Gamekeepers have warned that Scotland's £30m a year shooting season is under threat because freezing temperatures have forced thousands of grouse to abandon moorlands and killed many more.
Grouse, which eat heather and forage for berries and insects, have been unable to find food on frozen moorland blanketed in snow.
The worst affected estates are those in the Lammermuir Hills in the Scottish borders, Inverness-shire and Perthshire where up to 3ft of snow has fallen.
There have been reports of flocks of up to 1,000 grouse heading south. Unlike pheasants and partridges, which can be reared in captivity to meet the demand of shooters, red grouse only reproduce in the wild.
"That means if your population decides to up sticks and leave, then that's an issue," said Dr Adam Smith, head of policy for the Game & Wildlife Conservation Trust in Scotland.
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"The chances are they will head back to the moor but this is less likely to happen for some reason in the west of Scotland. That's why keepers are getting a little nervous."
One of the worst affected estates is Kettleshiel, near Duns, which has won awards for its grouse management. It said virtually all of its 1,000 birds had disappeared.
"Nearly all of them have gone," said Paul Percival, the estate's head keeper. "The snow was up to the waist at one point. We are also beginning to find birds in snow holes which have frozen to death . . . The worst case scenario is that there will be no shooting season this year."
The setback is a blow to sporting estates, which last year reported a surge in bird numbers after several poor seasons due to outbreaks of disease carrying parasites and bad weather. Numbers had risen by an estimated 25% to about 900,000.
It costs around £75 to shoot a bird. Grouse shooting is a vital source of income for many rural communities and supports 940 full-time jobs, according to a report from the Game & Wildlife Conservation Trust.
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Alex Hogg, chairman of the Scottish Gamekeepers' Association, said: "It's been devastating for us to see flocks flying off the frozen moors. Keepers have been doing everything they can to uncover areas of heather to save the birds that are left but they're finding more dead and weak grouse every day.
"Clearly we're concerned about the economic impact because all grouse moors are dependent on a good breeding stock before the surplus can be shot. Time will tell what the implications will be."