Bird deaths are one of the fiercest areas of dispute between wind farm developers and protesters. The ageing installation at Altamont Pass in California is often cited when it comes to showing the dangers turbines can pose to birds. Each year turbines there kill between 800 and 1300 birds of prey, including 75 golden eagles and several hundred red-tailed hawks, according to research carried out by the California Energy Commission. Wind-energy lobby groups, meanwhile, acknowledge the poor record of a few wind farms, but point out that only a tiny proportion of human-related bird deaths are caused by the wind-energy industry.

Hard evidence that could resolve the dispute is thin on the ground. Mark Avery from the UK's Royal Society for the Protection of Birds (RSPB) says it is large birds - eagles, vultures, storks and the like - that seem to be most vulnerable. "Large birds are not that nippy, and they can struggle to get out of the way of turbines, particularly in bad weather, or the dark, or if they're tired," he says. "We need to explore all this in more detail."

After re-analysing previous studies last year, researchers at the University of Birmingham, UK, concluded: "Available evidence suggests that wind farms reduce the abundance of many bird species at the wind farm site." But the most striking aspect of their report was how little evidence is available. The researchers found just 15 articles drawing on 19 datasets, of which only nine were complete. Lead author Gavin Stewart says that many studies are kept secret, sometimes for commercial reasons, with statistics on bird kills being kept from bird conservationists.

Ongoing research in Norway adds weight to the idea that turbines and large birds don't mix. The Smøla islands, 10 kilometres off Norway's north-western coast, have one of the highest breeding densities of white-tailed or sea eagles in the world. Smøla also has a new wind farm, most of whose 68 turbines started turning last summer. Between August 2005 and May 2006, researchers have found nine sea eagles killed by turbine strikes. The RSPB has been running a pilot study of eagle behaviour on Smøla, and has now stepped up its work to include checks on eagle deaths. "Breeding results on Smøla have been strikingly poor compared with the 30 years before the wind farm was built," says Arne Follestad from the Norwegian Institute for Nature Research. "We are not confident that white-tailed eagles will adapt to the turbines and return to the wind park area."