"Pollutants Increase Song Complexity and the Volume of the Brain Area in a Songbird" [1].
A team led by Katherine Buchanan of Cardiff University have uncovered startling evidence that pollutants consumed by starlings are leading to males singing more complex songs, improving their ability to attract females. But as their songs become more beautiful, they may have devastating consequences.

Linking pollution to bird-song may seem odd to those of us without a background in avian biology, but in fact it is quite logical. Bird song is regarded as a secondary sexual trait, like a peacock's tail. It is a demonstration of the male's health, evolving due to females' preferences - the more complex the song, the more the females are attracted to the singer. Being a gender-specific trait, the development of the centers in the brain responsible for bird song is heavily influenced by the levels of the hormones testosterone and estrogen. Since estrogen is a common environmental pollutant, Buchanan's team set out to see what effect this could have on the male birds' neural development, and in turn their songs. Additionally, they sought to measure the effect on their immune systems.

In order to conduct their experiment, the team needed to find a suitable location. The chosen spot was far from glamorous: "Natural and synthetic estrogens are known to both occur in concentrated amounts in sewage effluent. As part of sewage treatment processes worldwide, effluent is trickled over filterbeds rich in organic sediment, thereby supporting a complex community of micro and macro-invertebrates. These commonly occurring environments provide an essential foraging environment for a range of wild songbird species, including for one of the most invasive bird species on a global scale, the European starling (Sturnus vulgaris), particularly during the winter"

So the local invertebrates pick up the pollutants from the contaminated soil, and pass it up to food chain to the starlings. This gives a neat way to get a good estimate of the birds' exposure to pollutants: take samples of the invertebrates to see how much estrogen they contain, then observe the starlings to see how much they eat. It's good science, but crawling around in sewerage inspecting bugs probably isn't going to inspire the next generation of ecologists...

Having worked out the exposure of wild starlings to the pollutants, they set about creating their experiment in the lab. 36 starlings were randomly assigned to 3 groups, including a control group, and one exposed to the pollutants they found in the wild. (A third group checked the effect of one particular pollutant, 17-b estradiol, but found no significant results.) What happened?

"Treatment had a significant effect on the song output of the male starlings. Males in the group which received the mixture of chemicals spent more time singing, sang more song bouts, sang longer song bouts and had a larger repertoire size, a measure of song complexity, than males in the control group. The mechanism for this effect is clear as when examining the underlying neurobiology. There was a significant effect of treatment on HVC volume, the principal nucleus in the songbird brain associated with the production of complex songs, such that the HVC volume of the males in the mixture group was significantly larger than in males in the control group."

"In mate choice preference tests female starlings showed a significant preference for song playback from males dosed with the mixture of chemicals in comparison to control males."

The results were very clear, and confirmed the team's suspicions. Starlings exposed to the cocktail of pollutants were better singers, and more successful at attracting mates, but like many human artists they paid a price for their talents:

"Exposure significantly reduced both cell-mediated immune function and the humoral immune response of male starlings."

Both the adaptive and innate branches of the immune systems of the affected birds were suppressed. The net result of this is that the exposure of birds to excess estrogen from the environment leads to the least healthy birds having the most developed secondary sexual characteristics, and attracting the most mates.

The implications of this are highly disturbing - that pollution can interfere with the selective forces acting on populations. If females choose the least healthy mates, "then hatching and/or fledging rates could decline with detrimental consequences at the population level."

Further work is needed to quantify the importance of these effects in wild bird populations, but the image is a hard one to shake from the mind. Starling males singing at the peak of their species' ability, inadvertently contributing to its decline.

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"Mockingbirds don't do one thing but make music for us to enjoy... but sing their hearts out for us. That's why it's a sin to kill a mockingbird."
- Harper Lee, To Kill a Mockingbird

[1] Markman, S. (2008). Pollutants Increase Song Complexity and the Volume of the Brain Area in a Songbird.. PLoS One, 3(2), 1-6.