USDA Forest Service Research Note RMRS-RN-12. 2002 1

Abstract—To investigate the effects of military fixed-wing aircraft training on the behavior of the endangered Mexican spotted

owl (Strix occidentalis lucida), we subjected four adults and one

juvenile owl to low-altitude, fixed-wing, jet aircraft overflight

trials in Colorado in 1996 and 1997. Trials consisted of three

sequential fly-bys, each at a greater aircraft speed and noise level,

over owl territories in narrow and steep-walled canyons. All overflights were about 460 m above canyon rims, the lowest altitude

allowed during training flights. Maximum noise levels, measured

at one owl site, were 78, 92, and 95 dB (sound volume) for the

first, second, and third fly-bys, respectively. Behaviors of owls

during 25-second fly-by periods ranged from “no response” (no

body movements) to “intermediate response” (sudden turning of

head). The 25-second fly-by responses did not exceed behaviors (“no

movement” to “change of roost”) exhibited by the owls during

10-minute pre- and postflight observation periods before and after

each fly-by.

Keywords: fixed-wing, F-16 jet, Mexican spotted owl, disturbance, noise

Introduction

To develop and maintain flight proficiency of pilots, the

Colorado Air National Guard (COANG) makes approximately 3,300 year-round sorties (one flight, takeoff to

landing) by fixed-wing, F-16 jet aircraft on the Fremont and

La Veta military operating areas (MOA). The MOAs are

located on the southern slope of Pikes Peak (Fremont MOA)

and in the Wet and Sangre de Cristo Mountains (La Veta

MOA) adjacent to the Fort Carson Military Base in southcentral Colorado. In the montane portions of both MOAs,

the Mexican spotted owl occurs only in deep, steep-walled,

rocky canyons containing ponderosa pine and mixed-conifer

forests (Johnson 1997). When combined, the Fremont and

La Veta MOAs contained 72 percent (13 of 18) of all spotted

owl territories found in Colorado during studies (1989 to

1997) of the owl’s distribution and ecology at the northeastern

limits of their distribution (Johnson 1997). Due to the

importance of the MOAs to Colorado spotted owls, the few

reports on the effects of aircraft on raptors (Delaney and

others 1999; Holmes and others 1993), and an opportunity

to direct jet aircraft flights over the few Mexican spotted

owls in the MOAs, we subjected five Mexican spotted owls

in four canyons in the Fremont and La Veta MOAs to low

altitude F-16 flights in 1996 and 1997.

Study Area and Methods

The Fremont MOA, an area of about 490 km2 ranging in

elevation from 1,830 to 3,500 m, is located immediately west

of Fort Carson, El Paso County, Colorado, and includes much

of the south slope of Pikes Peak. The La Veta MOA, an area

of about 5,180 km2 ranging from 1,890 to 3,535 m elevation,

United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

Research Note

RMRS-RN-12

January 2002

Responses of Mexican Spotted

Owls to Low-flying Military

Jet Aircraft

Charles L. Johnson

Richard T. Reynolds

About The Authors

Charles L. Johnson was a wildlife biologist with the Rocky Mountain Research Station in Fort Collins, CO. He completed a B.S.

degree at Humboldt State University and an M.A. degree in biological science at the University of Northern Colorado. He is currently a wildlife biologist with ENSR in Fort Collins, CO.

Richard T. Reynolds is a research wildlife biologist at the Rocky

Mountain Research Station in Fort Collins, CO. He completed B.S.,

M.S., and Ph.D. degrees at Oregon State University.
2 USDA Forest Service Research Note RMRS-RN-12. 2002

is 24.5 km south of the Fremont MOA and includes portions

of the Wet and Sangre de Cristo Mountains. The montane

areas in both MOAs contained ponderosa pine (Pinus

ponderosa) forests at lower elevations and mixed-species

forests containing ponderosa pine, Douglas-fir (Pseudotsuga

menziesii), white fir (Abies concolor), blue spruce (Picea

pungens), quaking aspen (Populus tremuloides), and Gamble

oak (Quercus gamblii) at higher elevations. Both MOAs were

dissected with deep, narrow rocky canyons, some of which

were annually occupied by spotted owls while other canyons

were occupied only occasionally during our 9-year study

(Johnson 1997).

