The heron population in Britain is at risk from a mystery bone disease which leaves them unable to feed.

Birdwatchers say up to 75% of young herons in some colonies have been affected by the condition, which causes their legs and wings to break as they grow.

When they heal, the chicks often suffer crippling deformities, preventing them from being able to hunt for their own food and leading eventually to their starvation.

BBC One's Countryfile has discovered that a lack of calcium may be to blame, however, it is unclear why the birds are lacking the mineral.

Fatal condition

When the British Trust for Ornithology studied herons at a Nottingham nature reserve it found between half and three quarters of chicks were dying due to brittle bones.

Dr Nigel Clarke from the BTO says once a bird has developed the condition, it has little chance of surviving.

"Unlike humans, there is no chance of their bones being set and once they have broken 20 to 30 bones in their bodies there is not much chance of them getting into a position where they can feed and ultimately they starve," he told the BBC.

Some ornithologists fear the birds are becoming unable to absorb calcium due to pollution in their feeding grounds. Others think loss of habitat means they cannot find enough calcium-rich fish to eat.

Birdwatchers are calling for funding to properly research the problem.