Bloated rats and fat pigeons aside, the urban environment doesn't always super-size animals. In fact, cities might be having the opposite effect on one critter – the European dipper, a bird that's struggling with development problems linked to urban water pollution.
That's the theory of scientists at Cardiff University and elsewhere who've been poking around in the rivers of South Wales, which bear a rich history of industrial pollution. Compared to dippers that hail from rural environments, they say, the birds living downstream from cities are not doing so well. Specifically, they're having hormone issues: They hatch underweight and then grow up with chemical imbalances that cause them to produce fewer female chicks, a fact that casts a pall over the population's long-term survival.
The researchers believe the abnormalities stem from the fish and insects the dippers are eating. These waterborne creatures can act like sponges for polychlorinated biphenyls (PCBs) and flame-retardant chemicals (PBDEs) and, when consumed, pass the contaminants up the food chain. The Cardiff-led team has discovered what it deems a "strong correlation" between these chemicals and depressed thyroid levels in young dippers. As evidence, they note that one specific thyroid hormone was 43 percent lower in urban-river chicks compared to rural ones.
Certain endocrine-disrupting chemicals like PCBs can trigger a range of negative effects in birds, such as cognitive dysfunction, motor defects, and weakened immune systems. In fish, such substances are famous for wreaking havoc with reproductive systems, and were a suspect in the case of those male bass in the Potomac River that grew eggs.
What the physiological irregularities of British dippers spell for the wider ecosystem is unclear. Says one of the researchers, Steve Ormerod: "Wild birds, such as dippers, are very important indicators of environmental well-being and food-web contamination, and we need to know if populations, other species – or even people - are also at risk." Here's a bit more Cardiff:
Along the formerly heavily polluted rivers of South Wales, the same team showed previously how urban Dippers are exposed to a complex mix of chemical contaminants and dominantly, PCBs and PBDEs. Given the sensitivity of the endocrine system –responsible for controlling thyroid and other hormones – researchers have recognised that alterations in thyroid hormone levels are an important predictor in pollution-induced developmental effects in animals....
As top predators, Dippers are valuable monitors of river pollution that can help assess whether urban contaminants are influencing wildlife reproduction and development. Given the latest findings, scientists are now planning to examine whether effects on dipper sex ratios and thyroid hormones have consequences on individual survival and fitness which could alter wider population dynamics. They also plan to locate the exact sources of the pollution.
Top image: A white-throated dipper in County Kerry, Ireland. (snowmanradio / Wikipedia)