To examine the effects of jet overflights on spotted owls,

we recorded the behavioral responses of day-roosting owls

to low-altitude jet (F-16) overflight trials in Colorado in 1996

and 1997. An overflight trial consisted of three separate, but

same-day, fly-bys over each owl. Each fly-by was at

sequentially greater aircraft speeds and sound levels: “enroute

cruise” (speed approximately 300 knots), “2nd power setting”

(approximately 425 knots), and “higher-power setting”

(approximately 520 knots). We recorded the behavior of the

roosting owls subjected to overflight trails during (1) a 10-

minute period immediately prior to a fly-by, (2) during a 25-

second fly-by period, and (3) during a 10-minute period

immediately after each fly-by. All owls had been captured

with noose poles, mist nets (Forsman 1983), or leg-hold traps

(Johnson and Reynolds 1997) and fitted with tail-mounted

transmitters from 24 hours (one owl) to greater than 30 days

(all other owls) prior to the trials. On the day of a trial, owls

were located on their day roosts with hand-held receivers.

Owl behaviors were observed continuously during each flyby period with binoculars and 20x scopes by two to four

observers in at least two separate locations hidden by trees

and shrubs between 30 and 100 m from the owls. Observers

were at their observation positions more than 30 minutes

prior to overflights to allow the owls to adjust to any

disturbances related to our arrival. We used these observation

distances and pretrial arrival times because spotted owls

become accustomed to observers sitting quietly 25 to 50 m

away in only 10 to 15 minutes (Sovern and others 1994).

Planned elapsed time between each sequential fly-by was

10 minutes between the end of the 10-minute postfly-by

observation period and the beginning of the next 10-minute

prefly-by observation period. Aircraft safety factors

constrained our control of the speed sequencing of fly-bys;

thus, slower aircraft speeds always preceded higher speeds.

Pilots attempted each fly-by at 460 m above each canyon

rim, the lowest altitude allowed during flight training; we

could not determine the exact distances between owls and

aircraft. All flights were across or perpendicular to the length

of canyons and were directed via radio communication

between the pilot and a COANG controller on the canyon

rim directly above the owl’s roost. Owl behaviors during the

25-second fly-by periods were assigned to one of four

categories: (1) “no response” (no discernible body

movement); (2) “low response” (opening of eyes, slow head

turn); (3) “intermediate response” (sudden movement of

head, wing, or body); and (4) “high response” (flush or

change of roost). Owl behaviors during the 10-minute preand postfly-by observation periods were assigned to the

following categories: (1) owl asleep, no discernible body

movement; (2) eyes open, slow head turns, preening; (3) alert,

sudden head turns; and (4) movement to new roost. These

categories were equivalent to categories used for the owl

responses during the 25-second fly-by.

Four territories containing a total of five different owls

were subjected to overflight trials during the study: two adult

females in two different territories and one adult male and

his female fledgling in a third territory in 1996, and an adult

male and a 1-year-old female in a fourth territory in 1997.

Two of the five owls were subjected to overflight trails twice;

one female, overflown in 1996, was tested in 1997 in the

same territory where she was treated in 1996, and the

fledgling tested in 1996 was again overflown as a 1-yearold (AHY) in 1997 in the fourth territory (above) where she

had paired with an adult male. Thus, seven overflight trials

were conducted in which five different owls were treated

once and two owls twice. In 1996, a Larson-Davis Model

820 sound level meter, with an attached 1.3 cm condensor

microphone covered with a 9.0 cm wind screen, was used to

determine sound volumes (dB) and duration (seconds) of

three sequential fly-bys at one territory only. We reported Aweighted decibels (dBA), a logarithmic measure of the

magnitude of a sound that an average human hears. While

BA may not be representative of the noise energy that spotted

owls might hear (Delaney and others 1999), dBA occurs on

most sound level meters and is ubiquitously used. We used

likelihood ratio tests (exact methods due to small samples)

(CYTEL 1998) to compare observed frequencies of owl

behaviors during pre- and postflight 10-minute periods and

responses during the 25-second fly-bys among aircraft power

settings.

