Pollutants in UK rivers have been linked to hormone disruptions and underdevelopment in birds nesting nearby.

Eurasian dippers - river birds that feed exclusively on insects and fish in upland streams - was assessed in the study by an international team of researchers, who published their work in the journal Environmental Toxicology and Chemistry.

Eurasian dippers living near urban areas, where concentrated pollution is higher, were found to be underweight when compared to their rural counterparts.

The researchers also found depressed hormone levels in Eurasian dipper chicks living near urban rivers. Pollutants such as PCBs (polychlorinated biphenyls) and PBDE flame-retardant chemicals (polybrominated diphenyl ethers) acquired that the birds acquire through food are to blame, the researchers said.

"Our findings are important in showing that pollutants are still a source of concern for the wildlife along Britain's urban rivers despite very major recovery from the gross pollution problems of the past," Steve Ormerod, a professor of biosciences at Cardiff University, said in a statement. "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."

The research is an extension of prior work that suggested the same chemicals could cause developmental defects in river fish.

"We've known for some time that endocrine disrupting substances - the so-called 'gender-bending' chemicals from sewage and other waste water - can affect normal sex development in fish," said Christy Morrissey from the School of Environment and Sustainability at the University of Saskatchewan. "These are some of the first data to show that PCBs and PBDEs might be causing thyroid disruption in wild birds and interfering with normal animal development."

The researchers were able to pinpoint the effects of the pollutants to disruptions in the birds' thyroid systems. Tyroid disruptions in birds are often manifested as "impaired growth, cognitive dysfunction, compromised immune function changes in motor activity, and behavioural abnormalities that can persist into adulthood," according to Cardiff University.

"The return of Dippers to urban rivers is a fantastic outcome of pollution reduction in the UK. However, this study highlights the importance of birds as an indicator that some pollutants still persist in our rivers at harmful levels," said John Clark of the Royal Society for the Protection of Birds' Futurescape office. "We need to work in partnership with water companies, regulators, statutory agencies and communities at a catchment scale to address those practices that continue to introduce damaging chemicals to our rivers. The RSPB's Futurescapes conservation work program is doing exactly that - tackling environmental challenges at a truly large, landscape-scale, level."