United States 

Department of 

Agriculture Ecology of the Great Gray Owl 

Forest Service 

Pacific Northwest 

Research Station 

Evelyn L. Bull and Mark G. Henjum 

General Technical 

Report 

PNW-GTR-265 

September 1990 
Authors EVELYN L. BULL is a wildlife biologist, USDA Forest Service, Pacific Northwest

Research Station, Forestry and Range Sciences Laboratory, 1401 Gekeler Lane,

La Grande, Oregon 97850; and MARK G. HENJUM is a nongame wildlife biologist,

Oregon Department of Fish and Wildlife, 107-20th Street, La Grande, Oregon 97850.
Abstract

Preface

Bull, Evelyn L.; Henjum, Mark G. 1990. Ecology of the great gray owl. Gen. Tech.

Rep. PNW-GTR-265. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 39 p.

Information is needed on the great gray owl to understand its ecology and to consider this species in land management decisions. From 1982 to 1988, we studied

24 pairs and 107 juvenile great gray owls in northeastern Oregon. Forty-nine nests

were located; 16 were used more than once, so we observed 71 nesting attempts.

Seventy-eight percent of these nesting attempts were successful in raising 1 to 5

young (mean = 2.2). The nests were on stick platforms, on top of broken-off dead

trees, and on artificial wooden platforms. Nest trees occurred in a variety of habitats,

although most were in mature or older, unlogged stands of mixed conifer. Diet by

biomass consisted mainly of northern pocket gophers (67 percent) and voles (27 percent). The size of the home range for 16 adult owls and 19 juvenile owls averaged

67 square kilometers and 157 square kilometers, respectively. Management practices

enhancing habitat for great gray owls include providing artificial nest platforms, protecting existing nest platforms and large-diameter dead trees, providing dense tree

cover around or adjacent to the nest, and providing perches for recently fledged

young.

Keywords: Owl, great gray owl, management, conifer forest, Oregon.

This paper is intended to be a general guide for biologists, ornithologists, and others

interested in the ecology of the great gray owl. The detailed methods and statistical

analyses have been deleted to save space and to promote readability. For persons

interested in this information, it is available in the references by Bull and others listed

in the literature cited at the end of the text. Scientific names are given in the

appendix.
Contents 1 Introduction

2 Study Area

3 Locating Birds

3 Nesting Biology

6 Renesting Attempts

6 Nest Defense and Success

7 Breeding Age

7 Breeding Pair Density

7 Nest Site Fidelity

8 Diet

8 Prey Items in Pellets

10 Hunting Observations

10 Influence of Prey on Nesting

11 Habitat

11 Nesting Habitat

14 Perches Used by Nesting Females

14 Perches Used by Juveniles

16 Foraging Habitat

22 Movements

22 Movements of Adults

28 Dispersal of Juveniles

29 Dispersal of 2-Year-Old Birds

30 Mortality

31 Trapping and Radio-Tagging Owls

31 Trapping Techniques

35 Attaching Radio Transmitters

35 Management Practices

37 Acknowledgments

37 Literature Cited

39 Appendix
Many people have sought the great gray owl, a striking bird standing 56 cm tall with

a wing span in excess of 1.5 m. 1It is easily recognized by its gray plumage; large,

circular facial discs surrounding bright yellow eyes; and lack of ear tufts. Yet, this owl

is not easily observed because of its low numbers, nocturnal habits, and secretive

nature. Its range in North America is limited to some parts of Canada, Alaska, and

the northern Midwest States, with local populations in Yosemite Park (California),

south-central and northeastern Oregon, southeastern Idaho, and northwestern

Wyoming (fig. 1).

0

jiiiijiii BREEDING RAN;E

Ei3l .. OCCASIONAL OCCURRENCE IN WINTER

,

* STUDY AREA aFigure l-Distribution of the great gray owl in North America.

1 Scientific names for all plants, birds, and mammals are

given in the appendix.
People in other parts of the world have investigated this cryptic bird (Franklin 1988, 

Mikkola 1983, Nero 1980, Servos 1986, Winter 1986), but there were great gaps of 

knowledge pertaining to the great gray owl in northeastern Oregon. We did not have 

a clue as to actual number of birds in our forests. After investigating the literature, we 

still did not know how the habits of the birds in Oregon differed from birds in Canada 

or Scandinavia. Our forest structure in northeastern Oregon is in some ways much 

different from that in other regions. We wondered how the great gray owl has adapted 

to these forests and whether they use nests similar to those used elsewhere. Do they 

eat similar prey items? Is the local population successfully reproducing year after 

year? For these questions, we had no answers. 

Our study of the ecology of the great gray owl began in spring 1982 and lasted 6 

years. The information presented here is a result of this investigation of more than 

24 nesting pairs and 107 juveniles in four different study areas. We looked not only 

at reproduction, but also at the platforms and surrounding habitat used for nesting. 

We determined prey items used during nesting. We put radio transmitters on 23 

adults and 32 juveniles and followed them for up to 3 years to gather information on 

breeding biology, mortality, and dispersal. In this way, we started solving the riddles 

surrounding the survival of this elusive, little-known owl of the Oregon forests. 

Study Area Our study was conducted in four study areas called Spring, Sheep, Bowman, and 

Thomason in Union, Umatilla, and Wallowa Counties in northeastern Oregon (fig. 1). 

All study areas were in predominantly forested landscapes interspersed with shallowsoiled grasslands or deep-soiled meadows. The forests were entirely coniferous and 

contained ponderosa pine, Douglas-fir, grand fir, western larch, and lodgepole pine. 

Terrain ranged from gently sloping to steeply walled drainages (fig. 2). Elevations 

were between 930 and 1,500 meters. 

Figure P-Aerial view of great gray owl habitat in northeastern 

Oregon. 
Locating Birds

Nesting Biology

The first challenge was to locate these elusive owls. They are primarily nocturnal, like

most owls, so we searched for them after dark in March and April. We concentrated

our efforts where we had historical sightings. We walked routes through perspective

areas, stopping every 0.1 kilometer to listen for owls. If none was heard, we imitated

the territorial call of a male great gray owl (a series of 6 to 10 low “whoos”) three

times and listened for about 3 minutes for a response. Areas were searched up to

three times, with at least a week between surveys to increase our chances of hearing

owls if they were in the area.

All surveys were done from sunset to within 6 hours after sunset. More than 75 percent of the great gray owls were heard within 3 hours after sunset. On occasion, a

great gray owl would call before sunset, but this was the exception rather than the

rule.

