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.
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."
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.
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.