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By Victoria Gill Science and nature reporter, BBC News

The barn owl has become something of a conservation icon

Three-quarters of British barn owls now live in man-made nest boxes, according to conservationists.

Nest box installation schemes have helped boost the barn owl population. But in Britain the birds are now largely reliant on such measures.

According to the Cumbria-based World Owl Trust, people are now barn owls' custodians.

Without continued conservation effort, the organisation says, the once common birds could become a rare sight.

Natural icon

Three-quarters of Britain's barn owls nest in man-made boxes

Barn owls have become a something of a symbol of British wildlife and conservation.

Experts say the bird has been brought back from the brink of local extinction.

After the bleak picture of its decline became clear in the 1980s, regional and national projects, including the Barn Owl Conservation Network (BOCN) were set up to help the birds.

Between the 1930s and 1980s, according to two national surveys, their numbers fell from 12,000 breeding pairs to approximately 4,000 - a 70% decline.

These projects included the installation of nestboxes, which acted as replacements for traditional nesting sites that disappeared, as barns were renovated, and haystacks were replaced by silage.

Now, the BOCN and British Trust for Ornithology say that decline has been halted.

In some areas, they say, it has even been reversed.

"We almost lost them, but now it's looking hopeful," says the BOCN's Colin Shawyer, who carried out the second national barn owl survey in 1982.

Since then, an effort to restore suitable hunting habitat - namely the long, rough grass along riverbanks - has sent the population of small mammals on which the owls feed "through the roof".

"Last winter was quite a harsh one," Mr Shawyer told BBC News. "We had a lot of adult mortality, but the population is so much more robust than it was.

"They'll bounce back if we don't get a series of harsh winters."

Conservation controversy

But not everyone who champions the bird's cause agrees that its future is so promising.

The World Owl Trust says that barn owls are probably still in decline.

"The truth is that nobody knows what the population of barn owls in Britain is at the moment," says Tony Warburton, president of the trust.

Breeding pairs need to produce between three and four owlets in order to maintain the population

According to Mr Warburton, the most recent attempts to estimate its population have been based on monitoring nests - asking volunteers to count how many of them are occupied by breeding pairs.

"But you can't monitor barn owls where there are no barn owls," he says.

Extrapolating from these results, he suggests, may give us an inflated estimate.

Disappearing habitat?

BAD YEAR FOR BARN OWLS Although 2010 should have been a "peak vole year" according to predictions, this was not reflected in barn owls' breeding success. The harsh winter may have depressed the voles' breeding success - they are highly dependent on young grass shoots for food. Those barn owls that did breed did so relatively late in the year and produced small broods of one or two owlets. (Experts say each pair needs to produce an average of 3.5 owlets in order to maintain the population.) This does mean that British bird-watchers may still be able to see the barn owls feeding their chicks, some of which will not fledge until December. Colin Shawyer from the Barn Owl Conservation Network says: "Some of the best places to see barn owls are Cambridgeshire, west Norfolk, Lincolnshire and east Yorkshire mainly because populations here are relatively good, daytime feeding is not uncommon and these open fenland landscapes offer long range opportunities for the observer. More barn owl facts, sounds and videos from BBC Wildlife finder

Mr Warburton is a self-confessed "old-fashioned naturalist".

In his home county of Cumbria, he has installed nest boxes and even bought a few acres of meadow to create habitat for the birds.

But in many areas where barn owls used to be a common sight, he says their foraging habitat is disappearing.

"Ungrazed, unmown grassland has gone," he says. "It's not going, it's gone."

The deceptively beautiful green Cumbrian hillsides are heavily grazed. They are just green deserts, he says. "Field voles can't live there."

And the owls are extremely reliant on the abundance of the voles, which are their favourite food, almost to the exclusion of everything else. As vole numbers rise and fall, so do the barn owls.

The voles are weather-sensitive little mammals that need long, uncut grass for their food and shelter.

"So you can put a million nest boxes up, but if you don't put them in the right place where there's rough grass you're wasting your time," says Mr Warburton.

He and Colin Shawyer have compared notes and exchanged views on many occasions during their decades working to protect these much-loved birds of prey.

They have opposing views on the stability of the barn owl population, but they both agree that people should not feel complacent about the species - it still needs our help.

Wherever livestock graze, it is very difficult to preserve this ideal rough hunting ground.

And a lack of this type of land often means that barn owls feed along roadside verges, which is a risky pursuit.

According to the British Trust for Ornithology, 45% of their ringed barn owls are reported as road casualties each year.

And the World Owl Trust, which runs a small wildlife hospital, estimates that half of the barn owls born in Britain are hit by cars during their first year.

Flying future

For now, at least, there does not seem to be a shortage of bird-lovers willing to come to the aid of this ornithological icon.

Their hope is that saving the barn owl will not go out of fashion.

Mr Shawyer says: "In some areas, such as Salisbury Plain, nest box schemes have increased the barn owl population by up to five times.

"But the reliance on nest boxes is now so great, the birds will need constant conservation."

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