186 | Chapter 10
turbines rotating 20 times a minute, the blade’s tip is moving at close to
600 km/h, manifestly a danger to flying birds. The extent of the danger
will hinge on the heights at which birds fly and, if they fly at a vulner-
able height, whether the birds can detect and avoid the blades. Both
flight heights and avoidance behaviour are active areas of research. For
example Lesser Black- backed Gulls breeding in the United Kingdom fly
lower by night than by day. Fortunately, if fortuitously, this means that
the gulls are flying lower when they are less likely to be able to see the
Swords of Damocles or, more prosaically, the turbine blades.^11
Northern Gannets have been studied when setting forth to collect
food for their chicks from the Bass Rock colony in the Firth of Forth,
Scotland. Thanks to the exertions of scientists from the Universities of
Exeter and Leeds, the 55 adult gannets carried GPS trackers that revealed
their height above the sea, as did the atmospheric pressure sensors car-
ried by a subset of the birds. When commuting to feeding areas, often
over sea areas with planning consent for wind farms, the median height
of the gannets was 11 m above the sea. The danger from blades, whose
minimum permitted height is 22 m, would be small but not negligible.
However once the gannets found fish, they tended to circle and rise
higher, the better to scan the water below. This behaviour took them to a
median height of 26 m, well within the savage sweep of a turbine blade.^12
Undoubtedly the risk will vary among species. Within British waters,
a low- flying Little Auk is virtually always safe from impact with blades
whereas a relatively high- flying Black- throated Diver is in greater peril,
should it choose to fly through a wind farm.
Once inside a wind farm’s boundaries, the bird can avoid a rotating
blade by skirting around the turbine, or it can take emergency action at
the very last moment when within 10 m of the blade, so- called micro-
avoidance. Just how successful these avoidance behaviours are is being
studied by high- resolution digital cameras, both visual and infra- red,*
radar, and laser rangefinder technologies. The gannet project could bring
no direct observations of avoidance to bear. However, the researchers
assumed 99 percent avoidance, probably realistic, and predicted the death
- (^) To allow observation by night when many seabirds remain at risk.