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.

Practice Makes Perfect for Nest-Building Weavers

It has long been assumed that the nest building skills of birds are instinctive, but new research has revealed that building a nest could very well be a learned skill. Following and filming the activities of male Southern Masked Weavers in Botswana over a period of three months, researchers noted that not all nests are created equal. As the name of the bird suggests, Southern Masked Weavers use a weaving technique when constructing their nests from local grasses. However, the method of nest building varied between birds with some weaving from left to right and others weaving from right to left. It was also noted that they appeared to learn from their mistakes, and while a bird may regularly drop blades of grass when it first starts its nest building process, it soon learns to adjust its technique to prevent this.

The brightly colored African bird was chosen as the test subject for the study for a number of reasons. Their complex nests which hang from trees either as single units or multiple intertwined condominiums are seen as evidence of above average intelligence. Also, a single bird will build several nests in a season, allowing the research team to note the differences in nests built by the same bird.

Working with scientists from Botswana, researchers from the universities of St Andrews, Edinburgh and Glasgow noted that the fact that the Southern Masked Weaver birds displayed marked variations in their approach to nest building reveals that they may learn from experience. At this point, however, it is not clear whether they have the mental capacity to learn, or their improvement in skills can be attributed to repetition of a task. Researchers also pointed out that observing this behavior in one bird species does not imply that it would apply to all birds. One of the scientists taking part in the study, Dr. Patrick Walsh of Edinburgh University’s School of Biological Sciences, noted that if birds built their nests instinctively according to a genetic template, it would follow that all birds would build all their nests in the same way every time, but this has not been the case. Summing it up nicely, Dr. Walsh was reported as saying: “Even for birds, practice makes perfect.”

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