(The South River, Shenandoah Valley, Virginia, USA)Although mercury can be found naturally throughout the environment, local concentrations may be increased by pollution from industries and mines. Once mercury atoms join together with a carbon and three hydrogen atoms, they form a compound known as methylmercury, the most "bioavailable" type of mercury--in other words, the type most capable of doing damage to a variety of organisms. Concentrations of mercury increase at each subsequent stage in the food chain. So, for instance, a wren will contain more mercury than the spiders it eats, while a snake will contain more mercury than the wrens it eats. In habitats exposed to mercury contamination, this can be a big problem, since even relatively small concentrations of mercury can cause damage to the nervous system.
A team from the College of William and Mary recently demonstrated how this mercury-driven nervous system damage can lead to behavioral abnormalities. They studied birds along the South River in Virginia, an area that had been contaminated by industrial mercury during 1929-1950. Previous work in the area had shown elevated levels of mercury in both the blood and the feathers of resident birds. The levels were similar to those known to cause behavioral abnormalities in common loons (Gavia immer), inspiring the William and Mary researchers to take a closer look at their birds. Specifically, they investigated song performance in four species:
(Song sparrow, Melospiza melodia)
(House wren, Troglodytes aedon)
(Carolina wren, Thryothorus ludovicianus)
(Eastern phoebe, Sayornis phoebe)The first three species, all learn their songs as are juveniles. In the phoebe, on the other hand, song is innately programmed, and juveniles grow up knowing how to sing a "species-normal" song regardless of what they hear when they are young--even if they hear nothing at all.
For all three of the song-learning species, song parameters were significantly different in contaminated sites than in reference sites. Specifically, contaminated wrens sang shorter songs with fewer notes. They also sang at a lower peak frequency, meaning that they placed most of their volume emphasis at a lower pitch. Contaminated sparrows sang songs with fewer note types. Their "buzz note," a special syllable that is used by males to advertise their attractiveness, was lower-pitched and covered a smaller bandwidth--in other words, it was less complex, and therefore probably a bit easier to perform, than buzz notes sung by males at clean sites. Notably, mercury levels had no effect on any parameters measured for phoebe songs, indicating that song performance is not impacted by mercury contamination in this species.
Although it is possible that mercury contamination could somehow make song performance more difficult--for instance, by impacting muscle control--the marked difference between song-learners and the phoebe suggests that mercury has an effect during the song-learning stage. Previous laboratory work in monkeys has shown that mercury can lead to deafness in the upper frequency range. A similar trend in birds would certainly help explain why the sparrow and both wrens produced songs that emphasized lower frequencies--perhaps these are the only notes they could hear, and therefore memorize. The authors stress that future work will be needed to develop a more detailed understanding of how mercury could alter singing behavior by impacting growth and developmental pathways, or perhaps by disrupting birds' physiology.
Males use song to attract females and warn off rival males--two behaviors that have significant impacts on breeding success. Thus, regardless of the mechanism by which it affects individual birds, mercury contamination poses a clear risk to avian populations, and probably many other species that share their habitat--even as long as 60 years after the pollution has ceased.
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Hallinger, K.K., Zabransky, D.J., Kazmer, K.A., and Cristol, D.A. 2010. Birdsong differs between mercury-polluted and reference sites. The Auk 127(1):156-161.
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