We first heard great gray owls calling in late February and March, although the timing

depended somewhat on elevation. The lower the elevation of the area, the earlier

they called; for example, at the Spring study area (the lowest in elevation-930 to

1,140 meters), great gray owls started calling in late February and early March. At

Bowman (the highest elevation-l,380 to 1,500 meters), the owls began calling in

mid to late March.

When an owl was heard at night, we returned during the day to the exact spot and

searched for whitewash (fecal material), pellets (regurgitated fur and bones), and

nest platforms. We often found a nest within 30-250 meters of where we heard an

owl calling at night. We think these birds defend only the immediate area around the

nest site (Bull and Henjum 1987). Although concentrations of whitewash and pellets

were found near nest sites, none were found under the actual nest until a week

before the young left the nest. This nest sanitation probably reduces detection by

mammalian predators using olfactory signals.

From 1982 to 1988, we observed 71 nesting attempts and recorded information on

phenology (chronology of nesting) and reproduction at these nests (Bull and others

1989b). Courtship started in late February or March, depending on the study area,

with both the male and female in the vicinity of the nest. Both birds called, the male

giving a series of low whoos. The female more typically gave begging calls, panicularly just before the onset of nesting. During courtship, the two birds preened each

other and spent a great deal of time near the nest. Great gray owls do not build their

own nest or even add material to an existing one. Thus their nesting is contingent

upon the availability of a suitable nest platform.

During courtship the male presents the female with prey for her to consume. This

prey provided by the male enables the female to acquire food without expending

energy and thereby build her body reserves for egg production and incubation.

The female starts incubating after the first egg is laid; an egg is laid every 2-4 days.

The male continues to bring prey, doing most of the hunting at night. By day, the

male roosts in a tree close to the nest, often in a Douglas-fir, which provides shade

as well as concealment from avian predators.
INCUBATION STARTED

YOUNG FLEDGED

STUDY AREA

25 MAY

3 JUNE

THOMASON

7 APRIL 5 JUNE

I . ” - ’ n ‘a. ” * “I

MARCH APRIL MAY JUNE

DATE

Figure 3-Phenology of great gray owls nesting in three study areas in northeastern

Oregon, 1982-88.

The onset of incubation differed among study areas but not among years. It appears

that nesting can be delayed when deep snow remains. Snow depth may be a chief

determining factor for incubation. Owls in Spring went on nests about 7 days earlier

than owls in Thomason and about 10 days earlier than owls in Bowman and Sheep

(fig. 3). Females laid eggs from 17 March to 17 April. Once incubation began, females incubated the eggs almost constantly, leaving the nest once or twice during

the night to regurgitate pellets and defecate. We observed only two females off their

eggs during the day. Females diligently remained on the eggs, and we were careful

not to disturb the birds at this time. It is essential that females remain on the eggs

during the day, because unattended eggs are highly vulnerable to predation by

ravens.

Eggs hatched at the same interval that they were laid. This sequence of hatching

presumably has selective advantages if food is limited, because at least some of the

young (the largest) may survive. Females ate the egg shells; we found egg shell

fragments in pellets at 82 percent of the nests where we collected pellets. Males

continued to supply the female and young with food, and females fed small bits of

meat to the newly hatched young. Females constantly brooded the young. As the

nestlings grew they were able to swallow larger pieces of meat. With the increased

demand for food, males hunted during daylight hours in the morning and evening as

well as at night.

After about 3 weeks, the female no longer brooded the young during the day but

perched near the nest. After about 4 weeks, the young owls left the nest (color

plate A). Assuming, incubation lasted 36 days, as reported by Mikkola (1983) in

Finland, the mean number of days in the nest was 26 days. Young left the nest

between 14 May and 19 June. Nestlings raised at Spring left the nest earlier than

nestlings raised in the other study areas (fig. 3).

4
Seventy-eight percent of the nesting pairs successfully raised young. We observed

10 broods with 1 young, 20 broods with 2 young, 16 broods with 3 young, 1 brood

with 4 young, and 1 brood with 5 young. We calculated a mean brood size of 2.2 by

counting the number of young we saw in a nest within a week of fledging or by counting the number of young that had fledged.

The owlets in a nest fledged over several days, with the largest nestling usually the

first to leave. We never observed all young in a nest leave on the same day. Five

young in one nest left over 8 days. Weights of juveniles just out of the nest ranged

from 360 to 755 grams.

After young left the nest, the female stayed near to protect them. Any intruderperson, beast, or bird-coming near the young was immediately warned by the angry

hoots of the female. If her warning went unheeded and the intruder approached the

young, the female frequently attacked. The male continued to feed both the female

and young. After several days, the male usually took prey directly to the young, and

the female caught her own prey. Some females also caught prey for the young. After

3 to 6 weeks, 11 of 12 females had apparently left the care of the young to the male;

the females were at least 2 kilometers away, and we never saw them near the young

again. One of these 12 females remained near the male and young for several

months. Males continued to feed the young for up to 3 months after the young left

the nest. After this time, the young started to catch prey on their own.

We found one exception to this pattern, when a female took over the care of the one

young that survived. The two young that had been in the nest left the nest 30 days

after hatching. Their weights were 360 and 530 grams, which were well below the

average weight for young leaving the nest (609 grams). These low weights indicated

that the young were not getting enough food. After leaving the nest, the smallest

owlet died, probably of starvation. The male was alive and within 3.5 kilometers of

the nest after the young left it, but he apparently quit feeding the female and young

shortly before they left the nest. The female continued to feed the owlet for at least

2 months, and we never observed the male feeding them again.

We observed some unexpected behavior among several of the nesting birds. Twice,

we observed adult owls favoring the weakest of the fledglings. One of three young

that had just left the nest was blind in one eye and very uncoordinated. The female

stayed nearest the blind owlet, and the male fed it more frequently than the others. In

another case, one of two fledglings was temporarily injured by our handling and the

female stayed with that owlet for several days until it could fly again. This behavior is

contrary to some theories stating that the strongest young are favored and weaker

ones allowed to die if resources are limited. In our observations, food resources probably were not limiting, and it would seem advantageous to help the weakest offspring

so long as it did not jeopardize the survival of the others.

