Assessing Bird and Bat Mortality at
Wind Turbine
Ryan DiGaudio
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Assessing Bird and Bat Mortality at the McEvoy Ranch
urbine in Marin County, California
PRBO Conservation Science
Ryan DiGaudio and Geoffrey Geupel
(415) 868-0655
www.prbo.org
September 2010
PRBO Contribution #1754
the McEvoy Ranch
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Introduction
In the midst of recent public awareness of problems associated with climate change,
alternative energy sources have received strong public interest. While wind energy may
provide environmental benefits, and has generally been considered a favorable alternative
to fossil fuels, the negative impacts of wind energy development on birds and bats have
raised important ecological considerations. Recent studies of wind turbine effects on bird
and bat mortality have varied greatly, from having very little or no impact (Anderson et
al. 1996, Strickland et al. 1996) to documenting high mortality of raptors (e.g., Altamont
Pass; Orloff and Flannery 1992, Erickson et al. 2001). In order to address concerns
regarding potential conflicts between wind turbines and wildlife, preliminary site
screening should be conducted at proposed wind turbine sites to assess bird and bat use,
and also assess bird and bat mortality occurrences after a turbine is put into operation
(California Energy Commission and the California Department of Fish and Game 2007).
McEvoy Ranch, a 550 acre ranch located in northeastern Marin County, California
approximately 5 miles southwest of Petaluma, operates a single 225kW wind turbine with
a maximum rotor tip height of 148 feet. Prior to the construction of the turbine, PRBO
Conservation Science conducted a study during the winter, spring, and fall of 2006 to
assess diurnal bird use at the turbine’s proposed site. The 2006 study found that up to 28
species of birds used the airspace around the proposed windmill site, including 12 species
of raptors, 4 of which are special status species- Golden Eagle, Peregrine Falcon, White-
tailed Kite, and Northern Harrier (Cormier 2007, Shuford and Gardali 2008). Raptor
concentrations at the site, however, were not particularly high in comparison to other
sites in Marin County, and thus it did not appear that the proposed wind turbine would
pose a significant threat to local raptor populations (GGRO 2005, Cormier 2007).
Nonetheless, the study concluded that there would still be the potential for avian
mortality resulting from the turbine and therefore avian mortality should be carefully
evaluated once the turbine was built and put into operation (Cormier et al. 2007).
The McEvoy Ranch wind turbine was completed and commissioned in June of 2009, and
systematic carcass searchers were initiated in early July of 2009. This report summarizes
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the results from the first 13 months of carcass searches at the McEvoy Ranch wind
turbine from July 2009 through July 2010.
Methods
The methods used for carcass searches at McEvoy Ranch were derived from standardized
monitoring techniques recommended by the California Energy Commission and the
California Department of Fish and Game (2007), along with similar methods used in
other recent windmill-related bird fatality studies (Smallwood and Thelander 2008).
Carcass Search Plot: A circular 50 m radius search plot was established around the
wind turbine. A 50 m radius was chosen as an appropriate search area based on previous
studies at other wind farms, which showed that 85%-96% of carcasses are found ≤50 m
from turbines (Orloff and Flannery 1992, Smallwood and Thelander 2004). A flat,
sparsely vegetated area encircles the base of the turbine and extends asymmetrically 15-
50m out away from the turbine (Figure 1). This flat area was devoid of vegetation for
approximately the first 6 months of surveys, after which the area began filling in with
sparse grass cover. Beyond the flat area, the terrain slopes downward on 3 sides of the
search area and is vegetated primarily with a mix of grass species approximately 2 feet
tall.
Conducting Searches: In order to facilitate a systematic search of the entire plot area,
color coded survey flags were used to demarcate the search plot along 5 concentric
circles spaced away from the turbine at distances of 10 m, 20 m, 30 m, 40 m, and 50 m
(Figure 1). Searchers scanned for carcasses on the ground while slowly walking around
the turbine between the concentric circular transects outlined by the survey flags, such
that the searcher would encircle the turbine along a path at an approximate distance of
5m, 15m, 25m, 35m, and 45m. Searchers were instructed to meander slightly while
walking each concentric circle in order to maximize the area searched. In addition to the
colored survey flags, a GPS unit was used to help guide the searcher within the carcass
search plot. A survey log was maintained to record the surveyor’s name, date, start and
stop time, total minutes surveyed, weather conditions, and other field notes.
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Frequency of Carcass Searches: Through the first year of monitoring, PRBO
recommended that surveys should be conducted 5 days per week during peak migration
season (April and May for spring and September and October for fall), and at least once a
week during the non-peak migration season. Survey frequency, however, varied
somewhat and deviated from the recommended frequency depending on weather,
personnel availability, and other logistical constraints (Table 1).
