Seabrook Island is fortunate to have many nesting opportunities for Eastern Bluebirds. The golf courses have graciously provided space for nest boxes and they support the maintenance and replacement of the boxes. Individual Seabrookers also provide and monitor boxes. Recently, Glen Cox photographed some fresh eggs in one of his boxes. The eggs being white instead of
blue made this observation unique. Two questions come to mind. How often does this happen?And, why?
Glen knows that the norm for Eastern Bluebirds eggs is pale blue to a blue-green. Occasionally, a bluebird lays white eggs like Glen discovered. Rarely, in South Carolina and Georgia pink eggs pop up in reports. How occasionally or rarely, do these colors appear?
In a study in Lee County, Alabama females laid blue or blue-green eggs 98% of the time. Less than 2% of the eggs were white and less than 1% pink. If a female laid white or pink eggs, the whole clutch was white or pink. Glen noticed a relatively rare occurrence. Great observation!
Scientists know what creates the blue/green coloring of bird eggs, a pigment called Biliverdin-IX and Zinc Biliverdin Chelate. Note, the Italian word for green is “verdi”. Birds generate the pigment in their bile, presumably the derivation of the Biliverdin came from the pigment. In chickens, specific genes drive blue egg formation. A study of 108 species of birds commonly found biliverdin and zinc chelate in eggshells. Hence, blue eggs appear often in varied species-
American Robins, Gray Catbirds, Blue Jays, Blue-gray Gnatcatchers, Painted Buntings, Green Herons, and Great Blue Herons to name just a few.
The coloring of bird eggs happens in the uterus. Depending upon species, as the egg forms or just before female extrudes the egg, the pigment gets incorporated into the egg. In addition to blue other birds paint their egg various shades of brown (most common) or other colors. Why the rare white or pink ones? Who knows. Presumably, this female has not generated the Biliverdin. Is this genetic or a product of external conditions like stress or food sources?
Here is a challenge for our readers. When you monitor bluebird eggs, note the color and, more specifically, see if that color occurs annually in your local bluebirds. Assuming the Eastern Bluebirds using a nesting territory survive and return to the same territory year after year, we may learn if egg color stays consistent with the same female or changes year to year. This will indicate if genetics or environmental conditions create the coloration anomaly.
Answer submitted by Bob Mercer
Photo by Glen Cox
References
Gowaty, P. A. and J. H. Plissner (2020). Eastern Bluebird (Sialia sialis), version 1.0. In Birds of the World (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.easblu.01
https://amerpoultryassn.com/2021/10/blue-eggshell-what-pigments-create-this/ A study of pigmentation in chicken eggs.
https://bioone.org/journals/the-wilson-journal-of-ornithology/volume-131/issue-3/18-181/Migratory-return-rates-and-breeding-fidelity-in-Eastern-Bluebirds-Sialia/10.1676/18-181.short A study that found Eastern Bluebirds often return to the same location where they nested last year.
SIB 