Scientists at Cardiff University have found that one species of river bird is swallowing hundreds of microplastics every day through their insect prey.
The dipper birds then inadvertently pass on those plastic fibres to their chicks when feeding them.
The research is the first to find evidence that microplastics can pass from insects to predators in rivers.
Microplastics are pieces of plastic debris under 5mm in size that can be found polluting rivers.
The research - carried out by Cardiff University, Greenpeace and the University of Exeter - has been published in scientific journal, Global Change Biology.
Dippers prey on river insects, often by standing partly submerged in the water and putting their beaks under the surface to catch them. This new study found that if an insect is contaminated by plastic, the birds ingest the plastic along with the bug.
The scientists said the transfer of so many plastic fragments to the birds and their chicks was startling.
In almost 40 years of researching rivers and dippers, I never imagined that one day our work would reveal these spectacular birds to be at risk from the ingestion of plastics - a measure of how this pollution problem has crept upon us.
The amount of plastic particles dippers in the study were found to be ingesting each day.
Professor Ormerod added, “The same features that make dippers so wonderfully adapted as the world’s only songbirds able to dive and feed on river insects, also mean that they can have no escape from this enormous source of pollution for decades to come."
The researchers involved in the study examined droppings and regurgitated pellets from adults and nestling dippers.
They found microplastic fragments in about half of 166 samples, at 14 of the 15 south Wales sites studied. The greatest concentrations were found in more urbanised locations. Most were fibres from textiles or building materials.
More than 75 per cent of the fragments we found were less than 0.5mm in size, but some were up to several millimetres in length.
Although the plastics seem to pass through the birds after they are ingested, the researchers stressed the need to understand more fully any adverse or toxic effects from such a large daily intake.
Previous research by Cardiff University scientists in 2018 revealed that half of the insects in rivers in south Wales contain microplastics.