Two tonnes of plastic have been found in the stomachs of fulmar birds around the North Sea.
Helen Moncrieff, Shetland manager at RSPB Scotland, said every single dead bird they analysed as part of a study contained a “lunchbox full of plastic”.
She was speaking ahead of a summit in Oban next week on tackling marine litter, which will be hosted by Environment Secretary Roseanna Cunningham.
Scotland is home to a third of the European Union’s breeding seabirds, with the islands having traditionally hosted huge colonies.
However, concerns about the impact of plastic pollution are growing, and climate change has been blamed for a catastrophic collapse in seabird numbers.
Speaking at a fringe event at the SNP conference in Aberdeen, Ms Momcrieff said: “The mallies (fulmars), there is a long-term study going on in the north Atlantic and the North Sea particularly, with a whole lot of countries involved.
“Every month we go out and we check, do a beach bird survey, and any fulmar or mallie we find intact we take it, stick it in the freezer and then the stomach contents are analysed later on.
“What we find is there’s plastic in every single mallie’s belly.
“It’s like having a lunchbox full of plastic in your stomach. We calculated that there’s around two tonnes of plastic in fulmar bellies alone.”
Warming oceans and overfishing of sand eels were also linked to a dramatic decline in many species.
Of Arctic terns, Ms Moncrieff said: “One of the sites I went to on the database, 9,700 terns were counted during the seabird census in 2000.
“I went out a couple of weeks ago and there were 110. So there has been big, big numbers.
“They do vary year or year, they breed in different places, so that figure might be a bit big, but it indicates that there is a big problem going on.”
Puffins are difficult to count, but she said there were more than 30,000 recorded in Shetland in 2000, but last year fewer than 600 were found.
Paul Walton, head of habitat and species at RSPB Scotland, said: “Scotland’s seabirds are in trouble. They are in big trouble. There is no doubt about it.”