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It's a scene that's all too familiar in the south of Wales: a quaint, tree-lined river, flooded by the blight of human innovation. In the past 30 years, the country underwent a major initiative to clean up the industrial waste that clogged its waterways. The hulking factories and coalfields have moved away from the riverbanks, and dippers and wagtails have returned to nest in the beech trees. Decades after the pollution was tamed, the rivers are wild and full of life again.

But the water in Wales is still dirty. PCBs (polychlorinated biphenyls) and PBDEs (polybrominated diphenyl ethers) are difficult to mop up; they persist in the environment for many years and are the scourge of rivers systems around the world. Once the two hydrocarbons enter the food web, their effects are magnified. They invade organisms as small as diatoms and as massive as killer whales. Studies have linked PCBs in the Hudson River to changes in song quality among chickadees and sparrows. Historically, they've been attributed to the thinning of osprey and falcon eggs.

Now another study has emerged, connecting birds to the contagion of hydrocarbons. Scientists from the University of Saskatchewan and Cardiff University teamed up to examine the presence of PCBs and PBDEs in urban rivers in south and central Wales. For them, the "canary in the coal mine" is the white-throated dipper. The species is at the top of the riparian food chain; it eats mayflies, caddisflies, and small fish – all of which are rich in toxin-transporting lipids.

The study, which was published in , is breaking new ground because it doesn't only focus on the reproductive success of dippers; it considers the quality of their offspring as well. To do this, the researchers looked at thyroid hormones rather than sex hormones. "A lot of people were looking at the gender-bending dimension," says Steve Ormerod, professor of ecology at Cardiff University and chairman of the Royal Society for the Protection of Birds council. "We see this is as more of a hormonal and developmental issue."

Thyroid hormones are essential to growth and metabolism in most animals. Birds with low thyroid levels tend to be lighter and less fit for survival in the long run. The researchers from Cardiff and Saskatchewan noticed that chicks in urban habitats were small in size and stature, when compared to nestlings from rural areas. Additionally, the sex ratio was highly skewed toward males at the urban sites. Such an imbalance can cause breeding problems for white-throated dipper populations in the future.

Previous studies on white-throated dippers have shown that they hold a higher concentration of PBDEs in their eggs than any other passerine species. Thus, Ormerod says that the developmental burden must start in the egg. But dippers have a long incubation period, making it impossible for the researchers to discern whether the contamination takes root in the egg or the new-born chick.

Another interesting pattern that the scientists noticed was that nest success was higher in urban zones. In a comparison of 87 nests at 36 rivers, there was a higher likelihood that birds in mined areas had at least one fledgling. The reason for this positive uptick is a mystery, because the availability of food was similar in both settings.

Ormerod says that the study provides correlative evidence that characterizes pollution in urban areas throughout the world. According to him, dippers are a powerful indicator species, even in the Himalayas. "We've used dippers to indicate a whole range of different problems. They are closely tied to river systems, as pretty much all of their food comes from there. And they are very territorial, which makes it easy to pinpoint the source of contaminants."

The study does stop short at reviewing the effects of PCBs and PBDEs on bird populations as a whole. But with Christy Morrissey, the lead author of the study, reflecting the same research on North American dippers, a broader context is in the making.

Ormerod has been studying dippers for the past 30 years, with the intent to incite large-scale action for better land use and improved regulation of chemicals. His most recent work proves that we can't wash our hands clean of the past. We can however, plan for an alternative future: one where the rivers aren't rife with poison.