Nightjars nest on the ground, which can make them vulnerable

Nightjars, which are nocturnal, usually fly in each summer from Africa to nest on the bare ground in Sherwood Pines near Clipstone.

But experts say many female birds have been frightened off by quad-bikers.

Environment forester Paul Barwick said conservation work is being jeopardised by the riders.

Alarm bells

He said: "We've lost one or two nests through direct damage. Eggs and chicks have been destroyed.

"But more particularly the birds have in fact left the area entirely, they haven't nested here at all in some areas.

"So we're down to about two thirds of the population that we had expected this year."

Margaret Overend, of the Royal Society for the Protection of Birds (RSPB) said: "They (nightjars) on what we call the Red List - that means they're of the highest conservation concern.