Draft National Wind Farm Development Guidelines - July 2010

In species that are sedentary, the risk of colliding with turbines exists only for the local
portions of the overall population whose home ranges coincide with wind farms. Thus, for
example adult Wedge-tailed Eagles Aquila audax in temperate south-eastern Australia
generally reside permanently within stable home ranges (albeit that young birds may be
dispersive). So only those adult Wedge-tailed Eagles whose home ranges intersect with
wind farms are at risk of turbine collisions, while the great majority of the breeding
population is at no risk from this source at all.
Species that migrate seasonally from one part of their distributional range to another
present a different situation. These species vacate one area, such as their breeding range,
entirely for part of the year and take up seasonal residence elsewhere. Some migrate
along quite narrow flyways and outside of the breeding season may move about within a
non-breeding range. For such species it is possible that large numbers, or even the entire
population, might pass through multiple wind farm sites in the course of an annual
migratory cycle. The Orange-bellied Parrot Neophema chrysogaster and Swift Parrot
Lathamus discolour are examples of such migrants.
Other less predictable usage of habitats within an overall distributional range, such as
nomadism, is characteristic of some Australian birds and bats, but such behaviours are
difficult to quantify for individual sites and consideration of cumulative risks for such species
is not likely to be feasible.
Determining cumulative impact
For sedentary, year-round resident species, the cumulative impact on the entire species is
simply the sum of the impact experienced by those parts of the population that are at risk
of collisions. The first step will be to determine the annual survivorship rate for the species in
question for each wind farm within the species’ range. From those rates calculation should
be made of the mean survivorship rate for the portion of the population interacting with all
wind farms. The mean must be weighted according to the relative numbers of birds and
bats resident at the various wind farm sites. The cumulative impact of wind farm collisions
on the entire population of the species is found by multiplying the annual survivorship rate
for the portion of the population at risk of turbine collisions by the background survivorship
rate for the remainder of the population that is not affected by collisions. The measure of
cumulative impact is the difference between the newly derived rate and the background
survivorship rate for the species.
All or part of the population of a migratory species may encounter a number of wind farms
during the course of its annual cycle. The cumulative impact is derived by assessing the
probability of birds and bats surviving their encounters with one wind farm after another for
as many wind farms as it is believed they might pass through. The survivorship rate of each
wind farm provides a measure of the proportion of the population that survives annual
encounters with that particular farm and thus has potential to encounter another wind
farm, and so forth sequentially through the geographic spread of wind farms within the
range of the species. The cumulative population survivorship rate for wind farms will thus be
the product of the survivorship rates of relevant wind farms.
If the population is segmented into geographic portions during parts of its migration cycle,
or only portions of the population are believed to encounter particular wind farms, then
the process outlined above may be applied to the relevant portion(s) of the population
and to applicable wind farms.
A migratory population may encounter wind farms during only a portion of its annual
cycle. The effects of turbine collisions will then be a seasonal one. For the purposes of
calculating this in terms of an annual survivorship rate, this is no different from the seasonal
variations in survivorship that affect populations over the course of a year due to natural
variables of climate, breeding and non-breeding behaviours, fluctuations in predator and
prey numbers and the like. However, it is important to determine the seasonal duration of
the collision effect and factor that appropriately into the annual survivorship rate.
As for sedentary species, the cumulative population survivorship rate for wind farms should
be multiplied by the background annual survivorship rate that effects the entire population
Draft National Wind Farm Development Guidelines – 2 July 2010 Page 135