Collecting Carcass Data: Any carcass found within the carcass search area was to be
collected, placed in a sealed plastic bag with a unique specimen tag and identification
number, stored in a freezer, digitally photographed, and transferred to PRBO
Conservation Science. At the time of collection, the following data were to be recorded:
date, time, observer, species, sex, age, description of injury(ies), and distance from
windmill. A numeric codes for carcass condition and detectability were to be assigned to
each carcass as proposed by Anderson et al. (1999).
Bias Correction: In order to correct for searcher efficiency biases, we conducted random
on-site trials to test searchers by volitionally planting bird carcasses randomly within the
search plot. These tests were done without the searchers prior knowledge. Bird carcasses
used for the trials ranged in size, including small (e.g. warbler, sparrow, finch), medium
(e.g. pigeon, scrub-jay), and large birds (e.g. crows, pheasant). Due to concerns over
potentially conditioning scavengers to search for carcasses near the turbine, we limited
the searcher efficiency trials to 3 per searcher per year. For this same reason, we did not
perform extensive scavenger removal trials; artificially supplying a food source for
scavengers may have conditioned scavengers to systematically look for carcasses around
the turbine and thereby inherently bias our carcass surveys. However, we evaluated the
presence/absence of scavenger activity during the searcher efficiency trials by revisiting
carcasses volitionally planted for the searcher efficiency trials; such carcasses were
checked daily for one up to one week and then removed.
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Results
Search Effort: Between 7 July 2009 and 9 July 2010, a total of 79 carcass search
surveys were conducted (Table 1). A total of 3,322 minutes (2 days, 7 hours, 22 minutes)
were spent searching for carcasses during this period. The mean time spent searching
during each survey was 42 minutes (standard error = 1.45). Four different observers
conducted the surveys, where the primary observer (McEvoy personnel) accounted for
62% of the searches, a secondary observer (PRBO biologist) accounted for 27% of the
surveys, and the remaining two observers (McEvoy personnel) accounted for 11% of
surveys.
Carcass Detections:
Carcass searches within the turbine search area yielded no bird carcasses, however 2 bat
carcasses were found. The bat species were identified as Western red bat (Lasiurus
blossevilli) and Mexican free-tailed bat (Tadarida brasiliensis). Both individuals were
found in foggy conditions on the morning of 19 September, 2009 and both were found
relatively close to the turbine at a distance of 2.5m and 15m away from the base. The
bat carcasses were found in excellent condition and were both fresh and intact. The red
bat had an apparent head injury, whereas the Mexican free-tailed bat did not show any
obvious signs of external injuries.
Searcher efficiency: Each McEvoy Ranch personnel conducting carcass searchers
underwent searcher efficiency trials, where the trials were conducted unannounced to
searchers. Trials took place in August and September 2009 when grass was brown and
dry, and again in April 2010 when grass was green and relatively denser than during the
fall surveys. Searcher efficiency, defined as the percentage of volitionally planted
carcasses that searchers found, averaged 70% (standard error = 11.5) across all bird sizes.
According to relative bird size, all large bird carcasses were found, 70% of medium bird
carcasses were found, and 42% of small bird carcasses were found.
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Carcass removal by scavengers: We were cautious against conditioning scavengers to
search for carcasses near the turbine and therefore did not conduct standardized carcass
removal tests, as this would have required us to regularly plant large numbers of
carcasses near the turbine. However, we documented scavenger removal of carcasses
that were volitionally planted for searcher efficiency trials. In a 2 day searcher efficiency
trial in August 2009, 3 out of 5 planted carcasses were removed by scavengers within 2
days. In September 2009, 2 out of 12 carcasses were removed by scavengers within a 7
day trial period. In April 2010, zero carcasses out of 10 were removed over a 5 day trial
period. Of the 5 bird carcasses removed over the course of searcher efficiency trials, 4
were small birds and 1 was a medium sized bird; no large bird carcasses were removed
by scavengers. This anecdotal evidence suggests that scavengers preferentially removed
small birds over the large and medium sized birds, though more study is needed to
quantify the true bias imposed by scavenger removal.
