Bends blamed for strange bat deaths

Richard Collins on deadly encounters at windfarms

BIRDS fall foul of wind turbines but they’re not the only victims; windfarms also threaten bats in parts of Europe, North America and Australia. At some accident black spots, more bats are killed than birds. Turbine blades hit hapless birds and it was assumed that bats are killed in the same way. Now, research by a Canadian PhD student suggests this is not the case.

Erin Baerwald of the University of Calgary, interviewed on RTÉ’s Mooney Show, claims that bats who have close encounters with turbines are not collision victims but develop a condition similar to what divers call “the bends”.

As a diver descends, the pressure on his blood vessels increases and air has to be forced into the lungs at high pressure from an aqualung to enable him to breathe. With the extra pressure, abnormal quantities of nitrogen are passed into the blood stream. This is not a problem even when a diver is at a considerable depth. However, once he begins his return to the surface, the pressure falls and the extra nitrogen in the blood coalesces into bubbles. If he ascends too quickly, the expanding bubbles rupture blood vessels in the lungs, an extremely painful condition. Unless a decompression chamber is available, a severely affected diver will die.

Baerwald collected the carcasses of bats found under turbines. To her surprise, the animals had sustained no external injuries; it was clear that they had not been struck by the blades. So what had killed them? Post-mortem examinations led to an extraordinary conclusion: just like divers suffering from the bends, the bats’ lungs had ruptured. Huge pressure develops on the windward side of a turbine blade, with correspondingly low pressure downstream of it. The pressure difference is greatest just off the tips of very large rotors. Baerwald believes that a bat’s sonar is so efficient and its flying skills so advanced that it avoids collisions with the blades. However, sonar cannot detect pressure drops and so a bat flying between the pressure zones bursts its lungs. Nitrogen bubbles are not a factor, it’s just that the lungs expand too quickly.

This might seem an implausible theory. After all, birds don’t have lung problems when they encounter turbines, so why should bats? But Baerwald points out that the respiratory systems of birds are different from those of mammals such as bats. “Bird lungs are more rigid and more tube-like and they have continual air flow,” she says. “That makes them less susceptible to their lungs over-expanding.”

But why do bats approach turbines? The deaths occur mostly during the autumn migration. Researcher Paul Cryan, in a paper published in the Journal of Wildlife Management, claims that bats are attracted to tall trees at this time of year. Most species mate before hibernating. The females store the sperm in their oviducts, with fertilisation being delayed until the spring. For their mating display flights, males seek out the highest objects in the landscape. These are normally trees, but wind turbines are higher and so these are often the places where ‘bat discos’ take place.

Wind farms are hazard to birds of prey, but new turbine designs have reduced mortality. The older windmill designs had lattice structures. These attracted birds seeking locations on which to perch. Modern ones have smooth rounded surfaces. Birds can’t alight and so they avoid the turbines. It’s not a complete solution to the bird strike problem but it helps. So could something similar be done to help the bats?

Changing the design for them is much more problematical; if the pressure difference across the blades is reduced, the turbines will be less efficient. Windfarms could be shut down during bat migration periods but this is hardly a workable solution. Conservationists, who support renewable energy technology and also want to protect bats, are in the horns of a dilemma.

Bats have a very low reproductive rate; females produce only one youngster per year or one every other year. Births must keep pace with deaths and, if mortality rises significantly, a species is in trouble.

The problem seems to be confined to migratory species which roost in trees. No Irish bat migrates, so deaths at windmills don’t appear to be a problem here. But has anybody looked for carcasses at windmill sites?