Background
The key question that Wisocki et al.’s study addresses is the patterns of evolutionary color changes under the influence of certain environmental factors. It is essential because it allows us to evaluate the influence of climate change on birds. One of the essential directions of ecology and the theory of evolution is the study of the diversity and mechanisms of the formation of adaptations of organisms to abiotic and biotic environmental factors.
Biochemical, physiological, behavioral, and morphological adaptations are involved. The latter includes egg coloration in birds, which often performs a protective function. This is most clearly observed in species that arrange open nests on the ground or in the water (Samaš et al., 2021).
In case of danger, the birds leave them in advance, relying on the invisibility of eggs to protect the clutches. Various ways of preserving clutches (nesting in burrows and hollows, dense branches or grass, harboring incubating females with protective plumage coloration, and active repelling of predator attacks) and nest parasitism cause a wide variety of egg coloration in birds (Hamchand et al., 2020). However, this study is unique because it connects the color of eggs with a new factor – thermoregulation (Wisocki et al., 2020).
Methods
The purpose of the proposed technique is to accurately assess the level of eggshell pigmentation and its quantitative expression. The obtained data were statistically processed and used in the study of intracranial, intrapopulation, and interspecific variability of egg coloration and in the comparative analysis of eggshell pigmentation of larger taxa. The technique is based on a well-known spectrophotometric analysis in chemical research, which has been adapted to determine the degree of pigmentation’s darkening (Wisocki et al., 2020).
To obtain high-accuracy results, the shell of a whole egg was analyzed. The use of small areas of the shell cannot give an accurate idea of the quantitative content of pigments since they are often distributed unevenly over the surface of the shell. The selected area can be characterized either by an almost complete absence of pigments or by overestimation compared to its real content.
In any case, to carry out the pigmentation analysis, the egg measurements (length (L) and diameter (D)) were measured, and the eggs were removed from the egg using a caliper. Using these initial values, the egg’s volume and the egg’s surface area were calculated using known formulas. The parameters obtained are necessary for determining the volume of the extracting mixture for this sample and the subsequent mathematical processing of the material under study. In spectroscopic studies of egg pigments, a mixture of a solvent and concentrated hydrochloric acid in a ratio of 1:2 was used as an extraction reagent (Wisocki et al., 2020). This made it possible to compare the results of spectroscopic measurements of pigment extracts obtained by various solvents and determine the degree of darkening of the shell depending on the climate temperature.
Results
Scientists have studied more than 600 species of birds living in different parts of the planet (Wisocki et al., 2020). The analysis showed that the eggshell’s color depends on the bird habitat’s temperature, the sun’s brightness, and humidity. Naturalist Charles Bogert also studied the influence of thermoregulation on the color of birds in the middle of the 20th century, as reported in the publication (Wisocki et al., 2020).
Bogert suggested that the colder the bird’s habitat, the darker its color (Wisocki et al., 2020). The dark color of the plumage absorbs the sun’s rays and warms the animal. White reflects sunlight and does not allow the body to overheat. The authors of a modern study suggested that Bogert’s hypothesis can be applied to the color of the shells of bird eggs (Wisocki et al., 2020).
Scientists from the USA and Australia collected 634 types of eggs from various museum collections (Wisocki et al., 2020). Biologists compared the color of the shell with the habitat of birds and found a relationship. Birds living in cold climates have dark and brown eggs.
The color of the shells of birds living in different countries varies from brown to blue-green and white. According to scientists, the color of the shell affects the egg’s ability to overcook and not overheat: the development and survival of the future chick depend on it. Scientists have suggested that this is due to ultraviolet radiation and the need for thermoregulation.
Discussion
The results answer the question mentioned in the background section by stating that black egg colors are evolutionarily more advantageous for thermoregulation for birds of a cold habitat. The paper’s results indicate that living conditions in bird habitats affect the development of their eggs. Under the influence of a cold climate, the shell becomes darker. This is because the darker the color spectrum, the worse it reflects and the better it absorbs light radiation. This is why dark objects are hot and light ones are much cooler.
The light color of the shell, such as white or yellow, reflects most of the light and absorbs only a small fraction (L’Herpiniere et al., 2019). Dark shell colors, such as black, absorb most of the light and reflect a smaller fraction. If some material absorbs more light, it heats up faster.
Link to Evolution
Evolution has endowed birds with many protective tools to adapt to an aggressive environment. One of the main functions of the eggshell is to maintain a relatively high body temperature of the bird despite fluctuations in ambient temperature. Its color corresponds to the color of the environment and can become a good disguise and an auxiliary device in the struggle for existence (Abolins-Abolsa et al., 2019). However, the fact that the color of eggs changes not only depending on the surroundings but also on temperature demonstrates that climate change leads to a new stage in the evolution of birds.
Reference List
Abolins-Abolsa, M. et al. (2019) ‘Anti-parasitic egg rejection by great reed warblers (Acrocephalus arundinaceus) tracks differences along an eggshell color gradient‘, Behavioural Processes, 166(103902), pp. 1-6. Web.
Hamchand, R. et al. (2020) ‘Expanding the eggshell colour gamut: Uroerythrin and bilirubin from tinamou (Tinamidae) eggshells‘, Scientific Reports, 10(11264), pp. 1735-1740. Web.
L’Herpiniere, K. L. et al. (2019) ‘Unscrambling variation in avian eggshell colour and patterning in a continentwide study, Royal Society Open Science, 6(181269), pp. 1-16. Web.
Samaš, P. et al. (2021) ‘A meta-analysis of avian egg traits cueing egg-rejection defenses against brood parasitism‘, Frontiers in Ecology and Evolution, 9(25), pp. 335-343. Web.
Wisocki, P. A. et al. (2020) ‘The global distribution of avian eggshell colours suggests a thermoregulatory benefit of darker pigmentation’, Nature Ecology & Evolution, 4(1), pp. 148-155. Web.