By Roger Pinney BBC Wales environment correspondent

There are just 400 breeding pairs of barn owls in Wales

Now think bigger. Much bigger and you are getting there.

Almost tea chest sized. That's it, a dez rez for barn owls.

With their distinctive screech call, in the wild barn owls are more often heard than seen.

But there is growing concern about the future of this beautiful and enigmatic bird.

Numbers have tumbled in the last 50 years.

Like so many countryside birds, barn owls have fallen victim to disappearing habitats and in places, shortages of food.

It's a very worrying thought that if the rate of decline were to continue we might lose the barn owl as a breeding bird Alan Davies, RSPB

Barns are not attractive to nest in once they have been converted.

And intensive farming has hit the small mammals barn owls need to feed on.

But there is a new threat - traffic.

To a barn owl a road makes an ideal hunting ground.

Wide, straight and with no cover for prey.

The problem is, owls do not seem to see cars, buses and lorries coming.

"The barn owl is being forced to hunt closer and closer to motorways and bypasses,� said owl expert Bill Broughton from the North Wales Bird Trust, based at Bodafon Farm Park in Llandudno.

Scale of problem

"The barn owl weighs probably 10-12oz and flies about 4ft above the ground when it is hunting.

"You can imagine this along a bypass.

"A great big lorry comes thundering by and just pulls the bird under."

A 10-year study by the Barn Owl Trust has uncovered the scale of the problem.

It found that traffic was responsible for more than half of all barn owl deaths.

It also found that as many as 72% of barn owls which encounter a major road are killed.

Boxes have been installed for breeding owls

And that adult barn owls which nest within 500 yards of a road are almost certain to be killed on it during the nesting season.

In Wales only around 400 pairs of barn owls now remain.

But they can be given a helping hand. And that brings us back to the owl boxes.

On Anglesey where boxes are being used, barn owl numbers are up.

Alan Davies has installed one on the RSPB Conwy reserve where he is a warden.

There is nothing in it yet but he lives in hope.

"Barn owls will regularly use boxes," he said.

"It's a very worrying thought that if the rate of decline were to continue we might lose the barn owl as a breeding bird.

"But there are positive signs, particularly where nest boxes are being used."

No more barn owls. A sobering thought.