s not heard to understand the function of such coloration. The speckling of the eggs breaks up the outline of the egg. Such camouflaged eggs seem to disappear within a nest or in a scrape.
We believe that the ancestral condition for egg color in bird was immaculate white. Even now, species that nest in cavities where eggs are not visible are not camouflaged.
However, camouflage may not be the sole explanation for speckling in eggs. We know that the protoporphyrin pigment that makes up the speckling strengthens the eggshell. The strengthening effect may be particularly important where birds have trouble acquiring calcium in their diets.
But how then can we explain the blue-green eggs of an American Robin? Robins lay their eggs in open nests. Such eggs would hardly seem to be camouflaged.
As it turns out, many other thrushes, including bluebirds, produce blue-green eggs. Blue-green eggs are also laid by birds in many other families. So, the question of why some eggs are blue-green has some widespread application within the avian world.
There is no shortage of speculation on the reasons for blue-green eggs. These hypotheses include warning coloration (the eggs must be distasteful), advantages in absorbing heat and cryptic coloration. Unfortunately, support of these explanations is weak or lacking.
A recent hypothesis with some experimental support is the sexual-signaling hypothesis. The blue-green color in the eggshell comes form a substance called biliverdin, which has antioxidant properties in the mother. A healthier, more vigorous mother can produce more biliverdin for her eggs. Her male partner can judge the condition of his mate, and of his soon-to-be offspring, by the depth of the blue-green color. The male will then respond to this signal by bringing more food to the young once they hatch.
Recent work by Daniel Hanley and colleagues with Gray Catbirds showed that females with higher antioxidant capacity produced deeper blue-green eggs. They also showed that the amount of food provided by male catbirds was directly related to the richness of the egg color.
[First published on July 10, 2011]