5
Another situation occurred when a young great gray owl out of the nest about 2

weeks was brought to us because its parents could not be located.-Rather than keep

the bird in captivity, we hoped another great gray family group would adopt it. We

released this orphan near another owlet of similar age. Within 2 days the male was

feeding the orphan as well as his own young. This species apparently will accept and

feed young belonging to other pairs. This behavior may have an adaptive advantage:

if a male feeding fledged young dies, another male in close proximity may adopt the

orphaned young owls and feed them along with his own. We think this phenomenon

is possible because of the close proximity of some nests, and because we have

observed different family group interacting occasionally.

Renesting Attempts Some owls will try a second nesting attempt if the first one fails, but this depends on

the condition of the female, prey supply, and time of year. We know of three females

that renested after their first nesting attempt failed. During the first nesting attempt,

two nests failed during incubation and one failed with nestlings; these females renested after 17-20, 28, and 30 days, respectively. In a fourth case, a nesting female

failed during incubation, and a second female laid eggs on the abandoned nest 13-

15 days after the first female left it. We do not know if both females were mated with

the same male, because only one male was observed. Females that renested laid

their second clutch between 25 April and 23 May. Two of the renesting attempts

were successful, and young left the nest between 22 June and 18 July.

Nest Defense and

Success

The adult owls readily defended their nests with a ferocity that discouraged most

predators. We saw females pursue common ravens and red-tailed hawks on numerous occasions, and one female attacked and hit a coyote when it came near a nest

of young. Females frequently attacked and hit us when we handled their young or

climbed the nest tree. Aggressive behavior differed among individual birds and typically increased the more years we had worked with a particular bird. Although females were usually more aggressive than males, we observed two males that pursued ravens, a coyote, and a red-tailed hawk when they came near the nest.

In spite of aggressive nest defense, 22 percent of the nesting attempts failed. Of the

nesting attempts that failed, 63 percent failed during incubation, 26 percent failed

with nestlings, and 11 percent were unknown. Eggs apparently fell through two nests,

and young fell out of three nests. Two females abandoned their eggs after incubating

for 26 and 42 days. At the former of these nests, the male had apparently quit feeding the female, because we observed the male frequently catching and consuming

prey. The cause of nest failure at 12 nests was unknown, although we suspect common ravens were the primary predator during incubation because (1) ravens frequently flew over the nests and even paused in midair directly over nests, (2) the

owls reacted to the presence of ravens near the nest by hooting or pursuing them,

(3) ravens mobbed a female on a nest, and (4) broken egg shells were found within

150 meters of four deserted nests.

6
Breeding Age We followed nineteen l-year-old birds and three 2-year-old birds with transmitters,

and none of them nested. One of these radio-tagged males courted a female during

both his second and third year, but we never found them nesting. We also found a

2-year-old female and a 3-year-old male nesting that we had banded as young. The

female nested 8.5 kilometers from her natal site, and the male nested 33 kilometers

from his natal site. We suspect great gray owls rarely breed at 1 year, sometimes at

2 years, and more commonly at 3 years.

Breeding Pair Density Great gray owls regularly nested in close proximity to other great gray owls, which

supports the theory that they defend only the immediate nest site. The shortest distance between two active nests was 430 meters; two other nests were 460 meters

apart. We calculated a density of nesting pairs of great gray owls in 1984 in portions

of the Spring and Thomason study areas where we were reasonably certain we had

found all nests. The highest densities at Spring and Thomason were 7 pairs per 9.4

square kilometers (entire study area was 44 square kilometers) and 5 pairs per 2.9

square kilometers (entire study area was 34 square kilometers), respectively. These

are the highest nesting densities reported anywhere in the world for great gray owls.

Nest Site Fidelity We observed 18 nesting attempts by nine pairs where at least one member of each

pair was radio-tagged. Thirty-nine percent of these pairs nested on the same nest the

second year, 39 percent nested within 1 kilometer of the previous years nest, and

22 percent nested more than 1 kilometer away from the previous years nest. The

average distance between successive nest trees was 1.3 kilometers (range of 0.2 to

4.5 kilometers) (fig. 4). Four pairs nested more than 1 kilometers from the previous

years nest, and each of their vacated nest sites was occupied by another pair of

nesting great gray owls the next spring.

At least six pairs seemed to have at least one alternative nest platform. During the

month before incubation started, these pairs perched near one nest platform for a

time and then perched near the other platform, as though trying to decide where to

settle.

7
5 5

) 83.84

RENEST ATTEMPT

IN SAME YEAR

- FEMALES

s1-1 MALES

N

1 KM t

Figure 4-Location of nests of radio-tagged great gray owls in the Spring

study area, 1982-88. Lines connect nests used in successive years by

the same bird.

Diet

Prey Items in Pellets

The diet of great gray owls was determined from 1,923 pellets collected near 58

active nests (Bull and others 1989a). Most of the pellets were found under trees

where females had perched and regurgitated pellets. We separated the bones out of

the pellets and identified and quantified the skulls and jaw bones to determine prey

species. From these pellets, we concluded that great gray owls preyed primarily on

small mammals. The diet consisted of 15 species of small mammals, 4 individual

birds, 1 fish, and 1 insect. By frequency, voles made up 52 percent of the prey, but

northern pocket gophers made up 67 percent of the prey by biomass (tables 1 and

2). Of the voles, 59.5 percent, 6.6 percent, and 0.2 percent were mountain, longtail,

and Richardson voles, respectively; 33.7 percent could not be identified to species.

Some variation in diet occurred among study areas. A higher percentage of redback

voles was eaten in Bowman and Thomason (table 1). Owls at Thomason ate the

highest percentage of northern pocket gophers. Owls at Sheep ate the lowest percentage of northern pocket gophers but ate more flying squirrels than in any other

area. These differences in diet among study areas demonstrated the opportunistic

nature of the great gray owl.

8
Table l-Diet composition of great gray owls in northeastern Oregon, 

by study area, 1982-888 

Study area 

Diet Spring Bowman Thomason Sheep Total 

Percent 

Mammals: 

Voleb 

Northern pocket gopher 

Redback vole 

ShrewC 

Deer mouse 

Heather vole 

Yellow pine chipmunk 

Other mammals8 

Birds 

Fish 

Insects 

Prey items (N) 

67 

26.7 

2.3 

1.7 

trd 

7 

.8 

.6 

tr 

tr .2 

2,086 566 

34.4 

32 

21.6 

2.8 

.5 

6.4 

.4 

1.6 

.2 

39.9 

30.6 

24.2 

2.4 

1.4 

0.2 

.7 

.4 

.2 

tr 

tr 

1,697 

67 

22.8 

2.5 

3 

.5 

3 

.5 

.5 

197 

51.6 

28.8 

13.9 

2.2 

.9 

:Y 

.7 

.l 

tr 

.l 

4,546 

a Determined from 1,923 pellets. 