Discussion
During the first year of the McEvoy wind turbine operation, the observed avian mortality
rate was zero- a positive indication that this wind turbine is not having an adverse effect
on local bird populations. Variance in bird and bat fatalities between wind farms may be
attributed to a variety of factors, such as food availability and site topography, and where
raptors occur fatalities may be attributed to factors such as specific flight behavior of
individuals (Hoover and Morrison 2005). It seems that wind farms consisting of an array
of multiple wind turbines positioned in tightly spaced rows along ridge lines (e.g. the
Altamont Pass Wind Resource Area) pose much more of an obstacle and potential threat
to flying birds than do isolated individual wind turbines, such as the McEvoy Ranch wind
turbine (Millikin in review).
Despite the null finding of avian mortality events at the McEvoy wind turbine, it is
possible that a small number of bird carcasses could have been undetected and/or were
removed by scavengers between surveys. This is particularly true for small birds because
searcher efficiency was only 41% for small birds, and it appears that scavengers may
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preferentially remove small birds over larger birds based on scavenger removals that
occurred during the searcher efficiency trials. However, these apparent biases alone
cannot be used to estimate avian mortality rates given that no avian mortality events were
documented.
The singular event of finding 2 bat carcasses under the wind turbine over the course of
one year indicates a very low bat morality rate. The fact that both bats were found on the
same day (September 19th
, 2009) and none were found from any other survey suggests
there may have been a unique set of circumstances that led to their fatality. The species
found (western red bat and the Mexican free-tailed bat) are both migratory, and they
would have been on their southward migration in September. It is possible that these two
individuals were part of a larger group of bats migrating or foraging together, perhaps
tracking a concentration of aerial insects. The weather conditions at the wind turbine on
the morning of September 19th
were foggy and overcast with light winds (10-15mph),
conditions that maybe have contributed to their mortality. Inclement weather has been
associated with bird strikes on human-made structures, including windmills, though this
relationship has not been described for bats (Erickson et al. 2001).
Future Monitoring Recommendations
We recommend continued monitoring of avian and bat mortality at the McEvoy Ranch
wind turbine for an additional 2 years to complete the 3 years of monitoring previously
agreed upon by McEvoy Ranch. The first year of monitoring was a pilot year for the
search method and protocols. Given that in the first year of mortality monitoring, only 2
bat fatalities and no bird fatalities have been documented, we recommend scaling back
the search effort such that searches are conducted 3X week during peak migration (April-
May and September-October) and once every 2 weeks during the rest of the year.
Furthermore, the search area radius may be reduced from 50m away from the base of the
turbine to 30m. This smaller radius is in accordance with guidelines set forth by the
California Energy Commission and the California Department of Fish and Game (2007)
and will allow searchers to focus more effort in the smaller search area.
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Acknowledgments
We would like to thank McEvoy Ranch for their support in funding this project, Jeff
Creque, Shari DeJoseph, and Mark Rohrmeier of McEvoy Ranch for conducting the
carcass searches, and Wildcare wildlife rehabilitation center of San Rafael, CA for their
donation of bird carcasses used in the bias correction trials.
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Literature Cited
Anderson, R. L., J. Tom, N. Neumann, and J. A. Clecker. 1996. Avian monitoring and
risk assessment at Tehachapi Pass wind resource area, California. Staff Report to
California Energy Commission, Sacramento, CA, November 1996. 40pp.
California Energy Commission and California Department of Fish and Game. 2007.
California guidelines for reducing impacts to birds and bats from wind energy
development. Commission final report. California Energy Commission,
Renewables Committee, and Energy Facilities Siting Division, and California
Department of Fish and Game, Resources Management and Policy Division.
CEC-700-2007-008-CMF. www.energy.ca.gov/2007publications/CEC-700-
2007-008/CEC-700-2007-008-CMF.PDF
Cormier, R., K. Kretinger, G. Geupel. 2007. Assessing Potential Impacts of a Wind
Turbine Site on Avian Species at McEvoy Ranch in Petaluma, California. PRBO
Conservation Science. Report to McEvoy Ranch.
Erickson, W.P., G. D. Johnson, M. D. Strickland, D. P. Young, Jr., K.J. Sernka, R. E.
Good. 2001. Avian collisions with wind turbines: a summary of existing studies
and comparisons to other sources of avian collision mortality in the United States.
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summary 2005.
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Smallwood, K. S., and C. Thelander. 2004. Developing methods to reduce bird mortality
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Table 1. Number of carcass surveys conducted at the McEvoy wind turbine July 2009
through July 2010.
Month Year
Number of
Surveys
Jul 2009 5
Aug 2009 5
Sep* 2009 12
Oct* 2009 7
Nov 2009 3
Dec 2009 4
Jan 2010 2
Feb 2010 2
Mar 2010 4
Apr* 2010 16
May* 2010 14
Jun 2010 3
Jul 2010 2 *Peak migration months