assessing bird and bat mortality at the mcevoy ranch wind ...searcher efficiency: each mcevoy ranch...

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Assessing Bird a Wind Tur Rya 1 and Bat Mortality at the McEvoy Ra rbine in Marin County, California PRBO Conservation Science an DiGaudio and Geoffrey Geupel (415) 868-0655 www.prbo.org September 2010 PRBO Contribution #1754 anch

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Assessing Bird and Bat Mortality at

Wind Turbine

Ryan DiGaudio

1

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

2

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.

National Wind Coordinating Committee, Washington, D.C., USA.

Golden Gate Raptor Observatory. 2005. Golden Gate Raptor Observatory season

summary 2005.

Hoover, S. L. and M. L. Morrison. 2005. Behavior of Red-tailed Hawks in a wind

turbine development. Journal of Wildlife Management 69: 150-159.

Millikin, R. L. in review. Avoidance of wind turbines by night migrating birds. Echo

Track, Inc. Ottawa, ON.

Orloff, S., and A. Flannery. 1992. Wind turbine effects on avian activity, habitat use and

mortality in Altamont Pass and Solano County Wind Resource Areas. Report to

the Planning Departments of Alameda, Contra Costa and Solano Counties and the

California Energy Commission, Grant No. 990-89-003 to BioSystems Analysis,

Inc., Tiburon, CA.

Shuford, W.D., and T. Gardali. 2008. California Bird Species of Special Concern. : a

ranked assessment of species, subspecies, and distinct populations of birds of

immediate conservation concern in California. Western Birds Monograph Series

No. 1.

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Smallwood, K. S., and C. Thelander. 2004. Developing methods to reduce bird mortality

in the Altamont Pass Wind Resource Area. Final Report to the California Energy

Commission, Public Interest Energy Research–Environmental Area, Contract No.

500-01-019, Sacramento, California, USA.

Smallwood, K.S., C. Thelander. 2008. Bird Mortality in the Altamont Pass Wind

Resource Area, California. The Journal of Wildlife Management. 72(1): 215-223.

Strickland, M. D., W. P. Erickson, and L. L. McDonald. 1996. Avian monitoring studies

– Buffalo Ridge Wind Resource Area, Minnesota. Prepared for Northern States

Power, Minneapolis, MN. 31pp.

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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

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Figure 1. Schematic layout of wind turbine with topographic lines; concentric rings

represent the search transects out to 50m.

10m 20m 30m 40m 50m