A simple length of fishing line could help give Bermuda’s bluebirds a better chance of holding off house sparrows that aggressively take over their nesting sites.

Sparrows have threatened the native bluebird since they were introduced to the island in 1880. On their arrival, they pushed bluebirds out of favoured nesting sites such as building eaves and rock faces, beginning a steady erosion of the species’ breeding success.

Between the mid-1940s and early 1950s, the cedar blight devastated Bermuda’s cedar forests, eliminating another vital source of nesting cavities. Bluebirds persisted largely through conservation efforts, including the introduction of man-made nesting boxes in 1968.

When Stuart Smith began his conservation work with the Bermuda Bluebird Society about 25 years ago, it was believed there were about 500 nesting pairs of bluebirds on the island.

Today, the population is difficult to measure but has declined significantly because of sparrows, other competing birds, rats, lizards, feral cats and pesticide use. The species now depends almost entirely on man-made nesting boxes.

Keeping the sparrows out of them is a battle Mr Smith has fought with limited success. His latest effort involves placing 6-8lb fishing line about 1/8 inch on either side of the entrance hole, a method that has shown promising results in field tests. He is now urging homeowners to do the same.

Most important is that the line is installed now, before the male sparrow claims the box for the season.

“I'm beating my head against the wall because I'm not winning the war against these sparrows. We need to help,” Mr Smith said.

“I can do it on my own trails but I'm trying to reach the private box owners who might be able to help and install this deterrent. It's not foolproof but it's a deterrent. And if we can get that going before this month ends, then maybe we can get two or three broods of bluebirds in the season.”

Today many boxes are maintained by homeowners on private property. The Bermuda Bluebird Society maintains about 175 nesting boxes across the island.

Bluebirds historically produced three broods each breeding season, typically between March and July. According to Mr Smith, most now manage only a single brood later in the season, usually in June or July.

Recent weather extremes have further reduced breeding success. Periods of drought and heavy rainfall have made it harder for parent birds to find worms, which provide both food and moisture for chicks, which contributes to the problem.

In addition to taking over nesting boxes, sparrows also pose a direct threat to adult bluebirds and their chicks.

“What happens is the sparrow will come into the box with the mother sitting on the nest and jump on top of the bluebird and then peck. And the poor bluebird, because the beak is pointing downwards, cannot protect themselves. If they're in the air, if they're face to face, they can fight them off, but because the sparrow comes in over the top it's very difficult,” Mr Smith said.

“I've seen sparrows go in and pick out individual bluebird eggs and throw them out of the nest. I've also found chicks with their heads pecked. So these sparrows can be quite aggressive. Violently aggressive.”

Once a sparrow claims a nesting box early in the breeding season, it can effectively block bluebirds from using the site.

“The male sparrow will dominate that box for the whole season, once he's claimed it,” he said. “So he'll go out in February and find a female sparrow and they'll start building a nest. In a lot of cases, [a volunteer] monitor comes along, he finds a sparrow, pulls out the nest and throws it away. But then the male sparrow is dominating this box. So he goes out and gets another female. He keeps on trying and it's not good because the sparrow will not let the bluebird nest.”

The society began experimenting with monofilament fishing line as a deterrent last year, adapting an idea developed in the United States.

“The sparrows don't like it. They think it's either cobwebs or spider webs, or something and they're not very efficient flyers. The bluebird seems to miss that. They go in and they don't seem to have an issue with it.”

The boxes should be monitored at least twice a week during nesting season. It’s an effort that Mr Smith understands can discourage some homeowners.

“It's understandable that people who had the good intentions of having a bluebird box in their yard, after a while, they're like, this is too much work. And then they leave the box to sit and it ends up being a sparrow producer,” he said.

He acknowledged that some residents may also find it difficult to remove sparrow nests or eggs once they are established.

To encourage participation, the Bermuda Bluebird Society is offering to help homeowners install the deterrent.

“We'll come and install a monofilament line on your box. You're more welcome to do it yourself but we're quite happy to come over for a small donation of, say, $10 that will go back to the Bluebird Society,” he said.

Mr Smith is also working with students at Warwick Academy to explore new technology that could further protect nesting sites.

“I'm working with schools right now trying to produce a face recognition camera run by a solar panel that once it recognises a sparrow, it has a little speaker that sends off [a sound].

“They're going to have to be a few sounds, because sparrows are smart. They’ll get used to one single sound. And hopefully that's a start in making a sparrow-proof bluebird box.”

• For more information, visit bermudabluebirdsociety.com or call Stuart Smith on 777-9856