Linking plumage dimorphism and environmental stress

Besides the importance of pigmentation in species recognition, pigments protect the body against different biotic and abiotic features. Melanin, the dark coloration pigments in skin, hairs, and feathers, is necessary for protecting the body from UV radiation. On the other hand, the lighter a surface is, the more solar radiation is reflected. Based on these arguments, it is possible that pigment distribution would not be the same among all body regions of a unicolored individual due to the amount of solar exposure. Therefore, three species of the Egretta genus (E. dimorpha, E. gularis, and E. garzetta) in which all individuals are unicolored, and two of which are dichromatic (dark grey/black and white), were selected as model studies. We hypothesized that within dark and white unicolored Egrets, due to foraging and standing postures, body regions that are more exposed to the sun (like back and breast) would be darker or whiter compared to other body regions (vent and tail), respectively. Visual wavelength reflections of five body regions (breast, back, belly, vent, and tail) from 186 museum specimens were measured to indicate the concentration of pigments in each body region. Reflectance among body regions was compared using Kruschke's 'BEST' (Bayesian Estimation Supersedes the t test) framework to create Bayesian analogues for the paired t-test. The results show, regardless of the species, that the more exposed body regions to the sun (back followed by breast) are darker among dark individuals and whiter among white individuals in comparison to other body regions. Therefore, our hypothesis that the concentration of pigments is not similar in all body regions of unicolored individuals was supported.