’ Included mountain, longtail, and Richardson voles. 

’ Skulls that could be identified were vagrant shrew. 

d tr equals qO.05 percent. 

’ Included prey comprising ~0.3 percent of diet: northern flying squirrel, Pacific mole, red squirrel, 

bushytail woodrat, Columbian ground squirrel, and golden-mantled squirrel. 

Table P-Biomass of prey in the diet of great gray owls in northeastern 

Oregon, by study area, 1982-88 

Study area 

Diet Weight= Spring Bowman Thomason Sheep Total 

Mamqals: 

Vole 

Northern pocket gopher 

Redback vole 

Northern flying squirrel 

Yellow pine chipmunk 

Heather vole 

Red squirrel 

Other mammalsC 

Grams _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ percent _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

26 35.3 16.8 

115 60.6 69.1 

19 .9 7.7 

115 .3 1.5 

56 1 .4 

27 3.2 

323 .7 

1.4 1.3 

22 36.3 27.4 

74.6 54.7 67.3 

1 1 1.8 

.2 7.3 .7 

.a .6 .a 

.5 

.4 

1 0.1 1.1 

a Mean weight used for biomass calculations. 

’ Included mountain, longtail, and Richardson voles. 

’ Included prey comprising eO.3 percent of diet: deer mouse (16 grams), shrew (5 grams), Pacific 

mole (53 grams), bushytail woodrat (262 grams), Columbian ground squirrel (486 grams), and 

golden-mantled squirrel (164 grams). 

9 

I 
Collins (1980) and Nero (1980) in Canada and Hoglund and Lansgren (1968), Mikkola

(1972, 1981, 1983), Mikkola and Sulkava (1970), and Pulliainen and Loisa (1977) in

Europe report that great gray owls prey mostly on small mammals, voles in particular. Pocket gophers seem to be eaten in greater numbers in northeastern Oregon

and other parts of the Western United States than elsewhere in the range of the

great gray owl.

Hunting Observations We followed eight radio-tagged males during the nesting season for 229 hours to

observe the prey they caught and determine if the prey we observed them capture

corresponded with prey we found in the pellets. Ninety captures were observed. Rate

of successful prey capture per observed attempt averaged 33 percent; the mean rate

of successful capture ranged from 16 to 60 percent for individual owls. The prey

captured was 74 percent voles, 19 percent mammals larger than voles, and 6 percent shrews; 28 items could not be identified. The prey larger than a vole consisted

of northern pocket gophers, yellow pine chipmunks, and northern flying squirrels. Of

the prey captured, 75 percent of the shrews were eaten by the males with 25 percent

taken to the nest; 37 percent of the voles were eaten with 63 percent taken to the

nest; and 8 percent of the larger mammals were eaten with 92 percent taken to the

nest.

Males took a greater proportion of the larger prey to the nest and ate a greater proportion of the smaller prey. In this way, energy expenditure was minimized when

males returned to the nest with larger prey items; more food energy was delivered

with larger prey in fewer trips to and from the nest.

Because breeding male great gray owls consumed more of the smaller prey and took

more of the larger prey to the nest, analyses of pellets at the nest site may be misleading. We compared the prey composition found in 710 pellets collected at the

nests of the eight radio-tagged males with what we observed them catch. Pellet analysis revealed 79 percent voles, 20 percent mammals larger than voles, and 1 percent

shrews. The diet based on pellet analysis therefore underestimated the number of

small prey items and overestimated the number of larger prey items consumed by

the pair during the nesting period. Analysis of pellets at the nest did represent the

diet for the female and young.

We observed great gray owls eating birds on two occasions. The first was a female

eating a northern flicker, picking meat off the carcass. The second was a male eating

all the young in an American robin nest, swallowing each small, downy chick whole.

This observation explained the constant mobbing great gray owls received from

robins in May, June, and July.

Influence of Prey on

Nesting

In Canada and Europe, populations of great gray owls regularly experience years

when they do not breed, presumably in response to low prey numbers (Hilden and

Helo 1981, Nero 1980, Pulliainen and Loisa 1977). Between 1982 and 1988, great

gray owls in Union and Umatilla Counties nested every year except 1987, and great

gray owls in Wallowa County nested every year except 1988. We have information

on small mammal numbers in one study area (Spring) in Union County for 1985 and

1987 that suggests the lack of breeding corresponded with a crash in the number of

voles and other prey species.

10
Habitat

Nesting Habitat

We sampled the small mammal population in the Spring study area in August 1985

and 1987 by trapping in 18 plots. At each plot, we used a 5-by-5 grid with stations at

5-meter spacings and set two museum specials and one rat trap at each station. We

trapped for three nights at each plot for a total of 4,050 trap-nights each August.

A dramatic decrease occurred in the number of small mammals trapped from 1985 to

1987. We trapped 187 small mammals in 1985 and 23 in 1987-on the same plots,

at the same time of year with trapping done in the same manner. In 1985 and 1987,

the respective numbers trapped were 107 and 0 voles, 23 and 3 redback voles, 22

and 13 deer mice, 20 and 6 yellow pine chipmunks, 8 and 0 northern pocket gophers,

6 and 1 shrews, and 1 and 0 bushytail woodrats.

We think the absence of voles and the decrease in other small mammal populations

explained why none of the owls nested in the Spring study area in 1987. We have no

small mammal data for Umatilla or Wallowa Counties, so do not know what occurred

there. Because there was evidence from sightings, feathers, pellets, and whitewash

of great gray owls in Spring in April 1987, we believe that the birds were there but

did not nest and did not go elsewhere to nest. There simply was not enough prey for

the birds to raise young.

Because great gray owls do not build their own nests, they must use existing platforms. From 1982 to 1988, we located 49 nests used by great gray owls. The same

nest was often used more than once: one nest was used for 4 years, six nests were

used for 3 years, nine nests were used for 2 years, and two nests were used twice

in the same year. Fifty-one percent of the nests were stick platforms, 29 percent

were artificial wooden platforms, and 20 percent were natural depressions on brokentopped dead trees (table 3, color plates B-D). Of the stick nests, 68 percent were

originally made by northern goshawks, 12 percent were made by red-tailed hawks,

and 20 percent were natural platforms formed by dwarf-mistletoe brooms (fig. 5).

