Environmental Engineering Reference
In-Depth Information
or other patterns in the data may have provided indications of the factors involved
in collision risk, possibly including their cause. The lack of observed patterns in
these data suggests that collisions are random, with no one specifi c factor respon-
sible. Smallwood and Thelander (
2004
) were also unable to identify specifi c factors
in raptor collision risk at the Altamont Pass Wind Resource Area and suggested the
pattern was random. However, at some overseas wind farms seasonal patterns have
been documented (Barrios and Rodriguez
2004
; Smallwood and Thelander
2004
;
Rasran et al.
2008
). The differences in fi ndings across various wind farms reinforces
the suggestion that raptor collision risk shows species- and site-variability (Hoover
2002
). As there was no evidence of clustering in the collision data, no support for
the display period or cascade theories was found.
Some previous wind farm studies have hypothesized that predatory birds may
become fi xated on prey when hunting, resulting in less attention being paid to wind
turbines (Orloff and Flannery
1992
). No direct evidence was found for this specifi c
causal factor in collisions at the BPWF and SBWF. Of the two observed collision
events, one was not associated with foraging (a juvenile bird was chased through the
swept area by two adults). The other may have involved a bird following prey, but
this could not be confi rmed.
Both males and females have been involved in collisions at these wind farms.
While more males than females have been recorded colliding with turbines, small
sample sizes precluded statistical analysis. Similarly, both adults and younger eagles
(approximately half of each, see
Appendix
) have been involved in collisions, indi-
cating that inexperience of juvenile birds was not a key factor in eagle collision risk
at these sites. Studies of golden eagle collisions at Altamont Pass Wind Resource
Area found that adults were predominantly involved in collisions (Hunt
2002
). Hunt
(
2002
) speculated that this was related to the differences in foraging behaviour of
adults and juveniles, as juveniles rarely foraged for live prey, and they speculated
that foraging for live prey put eagles at higher risk of collisions with turbines.
There is no evidence to suggest that collision risk is related to bird age, poor
weather, foraging behaviour, territorial disputes or eagle displaying behaviour at
these sites. Until the factors involved in collision risk are better understood, it is
important that all potential theories are tested wherever possible. It is also prudent
to focus management interventions on demonstrated causal relationships with colli-
sion risk, not speculated ones, as only those that target the causes of collisions are
likely to be effective at reducing risk.
The breeding success rates of eagles at the wind farms was equal to, or possibly
higher than, those of eagles away from the wind farms and generally consistent with
or above that of WTE across Tasmania (see Forest Practices Authority
2013
).
Walker et al. (
2005
) found no effect on breeding success of eagles at a wind farm,
although a study of white-tailed eagles
H. albicilla
by Dahl et al. (
2012
) found that
productivity was decreased due to mortality and displacement effects of a wind
farm. It should be noted that white-tailed eagles in the latter study had nest sites
amongst the turbines, in contrast to BPWF and SBWF where eagle nests were
within the wind farm properties, but not amongst turbines.
