Background: Melanin-based coloration is often found to be associated with variations in physiological and behavioral traits that are essential in avian communication. However, the function of melanin-based traits to their bearers has been investigated less than that of carotenoid-based ones. It is commonly believed that expression of melanin-based pigmentation is genetically controlled. However, whether it could be modulated by physical condition has remained controversial. In this study, we demonstrate that the level of melanin-based plumage pigmentation could be negatively correlated with an individual's oxidative stress level. Methods: In this study, we used two groups of Black Bulbuls (Hypsipetes leucocephalus nigerrimus) obtained from a pet-shop in 2010 and 2011 to examine the correlation between the melanin-based plumage coloration and the ratio of lymphocytes to heterocytes, which is an indicator of the oxidative stress experienced by an individual. Results: We found a negative correlation between oxidative stress levels and expression of melanin-based pigmentation on the breast and scapular feathers, but the correlation only appeared in the 2011 group, individuals of which had higher oxidative stress level and brighter plumage (i.e. more melanins) than those of the 2010 group. Conclusions: Our data suggest that melanin-based plumage could reflect an individual's physical condition in certain situations. Therefore, melanin-based plumage coloration could have a function in Himalayan Black Bulbuls' communication.
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