Environmental Engineering Reference
In-Depth Information
The evidence for this includes:
Eagle carcasses are large and conspicuous and therefore easy to detect;
The search zone around the turbines was adequate to cover the fall zone of eagle
carcases (see Hull and Muir
2010
), and drive-bys conducted outside the formal
monitoring areas increased the probability of carcasses or injured eagles being
observed;
Eagle carcasses do not readily decompose in vegetation, with evidence from
these sites indicating that they last at least 12 months, and likely up to 2 years.
This means that if they are missed, there is a reasonable chance they will be
detected during future searches;
Avian scavengers leave evidence of the carcasses as they consume large car-
casses in situ. The existence of remains means that a collision event can be docu-
mented and it is possible to identify an eagle from feathers and skeletal remains;
A small number of eagles have survived a collision with a turbine, but in all cases
were unable to fl y, although they could walk. In addition, post-mortems con-
ducted on eagles that collided with turbines found that they suffered signifi cant
injuries, indicating that if they survived the collision they are very unlikely to be
able to fl y and therefore leave the site (which comprise large open areas, with
fenced remnant vegetation, see below) where they are likely to be observed;
Approximately one-third of the turbines on both wind farms are fenced with
predator-proof fences (buried fences, with fi ne mesh and an electrifi ed compo-
nent) which restricts access to the turbine areas by mammalian scavengers. These
fences were installed during the construction phase of both wind farms around
the one-third of turbines that were initially monitored. Since this time, large parts
of the turbine areas at both sites have been subsequently fenced to control wal-
labies entering the paddocks from the remnant vegetation, or cattle from entering
the remnant vegetation. While the latter fences do not prevent mammalian scav-
engers entering the wind turbine areas, they prevent them dragging a large bird
carcass out of the area where it will be observed; and
Additional observations (for other studies and work at the wind farms), including
the 838 days of daylight observations (see eagle behaviour studies) has indicated
that eagle collisions are rare events.
Time was counted in wind farm-years (to account for the staged development at
the BPWF). Temporal patterns were assessed using the estimated mortality date of
each bird, which was derived from all evidence available for when the collision
occurred (which is not biased, but uncertain). Tests were also run with actual obser-
vation date. This is when the carcass was found, not when the collision occurred,
which is logically after the mortality event, i.e. biased, but is also known with cer-
tainty. The results based on the estimated mortality date are reported.
Due to the nature of the data, a series of specifi c statistical tests were required.
The range of tests increased the likelihood that a pattern would be detected if it
existed, although this also increased the likelihood of a Type I error (false positive).
The null hypothesis was that the collision rate was constant over time, and that col-
lision events were independent (described by a Poisson distribution). Statistical
tests assessed whether the various assumptions of the Poisson distribution were