The owls seemed to use whatever type of platform was available in each study area;

for example, 72 percent of the nests in Spring and Sheep were on stick nests. Stick

nests were common in these areas, but large-diameter dead trees were scarce, and

there was only one artificial platform before 1985. At Thomason, where there were

38 artificial platforms available and an abundance of large-diameter, dead trees, all

but one of the nests were on broken-topped dead trees or artificial platforms. Stick

nests were uncommon in this study area.

If there was a preference for nest type, we think the owls favored artificial platforms,

perhaps because they were more stable than stick ones. These platforms were

readily used, even when there were natural stick platforms in the immediate vicinity.

In addition, the rate of successful nesting attempts was higher on artificial platforms

(83 percent) than on stick platforms (66 percent). Eggs or young fell through at least

three stick nests, and 36 percent of the stick nests disintegrated within a year.

11
Table 3-Characteristics of 3 types of great gray owl nest 

structures at 49 nest sites in northeastern Oregon, 1982-88 

Nest structure 

Characteristic Stick 

Broken-topped Artificial 

tree platform 

Number of nests: 

Spring 

Bowman 

Sheep 

Thomason 

Nest tree species: 

Western larch 

Douglas-fir 

Ponderosa pine 

Lodgepole pine 

Nest height (m) 17 5.05 11 3.88 12 3.01 

Tree d.b.h. (cm) 58 17.16 78 15.24 58 17.20 

Tree height (m) 30 4.98 11 3.65 29 8.73 

Bole height (m) 10 5.00 8 4.24 13 6.59 

Tree age (years) 151 35.07 173 25.40 129 51.73 

16 

3 

5 

1 

1 

2 

1 

6 

Percent 

76 

20 

4 

X SD X SD X SD 

10 

20 

70 

- 

6 

3 

- 

5 

43 

- 

43 

14 

Figure 5-Great gray owl nest on a dwarf-mistletoe 

broom. 

12 
Figure 6--Great gray owl nest platform constructed from 2.5centimeterthick boards. Platforms were stained with five parts linseed oil and one

part gray stain.

Because great gray owls readily used artificial platforms (Bohm 1985, Bull and others

1987, Nero 1982), we decided to see if the owls showed preferences for type and

location of these platforms. We put up 158 artificial platforms to determine preference

for platform type (open platforms versus nest box), for height off the ground (9 versus

15 meters), and for proximity to a clearcut (adjacent versus 100-200 meters away).

Great gray owls always nested on the open platforms (design shown in fig. 6), never

in the nest boxes. Given a choice, the owls always used the platform 15 meters

above the ground but used platforms 9 meters above the ground if none higher were

available. Given a choice between a platform adjacent to a clearcut versus one 100

to 200 meters away in a forested stand, 80 percent of the owls (N = 5 nests) nested

on the platform in the forested stand.

There was great variability in the type of nest as well as the type of stand used for

nesting. Because these owls do not build their own nests, the nest location really

reflected the choice of the nest builder, the branch structure of the tree, or the choice

of whoever put up the artificial platform. Most of the stick nests were in large-diameter

western larch, because these trees provided heavy branches capable of supporting a

nest structure built by northern goshawks or red-tailed hawks. These stick nests used

by great gray owls were fairly large and averaged 74 centimeters long, 65 centimeters

wide, and 27 centimeters high. In contrast, most nests on broken-topped trees were

in dead ponderosa pine, 46-94 centimeters in diameter at breast height (d.b.h.), and

6-18 meters tall. The trees chosen for nests were large in diameter because the top

had to be wide enough to accomodate a family of owls. One of these nest depressions was 56 centimeters in diameter and 26 centimeters deep. These dead trees

also had partially decayed wood at the top where the female could scratch out a

depression for the eggs.

The habitat surrounding nest trees differed greatly (color plates E-G), and we found

nests in the four forest types that we searched. Fifty percent of the nests were in

Douglas-fir/grand fir forest types, 29 percent in lodgepole pine/western larch, 15 percent in ponderosa pine/Douglas-fir, and 7 percent in ponderosa pine. The majority

(74 percent) of the nests occurred in stands with trees greater than 49 centimeters

d.b.h.; 26 percent occurred in stands with trees 30-49 centimeters d.b.h.; and none

occurred in stands of trees less than 30 centimeters d.b.h.

13
Perches Used by

Nesting Females

Perches Used by

Juveniles

Most (72 percent) of the nests were in unlogged stands, while 19 percent were in

stands with a partial removal of the overstory, and 9 percent were within 200 meters

of a clearcut. This suggests that the owls favored unlogged stands, because 60-80

percent of the stands in each study area had been logged; however, we do not know

if this reflected actual preference or the availability of nest sites. We think most goshawk nests and large-diameter dead trees occurred in unlogged stands because

logging activity typically eliminates both.

Forty-seven of the 49 nest sites had two or more canopy layers with a canopy closure in excess of 60 percent at most nests. Western larch comprised the dominant

crown class at 50 percent of the nest sites. Density of live trees l0-49 centimeters

d.b.h. ranged from 5 to 64 stems/0.1 hectare, and live trees greater than 49 centimeters d.b.h. ranged from 0 to 10 stems/0.1 hectare.

Sixty-nine percent of the nests occurred on slopes, 22 percent on flat ground, and

9 percent in draws. Nest sites on slopes were in areas having a fairly gentle slope

(averaged 13 percent slope gradient). Most of the nests (65 percent) occurred on

north-facing slopes; however, these slopes usually contained denser stands than

south slopes and were preferred nesting habitat of the northern goshawk (Reynolds

and others 1982), the primary builder of nests used by great gray owls.

The percentage of area in forest within a 500-meter radius of each nest ranged from

52 to 99 percent, and the percentage of that forested area that had been logged

ranged from 0 to 97 percent.

Females with eggs or small young left the nest infrequently to defecate and regurgitate pellets. If we found a perch with a large accumulation of pellets, it was very likely

that there was a nest nearby. One or several perches 30-200 meters from the nest

were used repeatedly for this function (color plate H). If several perches were used,

they were usually in the same general vicinity and often in a straight line away from

the nest. Perches were usually uphill or on the same contour as the nest, presumably so the female could watch the nest and return to it quickly if necessary.

