By Richard Black
BBC News environment correspondent
Migrating birds are unlikely to be seriously affected by offshore wind farms, according to a study.
Birds simply fly around the farms or between the turbines, says the study
Scientists found that birds simply fly around the farm, or between the turbines; less than 1% are in danger of colliding with the giant structures.
Writing in the Royal Society's journal Biology Letters, the researchers say previous estimates of collision risk have been "over-inflated".
However, conservationists warn that turbines pose other risks to birdlife.
The research project involved one of Denmark's two large offshore wind farms, Nysted in the Baltic Sea, which contains 72 turbines each measuring 69m to the top of the nacelle or hub. It started operating in 2003.
"This is the first such study involving a large-scale offshore wind farm," researcher Mark Desholm, from the Environmental Research Institute in Ronde told the BBC News website.
"There has been other data from farms with fewer than 10 turbines, but we thought this issue was so important because the potential for offshore wind power is so huge."
Globally, offshore projects currently generate around 600 MW, less than 2% of the overall total for wind.
But the potential is huge, because there is less competition for space at sea, turbines are less visible, and the wind there is often more reliable.
Threading a path
From a conservation point of view, the two options present different issues. Land-based turbines may affect birds when they are nesting; whereas at sea, blocking migration routes could be a bigger problem.
Mark Desholm and his colleague Johnny Kahlert began their study in 1999, before building at Nysted began; so they have been able to compile a long-term picture of how turbines have affected the flight-paths of migrating ducks and geese.
"We need a stable platform for the radar, which is usually used for detecting ships," said Mr Desholm.
"We have an 8m-high tower which rests on the sea bed - the little cabin at the top is just 2.5m by 2.5m - and during the migration season we spend three days out there every week, so you have to like each other."
The radar plots the paths of ducks and geese as they migrate to the Arctic each Spring to breed, and again in the Autumn when they return with their young to feeding grounds largely around northern Germany and Holland.
The results clearly show that most of the birds just fly around the Nysted farm; most of those that go through appear to thread a path between the turbines.
Desholm calculates that less than 1% of the birds fly close enough to the turbines to be at risk of collision.
"And these are heavy birds; they're not easily manoeuvrable, so we were quite a bit afraid that they would not be able to avoid a collision," he said.
For the industry, the finding could mean a relaxation of regulations on offshore wind farms, perhaps enabling them to be built routinely along migration routes.
"Offshore wind power is only two or three years old, so there's lots to be learned," according to Gordon Edge, head of offshore wind at the British Wind Energy Association.
"There's been a precautionary approach so far. As experience grows, the current stringent conditions may relax, and we'll find that some of the anticipated problems aren't so serious," he told the BBC News website.
Wind power is sometimes a tricky issue for conservation groups, who have to pit the consequences for wildlife of constructing huge structures in pristine areas against the likely extra impacts of climate change if those turbines are not built and fossil fuels are used instead.
David Gibbons, head of conservation science at the Royal Society for the Protection of Birds (RSPB), told the BBC News website that this study suggested the risks to birds were smaller than had been feared.
"It's a nice, clear picture of research; there's always been concern about turbines as 'mincers', but this study is suggesting that the birds fly around or go through.
"So on the face of it, this is pretty good news for wind farms; but there are other issues when you look at the much larger farms which are coming, and different ways in which they could affect birds.
"The proposed London Array farm in the Thames Estuary would, for example, cover more than 200 sq km. This is a very important feeding area for the red-throated diver, which could suffer from being displaced."
Dr Gibbons says that further research is the key, and that view is endorsed by Mark Desholm.
"For example, we presume that the birds are avoiding the turbines by seeing them - in the night they have red lights on," he said.
"But are they more scared off by flashing or still lights? Can they sense turbulence of the wind? Will they become habituated to the wind farm?"
The current study continues for one more migration season, and may provide answers to some of these outstanding questions.