The velvet scoter and the long-tailed duck, both wintering sea ducks, are dwindling in such rapid numbers experts are worried for the survival of the species.
Their warning is contained in the State of UK's Birds 2012 report, which shows some species nearing extinction here. The experts who compiled the survey have called for increased international co-operation to reverse the trend, with pollution and climate change seen as likely causes.
The report is published by a coalition of conservation organisations and charts the ups and downs of the nation's bird populations over recent decades.
Neither of the two threatened species breeds in Scotland, but both are winter visitors particularly to the east coast and are vulnerable to oil pollution at sea on their journey from northern Europe. The velvet scoter has also been suffering from depleted fish stocks.
Massive declines have been recorded in the Baltic Sea and have been mirrored in Scotland, where the bulk of the UK population is found.
Numbers have fallen so sharply, 65% and 60% respectively since the first Baltic Sea survey in 1992, that both species are now considered threatened with extinction globally.
Other species to have suffered particularly significant declines are seabirds, of which Scotland holds 45% of Europe's breeding population.
Since 1986, when a national seabird-monitoring programme began, 10 of the 18 monitored seabird species have suffered long-term declines, with populations of Arctic skua and roseate tern declining by around three-quarters, 72% and 75% respectively, during that period. As such, both species are red-listed as of high conservation concern, with the roseate tern edging close to extinction in Scotland.
Similarly, kittiwake numbers have more than halved (55%) over the same period, while other once-abundant gull species, such as the herring gull and great black-backed gull, have declined by 24% and 35% respectively.
Stuart Housden, director of RSPB Scotland, said: "There have been many changes across the UK that have affected birds, including shifts in land use, habitat loss, climate change, the rise in some non-native species and a lack of food."
He said it was vital the declining species are supported.
Richard Hearn, head of species monitoring at the Wildfowl and Wetlands Trust, said: "Sea duck numbers in Europe have crashed and they urgently need conservation. Velvet scoter overwintering in the UK have gone from several thousand birds to less than a hundred in just a few years, and the picture for long-tailed duck is similar."