Females perched on branches (61 percent), leaning trees (26 percent), and brokentopped dead trees (13 percent). Branches used as perches were usually dead limbs

in a large-diameter ponderosa pine; leaning trees used as perches were generally

small-diameter western larch or lodgepole pine. Perches were typically close to the

ground (most were less than 6 meters above the ground) in sites with a relatively

open understory to facilitate easy and fast maneuvering, yet with a fairly dense canopy closure (average of 59 percent) overhead to provide cover.

Owlets left the nest before they could fly but were capable climbers; they used talons,

bill, and wings to claw and flap their way up tree trunks. For the first few days, leaning

trees with bark were easiest for the young to climb to get off the ground and away

from terrestrial predators (color plate I). After several days, juveniles could climb some

vertical trees, particularly those with branches or deeply fissured bark because the

bark provided a gripping surface for the owlets. After leaving the nest, juveniles invariably headed for dense forest cover (color plate J), if the nest was not already in such

cover. As the owlets aged, they perched higher in the canopy and thus had more

cover and shade.

14
] O-10% CANOPY CLOSURE 1 .‘.; by:

1 1149% CANOPY CLOSURE

] 1 50% CANOPY CLOSURE

JUVENILE PERCH

.., ,, .::i:::

:,.<+ .’

I.EU I . .

: .‘...: ,:

, . . . . .

0

.: . . ,, :, ;;; .’

Figure 7-Location of nests and perches used by juveniles of four

great gray owl family groups in Spring study area. Juveniles were

located during the first 2 months after fledging. Family groups A

and 6 nested in 1983, and family groups C and D nested in 1985.

Within the first week after fledging, all the juveniles stayed within 200 meters of the

nest. Sixty-seven percent of the perches they used were either leaning trees or trees

touched by a leaning tree that the owlets crawled up in order to get to the perch.

Leaning trees used as perches were typically small-diameter, dead lodgepole pine or

ponderosa pine with an average of 87 percent of the bark remaining. The branch

perches were generally in live Douglas-fir with branches within 0.5 meter of the

ground.

Within 2 weeks after leaving the nest, juveniles could fly, thus increasing their mobility; however, owlets generally restricted their movements to forests having at least

60 percent canopy closure (fig. 7). We think the owlets remained in these dense

stands because of the cover provided against avian predation.

15
Foraging Habitat

16

Great gray owls usually hunted from perches close to the ground. They flew from

perch to perch, listening and watching the ground intently. Even after hearing something, they would often listen or watch for several minutes before attacking. The average distance from the perch to the prey was 10.5 meters. If they did not catch prey,

they usually flew to a perch quickly. If they caught prey, they often remained on the

ground for several minutes and eventually swallowed the prey or carried it to the nest.

If the prey captured was as large as a woodrat or flying squirrel, they flew to a perch

and picked meat off the carcass. The great gray owls we observed usually hunted in

forested stands having an open understory, thus enabling them to fly without maneuvering around trees. Most of the stands had been logged, thereby leaving an open

stand of trees with a dense cover of grass on the ground (color plate K).

We followed eight male great gray owls (with radio transmitters) for 229 hours during

daylight hours while they were foraging, and we recorded habitat characteristics of

the areas used for hunting (Bull and others 1988b). The males hunted from perches

averaging 5.5 meters aboveground. At sites where the owls either caught prey or

attempted to catch prey, vegetative cover averaged 88 percent with a mean height of

21 centimeters; grasses dominated (color plate L). At 77 percent of the sites, downed

wood was within 1 meter of the point of capture (color plate M). This downed wood

presumably provided cover for small mammals.

For five of the eight radio-tagged males, we compared habitat they used with expected use based on availability. Males preferentially foraged in stands with 11-59

percent canopy closure and avoided clearings. Four of the males avoided stands with

at least 60 percent canopy closure; one male used such stands in proportion to their

occurrence. Use of edge was greater than expected with two males (color plate L),

less than expected with two males, and no different than expected with one male.

This variability again reflects the opportunistic nature of the species; they go where

the prey is.

There were differences in habitat used between five foraging males at Spring and

three at Sheep and Bowman (table 4). Males at Spring hunted more often in open,

logged stands with one or two canopy layers and containing more ponderosa pine

(fig. 8). Males at Sheep and Bowman hunted more often in stands that were unlogged

with two or three canopy layers and containing more Douglas-fir and lodgepole pine

(fig. 9).

Maximum distances from the nest that the eight radio-tagged males foraged ranged

from 0.7 to 3.2 kilometers. The home range of five males (with more than 90 location

points) during the nesting season averaged 4.5 square kilometers (range, 1.3 to 6.5

square kilometers). The home range was calculated by connecting the outermost

points of observation and determining the area of the polygon (minimum convex

polygon method).

The areas used by several males for foraging overlapped. We observed three instances of males hunting within sight of each other with no aggressive behavior

exhibited. The occurrence of shared hunting grounds supports the theory that pairs

defend only the immediate nest site. This behavior is unlike that of many owl species

that defend their entire territory all year to ensure that other owls of the same species

do not hunt in the same area (Mikkola 1983).
Table 4-Characteristics of foraging sites of 8 nesting male

great gray owls in northeastern Oregon, 1985

Characteristic

Spring Sheep/Bowmara

(N=357) (N=265)

Forest type:

Ponderosa pine

Ponderosa pine-Douglas-fir

Douglas-fir-grand fir

Lodgepole pine-western larch

Successional stage:

Subclimax

Mature

Over-mature

Remnant

Physiognomy of stand:

Open forest

Edge

Dense forest

Logging status:

Unlogged

Partial cut

Clearcut

Number of stand layers:

1

2

3

Perch location:

Branch

Trunk

Leaning tree

Tree species of perch:

Ponderosa pine

Lodgepole pine

Douglas-fir

Other

62 3

25 5

11 60

2 32

23 17

61 58

6 6

10 19

84 51

14 30

2 19

25 49

74 46

1 5

46 13

52 54

2 33

Percent

68 55

27 25

5 20

82 7

9 55

7 25

2 13

a

 The 3 birds in Sheep and Bowman were combined because of the

small sample size.
F

I

I

I

--J o-,o% CANOPY CLoSURE $ . ...,;

59% CANOPY CLOSURE .: ,‘-:j.

,... ,.i’,

> 60% CANOPY CLOSURE ‘. :!.: ’ .’ :::

. .

::. ‘,’ . .

: ‘Y..;

,: . ::

.: ., “::. .. ::. ::;t

. HUNTING :.

