Scotland's seabird numbers plunged by 19% between 2000 and 2008, a new report has said.
Scottish Natural Heritage (SNH) said the major cause was almost certainly a shortage of food due to a drop in the number of small fish, such as sandeels.
SNH said the fish were probably being affected by rising sea temperatures.
Declines have been greater in areas such as the Northern Isles and down the east coast. RSPB Scotland said the figures were "deeply worrying".
SNH said lower fish numbers led to lower numbers of adult birds surviving from one year to the next, and not enough chicks being produced and surviving to replace them.
SNH director of policy and advice, Prof Colin Galbraith, said: "While it's always disappointing to witness declines in important species, we are not entirely surprised at these findings.
"After several decades of increasing seabird abundance, we are now witnessing a period of decline. Key reasons are likely to be linked to food availability, weather, and predation.
"It is important that we are now able to monitor seabird numbers much more effectively than in the past, to inform policy and action. We need to keep a close eye on seabird trends and try to understand what is driving them."
SNH said there were now 55% fewer black-legged kittiwake and 71% fewer Arctic skuas breeding in Scotland than in the mid 1980s. Arctic terns declined by 26% over the same period.
Deryk Shaw, warden of the Fair Isle Bird Observatory in Shetland, said: "Breeding kittiwake numbers have been falling for many years now and there was no reprieve in 2008.
"A whole island count for the Fair Isle found that the number of nests is only half of that counted as recently as 2005 with many birds just standing on bare ledges."
Douglas Gilbert, of RSPB Scotland, said: "If the declines continue at this alarming rate, then many of Scotland's famous seabird cities could be virtually deserted within a decade.
"In the past decade the hopes of a good breeding season have been crushed, as eggs are deserted or young chicks starve in their nests because the adult birds cannot find enough fish."