Results

During each overflight trial, owls were day roosting in

trees (for example, Douglas-fir, quaking aspen, Gambel oak)

or on rock walls of canyons between 200 and 300 m below

the canyon rims. All overflights occurred between 1200 and

1600 hours Mountain Standard Time and between 27 and 29

August in 1996, and on July 4, 1997. Elapsed time between

the end of a 10-minute postfly-by and the beginning of a 10-

minute prefly-by observation period during fly-bys in a

canyon varied from 10 to 40 minutes. Ambient temperatures

during all overflight experiments ranged from 18 to 29 oC,

and winds were calm to light. At the one canyon territory

where durations and sound levels of fly-bys were measured

with a sound level meter, sound duration for “enroute cruise”

was 15.5 seconds and volume was 78 dBA, for “2nd power

setting,” 19.0 second and 92 dBA, and for “higher-power

setting,” 22.5 second and 95 dBA. During overflights trials

while the sound levels were being recorded in this territory,

a thunderstorm was in progress; intermittent light-to-heavy

rain was falling, and numerous thunderclaps were recorded.

Close thunderclaps had durations of 4.5 to 6.0 seconds and

sound volumes between 62 and 87 dBA. Thunderstorms did
USDA Forest Service Research Note RMRS-RN-12. 2002 3

not occur during any of the other fly-by trials. Mean ambient

or residual sound in the canyons was 61.2 dBA (range =

56.4 to 69.1 dBA, n = 4), excluding the thunderclaps

measured in the one canyon.

In 17 (81 percent) of the 21 total fly-bys, only two (9.5

percent) owls showed a higher level of alertness during the

post- than preflight 10-minute periods while two (9.5 percent)

showed greater movement during the preflight than in the

postflight 10-minute periods (table 1). While the numbers

of same levels of behaviors by owls during the pre- and

postflight periods increased with power setting, the increase

was not significant (P = 0.34, df = 4). Of the two owls that

showed greater movement during postflight periods, one

changed roost trees (a 5-m flight) 5 minutes into the 10-

minute postflight period for the “enroute cruise.” This change

of roost appeared to be induced by the thunderstorm rain.

The second owl also changed roost trees (a 15-m flight) 7

minutes into the 10-minute postflight observation period

(“2nd power setting”). This owl moved from a roost in direct

sunlight to a shaded roost site. Two other roost changes, both

made by the fledgling owl in the 1996 trials, were observed

during the 10-minute prefly-by period for the “enroute

cruise.” Both of these roost changes appeared to result from

the owl’s seeking protection from the thunderstorm rain and

to escape the mobbing of small birds.

During the 25-second fly-by period for “enroute cruise,”

one (14.3 percent) owl showed no response, four (57.1

percent) showed low responses (slow head turn), and two

(28.6 percent) showed intermediate responses (sudden

head turn toward origin of the sound). For the “2nd power

setting,” two (28.6 percent) owls showed no response,

three (42.8 percent) owls showed low response, and two

(28.6 percent) showed intermediate response. For the “higher

power setting,” two (28.6 percent) showed no response, two

(28.6 percent) showed low response, and three (42.8 percent)

owls showed intermediate response. None of the owls showed

high response (flush) during any of the 25-second fly-by

periods. Overall, there were no differences in observed and

expected response levels during the 25-second fly-bys (P = 0.92,

df = 4).