; ./’

* NEST .’ .: :& ‘. ..

..,., -:y:;:,.. ‘,$,.

.; .‘: .,‘., ? ..,., ..:.:..;,

I

::. :.::

Figure 8-Locations at 15-minute intervals of a hunting male great

gray owl in the Spring study area during daylight. Observations

were made on 10 days between 1 April and 22 July 1985.

@j _> BC% CANOPY CLOSURE

Figure g--Points of observation at 1Bminute intervals of a male

great gray owl in the Sheep study area during daylight hours.

Observations were made on 14 days between 13 May and

26 June 1985.
Color plate B-Great gray owl nesting on vacated 

northern goshawk nest in western larch tree. 

Color plate A-Juvenile great gray owls can be 

idenbfied by their fluffy gray plumage, yellow eyes, 

large size, and lack of ear tufts. 

Color plate C-Great gray owl nesting on artificial platform. 

Color plate C-Female great gray owl sitting on nest 

on top of dead tree. 

19 

2: 
Color plate F- Partially logged stand containing a great gray owl 

nest. 

Color plate E-Unlogged. old growth stand surrounding great gray owl nest. 

Color plate H-Leaning tree used for a perch by the 

nesting female for regurgitating pellets and defecating. 

Color plate G-Great gray owl nest adjacent to small 

clearcut. 

20 
Color plate I-Leantng tree used as a perch by 

juvemle great gray owl just out of the nest and 

unable to fly. 

Color plate K-Partially logged stand of pondsrosa pine 

used for foraging in the Spring study area. 

Color plate J-Forest stand with closed canopy and 

leaning trees used by juveniles for several weeks after 

they leave the nest. 

Color plate L-Deep-soiled meadow edges used for 

foraging In the Bowman study area. 
Color plate M-Prey were often captured in areas with a dominant 

grass cover and abundant downed material. 

Color plate N-Great gray owl caught on a bal-chatri. 

After foraging in the morning, the males roosted during the day in stands with 11-59 

percent canopy closure (71 percent of the males) or denser (29 percent of the males). 

These stands typically had two or more canopy layers, were unlogged, and were 

mature or older. Owls roosted at least 7 meters above the ground 56 percent of the 

time, 3-7 meters above the ground 38 percent of the time, and below 3 meters 6 percent of the time. 

Movements 

Movements of Adults 

We wanted to determine where great gray owls went when not breeding. To monitor 

owl movements, we attached radio transmitters to 10 males and 13 females and replaced the transmitters each year when possible (Bull and others 1988a). Birds were 

located from the ground every 2-3 weeks and locations marked on aerial photographs. We followed 3 owls for 3 years, 7 owls for 2 years, and 13 owls for 1 year. 

The recorded locations comprised the owls’ home ranges, which were calculated by 

connecting the outermost points of observations and determining the area inside the 

polygon (minimum convex polygon method). 

22 
From 1983 to 1986, the maximum distance adults traveled from nest sites averaged

13.4 kilometers (range, 2.4-43.2 kilometers), and home range size averaged 67.3

square kilometers (range, 4-312 square kilometers). Adult owls from Bowman and

Sheep traveled farther than owls from Spring (figs. 10 and 11). Average distance

traveled from the nest and home range were 22.3 kilometers and 78.7 square kilometers (N = 3) from Bowman and Sheep and 11.3 kilometers and 8 square kilometers

(N = 10) from Spring. We think this difference in adult movements was largely due to

snow depths, because none of the birds wintered at Bowman where snow depths

were 70-90 centimeters. Birds from Bowman moved to areas with shallower snow,

except one female located for two winters in an area 43 kilometers from Bowman

that had more than 150 centimeters of snow. In contrast, 6 of 13 adults remained in

Spring for at least one winter, where snow depths averaged less than 50 centimeters.

The seven adults that left Spring in winter went to areas with a mean snow depth of

24 centimeters.

The majority of the owls either stayed in areas with less snow or traveled to areas

with less snow, presumably because of prey availability. We think deep snow made

prey less available, because these birds plunge through snow to capture prey on the

ground surface (fig. 12). The fact that one female spent two winters in an area with

150 centimeters of snow suggests that at least some birds can survive in areas with

deep snow. We think this female was preying on squirrels, hares, rabbits, and birds,

because small mammals beneath the snow were probably not accessible.

Of eight adults followed two or more winters, six returned to the same area or even

the same stand for at least a portion of more than one winter. But considerable differences occurred in movements and home range size among birds and even between

years for the same bird (table 5). Pairs did not stay together during nonbreeding

periods; however, pairs did return to the same area to nest.

Much greater movements have been reported for the great gray owl in Canada and

Scandinavia than we observed (Mikkola 1981, Nero and Copland 1981, Oeming

1964). We think the relatively short distances we observed the birds traveling were a

function of topography. Owls had to travel only a short distance to change elevation,

snow depth, and probable availability of prey. Owls in Ontario, Manitoba, and

Minnesota must travel long distances to change elevation and snow depth.

During the winter, 11 juveniles from Bowman and Sheep moved to areas with fess

snow accumulation (fig. 13); 12 of 14 juveniles from Spring spent most of their first

winter 6-13 kilometers to the east in an area characterized by open ponderosa pine

stands (fig. 14). One juvenile spent the winter in Spring, and another went 6 kilometers north of Spring to an area being logged. We were surprised to find this owl

adjacent to active logging operations, but tree falling and soil disturbance may have

displaced many small mammals, thereby making them easy prey. Both this juvenile

and an adult female in another winter remained close to logging operations; as the

logging operations moved to different stands, the owls followed.

23
0 NEST

- AREA BOUNDARY OF FEMALES

- AREA BOUNDARY OF MALE

L

3-5B FEMALE

3-5A FEMALE

Figure l0-Area where three adult great gray owls were located in the Spring study area. A letter refers to a particular bird or pair;

numbers preceding a letter refer to the years that bird was located (3 = 1983, 4 = 1984, 5 = 1985. and 6 = 1986). A letter preceding

a dot (nest site) refers to the bird that nested there, and the number preceding it refers to the year that nest was used. Number of

locations was 38 for 5A male, 70 for 3-5A female, and 40 for 3-5B female.

24
d Z-
Figure 12-Plunge hole where an adult great gray owl caught a vole 

under the snow. 

