Significance of Egg Coloration to Embryo Development
Researchers continue to debate the purpose of bird egg pigmentation, with the most popular theory being that camouflage is the main reason for the variation in eggshell colors, with the speckles and splotches of color providing protection from predators. This was the theory put forward by renowned biologist Alfred Russel Wallace in the late 19th century, a position that was challenged by naturalist Alexander M’Aldowie who believed the pigmentation of eggshells served to shield developing embryos from harmful radiation.
Wallace’s theory was the more widely accepted argument, and later research studies on this topic would be based on the fight for survival aspect that supports the necessity for Alexander camouflage. Other accepted theories to arise in subsequent years included the role that pigmentation plays in retaining heat and continuing the incubation process when the eggs are unattended, as well as signaling unpalatability to would-be egg-eaters and serving as identification for host birds that have parasitic eggs laid in their nests.
Although all these theories have merit, in a recently published review of a series of studies in the Journal of Avian Biology of 14 September, biologists Phillip Cassey and Golo Maurer of the University of Adelaide in Australia take the embryo’s view of its protective covering in offering possible explanations for the purpose of eggshell coloration. One of these explanations validates the original theory put forward by Alexander M’Aldowie where the pigmentation of the eggshell helps to filter, but not block, ultraviolet light for the developing embryo. It is interesting to note that even the darkest eggs, using the emu as an example, allow some light to filter through. It was also noted that pigmentation differs at each end of the egg, most likely to provide directional cues to the embryo, as well as to assist cells and structures in their early alignment.
As in the case of gulls, which commonly lay a clutch of three eggs, with the second egg laid being noticeably and consistently darker than the other two, the variations in eggshell pigmentation could be a significant factor in facilitating staggered hatching. Moreover, it has been proposed that the variations in eggshell coloring could assist the embryo in learning to recognize the difference between light and dark, calibrating circadian rhythms, as well as encouraging DNA repair and shaping bacterial communities within the egg. Researchers have also noted that pigmentation is affected by rainfall and weather, possibly compensating for local and seasonal conditions. Certainly, there is much still to learn about the marvelous world of birds, and with advancing science, more of these mysteries will no doubt be solved.