Signs of spring are starting to appear on Dorset heathland but the question worrying wildlife experts is how many scarce Dartford warblers have survived.

Harsh winters have in the past decimated the numbers of the perky small birds whose song from the top of flowering gorse is a harbinger of warmer weather.

The birds feed on insects and in the 1960s arctic blasts reduced numbers to a few breeding pairs. However in recent years they have been a conservation success story.

Dorset heathlands are carefully managed with these and other rare plants and reptiles in mind, but it is not yet known how many of the ruddy-breasted warblers have survived the coldest December for 100 years.

Those that did sit out minus 10 temperatures are starting to build their nests.

The warblers stayed on the heath, woodlarks flocked to the countryside and nightjars flew to sub-Saharan Africa for the winter.

Experts are not too optimistic for survival rates but want to give every bird the best chance of breeding this year.

Chris Dieck of the RSPB, who has been monitoring the numbers of heathland birds over the past few years, said: “By keeping your dog on a lead from February to August whilst on the heath, you will be helping Dartford warblers and other ground nesting birds like woodlarks to build future generations. Any disturbance by dogs or humans will potentially prevent them from breeding.”

The nightjar nests on a patch of bare ground between the heather, a place where a lizard might be seen basking later in the year. The Dartford warblers nest is low down in gorse or heather and woodlarks nest on the ground next to broad-leaved woodland.

Nicky Hoar from Dorset Wildlife Trust said the first Dartford warblers had been seen at their Upton Heath reserve in the past week.

“We think we might have done a little better because it’s more sheltered,” she said.

The Urban Heaths Partnership’s Dorset Dogs website has information where dogs can be exercised off-lead without disturbing rare birds.

If anyone sees wildlife deliberately disturbed, it can be reported to Crimestoppers on 0800 555 111.