Birds which have returned to the former industrial valleys of south Wales are being exposed to extremely high levels of hidden pollutants, experts warn.
Tests on eggs of dipper birds found chemicals used as flame retardants in furniture and motor vehicles.
The birds have fed on local rivers in growing numbers since heavy industries such as coal mining declined.
Natural Resources Wales says water quality has dramatically improved in the last 30 years.
Prof Steve Ormerod from Cardiff University's school of biosciences said PBDE and PCB compounds they found may be potentially hazardous to the birds, but the impact is not yet fully known.
"It's a paradox that on one hand dippers are progressively colonising rivers that used to be grossly polluted and they are being exposed to new, novel pollutants whose impact we don't fully understand," he said.
Sign of cleanliness
The tests found that urban rivers in south Wales contain some pollutants at levels, on average, over four times greater than in adjacent rural rivers.
"What we're learning here is that urban areas have increased concentrations," he added.