The female fledgling showed an intermediate response

during each 25-second fly-by in 1996. However, when this

owl was overflown on its new territory in 1997, it showed a

low response during the 25-second “enroute cruise,” and no

responses during the 25-second fly-bys at the “2nd power”

and “higher power setting.” While the fledgling changed

roosts twice in the 10-minute prefly-by period (see above),

it exhibited a mix of no responses and low responses to

thunderclaps that occurred during the pre- and post- 10-

minute observation periods in 1996. However, it responded

at the intermediate level to each 25-second fly-by in that

year. The greater responses to fly-bys versus thunder may

be related to the duration of noise: 4.5 to 6.0 second for

thunder verses 15.5 to 22.5 second for overflights. Noise

duration was greater for overflights due to the approach and

departure of the aircraft. The adult female owl treated in both

years showed only a single response (“low” to “2nd power

setting”) during the three 25-second fly-by periods in 1996.

In 1997, however, she showed an intermediate response

during all three 25-second fly-bys.

Discussion

The behaviors displayed by the five Mexican spotted owls

between the time of our arrival near a roosting owl (always

more than 30 minutes) and the beginning of the first 10-

minute prefly-by observation periods in each overflight trial

suggested that the owls had adjusted to our presence before

10-minute prefly-by trials were begun. With the possible

exception of two roost changes made by two of the owls in

the latter half of the 10-minute postfly-by observation

periods, the behaviors of the owls during the pre- and postflyby periods of the sequentially louder fly-bys, showed that

the owls that responded at the second or third levels all very

quickly returned to normal day-roosting behavior (sleeping,

awake but quiet, preening). Delaney and others (1999) also

noted a quick return to normal behavior by Mexican spotted

owls after disturbance by helicopters. Because the two owls

that changed roosts in the postfly-by periods in our study

showed only “low” responses to the preceding 25-second

fly-bys, we believed the two postfly-by changes of roosts

were not directly caused by the fly-bys. However, the roost

changes may have resulted from a residual alertness of the

owls caused by the fly-bys. We expected the greatest

responses from the owls during the 25-second fly-by periods.

However, five (23.8 percent) of the 21 fly-bys produced no

responses, nine (42.8 percent) produced low responses, and

Table 1—Number of behavior categories of five (two treated twice) Mexican spotted owls preceding and following three sequential jet fly-bys of increasing power

and noise that (1) were greater in the 10-minute preflight observation period

than in the 10-minute postflight observation period, (2) were the same in both

periods, and (3) were greater in the post- versus preflight periods.

Fly-by Owls Pre > post Pre = post Post > pre

Enroute cruise 7 2 4 1

2nd power setting 7 - 6 1

Higher power setting 7 - 7 -

2 (9.5 %) 17 (81.0 %) 2 (9.5 %)
4 USDA Forest Service Research Note RMRS-RN-12. 2002

seven (33.3 percent) produced intermediate responses; none

produced high (flush) responses.

There was no consistent year-to-year pattern of responses

during the 25-second fly-by periods of the two owls subjected

to overflight trials in both study years. The adult female tested

in both years exhibited higher response levels in 1997 than

in 1996 during the 25-second fly-by periods. The higher 1997

responses of this female may have resulted from the owl’s

different roost locations in the 2 years. In 1996, the owl was

roosting in the bottom of the main canyon 300 m below the

canyon rim. Her 1997 roost was in a smaller side canyon, 55

m closer to the canyon rim and the aircraft. The fledgling

that was overflown in 1996 responded at the intermediate

level to all three 25-second fly-bys in that year. However,

during the 1997 fly-bys, this 1-year-old owl responded with

a “low” response to only the “enroute cruise;” she showed

no response to the other two louder fly-bys. This owl’s low

responses in 1997 may have resulted from a year’s

habituation to military flights over the MOAs (see Delaney

and others 1999 for possible habituation in spotted owls to

helicopters and chainsaws). Alternatively, the higher response

levels of this owl in the first year may have been due to an

increased alertness of the owl caused by thunder, rain, and

the mobbing by small birds during the overflight trials in

1996. It is not uncommon for spotted owls to change roosts

during daylight hours in response to weather (rain, sun,

temperature), mobbing birds, or attempts to capture prey

(Forsman and others 1984; Johnson 1997; Sovern and others

1994).

