MILD summers and mild winters have taken their toll on North Yorkshire's grouse population, meaning the Glorious 12th today will not be as glorious as in the past.

George Winn-Darley, a North York Moors landowner, said: "The mild winters that we've had for three years with not a proper winter or a proper summer have resulted in a build up of trichostrongylus worm, a parasitic gut worm which builds up in the grouse's belly.

"If it increases beyond 3,500 worms per bird it reduces the hen's ability to incubate and breed her chicks, so she loses them.

"It's also ideal conditions for sheep tics, which are blood-seeking parasites and they spread a whole host of diseases to the grouse and all the other ground-nesting birds.

"This includes a blood-borne virus called louping-ill which is 80 per cent fatal to birds and found on the North York Moors.

"We are working hard to try to control and eradicate it."

Meanwhile, sportsmen planning to shoot grouse in Yorkshire were warned they faced protests from hunt saboteurs threatening to spoil the traditional opening of the season.

Members of the Hunt Saboteurs' Association were planning to prevent shooting on moors in Yorkshire, Lancashire, Cumbria and Scotland. Saboteurs plan to either occupy the shooting butts and force shooters to pack up, or "beat" the birds away from the guns.

Along with the shortage of birds it is a combination which could have a serious impact on an industry estimated to bring in up to £30 million of tourist money every year, and where a day's shooting can cost up to £1,500. Many estates have already experienced cancellations as word of poor shooting spreads, while others expect to miss out on the pinnacle of the sport, driven grouse, where a line of beaters forces the birds into the air.

"There are just not enough birds to justify driven shooting on many moors," said Colin Sneddon, spokesman for the British Association for Shooting and Conservation.

"There are a number of places where the heather beetle seems to have caused a number of problems. It has led to a bad outlook in some areas. It eats the young shoots of heather, which is where the insects young birds eat live."

The effect is to reduce the heather cover and the food sources for the birds, making them more vulnerable and meaning lower numbers surviving from their hatching in the spring, said Mr Sneddon.