A consequence of the color change is the males are becoming less attractive to females. The metals causing this negative reaction are mercury, chrome and copper. This is affecting great tits (Parus major) in particular, especially in areas close to factories.
With the different metals, mercury appears to affect carotenoids. These are responsible for the pigmentation of the brightest colors. This leads to a loss of the characteristic yellow hues on the great tit’s breast, or leads to a reduction in the intensity of the yellow color.
To a female great tit the yellow color signifies the state of health of a male bird, especially its ability to find food, which is important once the female is nesting.
With another metal, copper, this increases the “black tie” markings on the bird, which are symbolic of the male bird’s dominance. This is because the metal leads to an increased production of melanin. Chrome leads to a loss in size of the black. The impact of this depends on where the birds spend their time. In the countryside, a large black tie is seen as being attractive to females, whereas in urban areas the opposite occurs. Whether this will lead to any major adaptive behaviors in the future (such as more great tits living in robin areas, assuming air pollution levels remain the same) remains to be seen.
The research was conducted by the Museum of Natural Sciences in Barcelona (Spain), who operate the Spanish Foundation for Science and Technology. The study drawing causation between levels of air pollutants and the color changes to the birds is published in the journal Science of the Total Environment. The paper is titled “Metal exposure influences the melanin and carotenoid-based colorations in great tits.”
