Sparrow-proof bluebird nesting boxes could help protect the species from invaders, an expert said yesterday.

Stuart Smith of the Bermuda Bluebird Society, said bluebird boxes were often taken over by sparrows.

But the addition of a perspex top and “sparrow spookers” can help scare off intruders and allow bluebirds to hatch their brood in peace.

Mr Smith said: “It’s politically correct to have a bluebird box, but if you don’t monitor it, it becomes a sparrow breeding box.

“The bluebirds used to have three broods in a season from March to July, but because of the sparrows they have cut back to mainly June and July.”

“The male sparrow comes, claims the box as his own, and then he goes to find the female. If we don’t get rid of the male sparrow, he’s just going to keep the nest.”

Mr Smith said a section of clear plastic in the roof of the box and small pipes and reflective strips on the top deterred sparrows.

He explained: “Sparrows don’t like the direct light. Generally when they build their nest, they put in an overhang, something that goes over them.

“This clear Plexiglas roof is the first line of defence because it deters the sparrows and it has worked so far in my trials.”

But Mr Smith warned that the hole must be covered once the bluebirds lay their eggs to prevent the nest from becoming too hot.

The pipes and reflective strips then scare off any invaders.

He added: “Sparrows can be very aggressive, and sometimes they attack the bluebirds. Sometimes they kill the chicks and lay their nest on top.

“This dissuades them for the next three weeks, giving the bluebirds time to fledge.”

Mr Smith said: “Once they have fledged, you have to take this down because you don’t want the sparrows to get used to it.”

Mr Smith said that the Bermuda Bluebird Society had tried a range of different designs over the years, but the latest seemed to be the most effective.

He added: “I tried it on a few golf courses last year and it worked very well. I had successful broods of birds in two very sparrow-oriented boxes.”

Andrew Dobson of the Bermuda Audubon Society said bluebird boxes had become vital to the survival of the species in Bermuda.

He said: “Bluebirds now rely totally on nest boxes and these should be monitored carefully during the breeding season.

“Their historical natural nest holes were lost following the cedar blight and the rapid colonisation by starlings, which occupy the remaining natural tree nest holes.”

Mr Dobson said kiskadees are also aggressive towards bluebirds but cannot enter a bluebird box because of their larger size.

• For more information about the new nest design, visit bermudabluebirdsociety.com