Our observations of spotted owls exhibiting alert behavior

(for example, sudden head turns), or even changing

dayroosts, in the 10-minute prefly-by observation periods

showed that owl responses to low altitude F-16 overflights

did not exceed, and were often less than, responses to

naturally occurring events. The relatively minor responses

of spotted owls to fixed-wing jet aircraft flights near or at a

minimum height of 460 m (plus distance from owl roost site

to canyon rim) observed in this study are consistent with

Delaney and others’ (1999) finding that Mexican spotted owls

in New Mexico exhibited no flush (change of roost) responses

to noise when helicopters were farther then 105 m from

roosting owls, and no alert (sudden head turn) responses when

helicopters were greater than 660 m distant. Delaney and

others (1999) did show, however, that as the distance from

helicopters to owls decreased, the severity of owl responses

increased. The 460-m minimum flight altitude for fixed-wing

jet aircraft training flights in this study prevented us from

determining the effects of closer flights on the behavior of

spotted owls or to determine the distance at which Mexican

spotted owls consistently flush from low flying aircraft.

Acknowledgments

Eric Forsman, Teryl Grubb, Suzanne Joy, Rudy King,

Brian Linkhart, Hildegard Reiser, and several unidentified

reviewers provided useful comments on the manuscript. We

thank Dick Masse, John Dronenburg, and pilots of the

Colorado Air National Guard for the opportunity to

experimentally treat owls, and T. Grubb for suggestions

regarding table 1. Colorado Air National Guard and Fort

Carson Department of Defense provided funding for this

study.

Literature Cited

CYTEL Software Corporation. 1998. StatXact 4 for Windows.

Cambridge, MA.

Delaney, D. K.; Grubb, T. G.; Beier, P.; Pater, L. L.; Reiser, M. H.

1999. Effects of helicopter noise on nesting Mexican spotted

owls. Journal of Wildlife Management. 63: 60–76.

Forsman, E. D. 1983. Methods and materials for locating and studying spotted owls. Gen. Tech. Rep. PNW-162. Portland, OR:

U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 8 p.

Forsman, E. D.; Meslow, E. C.; Wight, H. M. 1984. Distribution

and biology of the spotted owl in Oregon. Wildlife Monograph.

87: 1–64.

Holmes, T. L.; Knight, R. L.; Stegall, L.; Craig, G. R. 1993. Responses of wintering grassland raptors to human disturbance.

Wildlife Society Bulletin. 21: 461–468.

Johnson, C. L. 1997. Distribution, habitat, and ecology of the

Mexican spotted owl in Colorado. Greeley, CO: University of

Northern Colorado. 78 p. Thesis.

Johnson, C. L.; Reynolds, R. T. 1997. A new trap design for capturing spotted owls. Journal of Raptor Research. 32: 181–182.

Sovern, S. G.; Forsman, E. D.; Biswell, B. L.; Rolph, D. N.; Taylor, M. 1994. Diurnal behavior of the spotted owl in Washington. Condor. 96: 200–202.

You may order additional copies of this publication by sending

your mailing information in label form through one of the following

media. Please specify the publication title and series number.

 Fort Collins Service Center

Telephone (970) 498-1392

FAX (970) 498-1396

E-mail rschneider@fs.fed.us

Web site http://www.fs.fed.us/rm

Mailing Address Publications Distribution

Rocky Mountain Research Station

240 West Prospect Road

Fort Collins, CO 80526

Natural Resources Research Center

2150 Centre Avenue, Building A

Fort Collins, CO 80526