Table 5-Maximum distance traveled and home range size of radio-tagged 

adult great gray owls in northeastern Oregon 

Bird 

1983-84 1984-85 1985-86 

Na Distance Area Distance Area Distance Area 

km kd km kd km kd 

Female A 72 2.9 4 12.5 18 2.4 5 

Female B 40 6.9 10 28.2 151 18.7 115 

Female D 89 18 135 17 43 10.4 26 

Female C 42 8.2 27 6.7 16 

Female H 27 41.2 91 43.2 68 

Mean 9.3 49.7 21.4 66 16.3 46 

Male C 60 22.8 37 19.2 120 

Male F 35 1.6 2 2.4 2 

Male G 38 12.3 84 2.4 2 

Mean 11 68.7 

a Number of locations. 

26 
Figure 13-Area where three juvenile great gray owls from the Sheep and Bowman study areas were located from August 1985 until June

1986. A dot with a letter refers to a nest site. A letter with a number points to the area used by each juvenile raised at the nest with the corresponding letter. Number of locations ranged from 9 to 20 for each bird.
-

Figure 14-Area where six juvenile great gray owls from the Spring study area were located from August 1985 until June 1986. A

dot with a letter refers to a nest site. A letter with a number points to each juvenile raised at the nest with the corresponding letter.

Number of locations ranged from 12 to 21 for each bird.

Dispersal of Juveniles To determine the dispersal of juveniles, we attached transmitters to 32 juvenile owls

after fledging and located them every 2-3 weeks for a year. Data on 15 of them have

not been included because 11 died within 6 months and 4 were located less than

10 times. We labeled these movements “dispersal” because they were movements of

the juveniles away from natal sites.

Maximum distance 17 juveniles traveled from natal sites in their first year averaged

18.5 kilometers (range, 7.5-32 kilometers); home range size averaged 157 square

kilometers (range, 20-637 square kilometers). Juveniles from Bowman and Sheep

dispersed farther than juveniles from Spring. Mean dispersal distance from Bowman

and Sheep was 29 kilometers; from Spring, it was 16 kilometers.

28
Dispersal of 2-Year-Old

Birds

We followed one female and two male juveniles for 2 years. These birds had a larger

home range their first year (mean, 167 square kilometers; range, 23-245 square kilometers), then restricted their movements to a smaller area the second year (mean,

13 square kilometers; range, 6-22 square kilometers) (figs. 15 and 16). These birds

seemed to spend their first year traveling and then settled down in one area their

second year.

N

Figure 15-Area where two juvenile great gray owls from the Spring study area were located from August 1984 until June 1986. Their

natal site was at A.

29
Figure 16--Area where one male juvenile great gray owl from the Bowman study area was located from August 1984 until May 1986. His natal

site was at H, and there were 34 locations.

Mortality During the 3 years when we had adults radio-tagged, 1 of 10 males and 3 of 13 females died. The annual probabilities of survival (Mayfield 1975) for adult nesting

males and females were 0.91 and 0.84, respectively. All deaths occurred in fall or

winter. Cause of death could not be conclusively determined.

Of 32 juveniles with transmitters, nine died before they became independent of the

adults, five died in the fall, and one died after it was a year old. The probabilities of a

juvenile surviving 12, 18, and 24 months are 0.53, 0.39, and 0.31, respectively. We

concluded, based on plucked feathers and fecal material at the kill site, that great

horned owls and northern goshawks killed seven of the dependent juveniles. We also

saw a great horned owl fly from a tree containing the carcass of a dead juvenile. Two

juveniles apparently starved to death while the parents were still feeding them. We

30
saw red-tailed hawks hit juveniles on two occasions. One of the juveniles was carried

a short distance before it was dropped unharmed. Of the five independent juveniles

that died, two died of avian predation and three of unknown causes.

The high rate of juvenile mortality observed in this study seems to agree with data on

great gray owls in Canada and Finland (Mikkola 1981, Nero and Copland 1981). If

juveniles survive to breeding age, survival rates are then quite high.

Trapping and Radio- We used eight different traps to catch great gray owls (Bull 1987). The variety was

Tagging Owls necessary because these owls were extremely wary and were rarely caught by the

same trap twice. To further complicate matters, the owls we dealt with seldom

Trapping Techniques seemed to be stressed for food. Starving birds are much more likely than satiated

ones to hit a trap with prey.

Initially we used a bal-chatri to catch great gray owls (color plate N). This large domeshaped wire mesh cage (15 centimeters high) with a rectangular frame base (45 by

25 centimeters) was made of re-bar to add weight so that the owl could move the

trap but not fly off with it. We attached 50-75 slip-nooses (6-10 centimeters in diameter) made from 18-kilograms monofilament line to the bal-chatri. We put two gerbils

in the bal-chatri for bait, because two moved around more than one. The owls

seemed to prefer gerbils, although we tried domestic mice, deer mice, and voles for

bait.

Bal-chatris were easy to use, very portable, and safe for the bird. If the bird did not

get caught, however, it quickly got discouraged when it could not get the bait.

In the second year of trapping, none of the previously caught birds would go to a

bal-chatri, so we used two other methods to catch females that were no longer brooding their young but were perched nearby. We baited the female by tethering a mouse

on a board (30 centimeters square) placed on the ground within 30 meters of the female. We returned for the next 2 days and repeated the procedure if the female ate

the mouse. On the fourth day, we returned and set up two mist nets (2 meters high

by 5 meters long, lo-centimeters mesh), parallel to each other and 2 meters apart,

within sight of the female and at right angles to the perch she was on. Then we tethered a mouse on the board and placed it between the two mist nets. We watched

from a distance but close enough to get to the net quickly. The net was set so that

when a bird hit it, the net came off the poles and fell over the bird.

We used a second technique shortly after the young left the nest but before they

could fly. We visited a nest area in the evening to locate the young. If we could not

find them, we imitated their begging call and invariably they responded by begging. If

we located a young bird within 12 meters of the ground, we raised toward the bird an

aluminum pole (in 1.2-meter sections) with a perch on the end. Juveniles readily

climbed onto the perch, and we quickly lowered them to the ground.

31
32

At this point the female usually became quite concerned and flew in for a closer look. .

If we kept the young on the perch above our head, the female rarely attacked us. A

second person moved over to the female with a 6-meter telescoping fishing pole with

a 25-centimeters noose made of 3-millimeters thick plastic cord. The noose was

slipped over the owl’s head and tightened around its neck; the bird was quickly lowered to the ground. The first 8 centimeters of the noose was taped to the pole to ensure that the neck would not be broken. Nooses were not used on juveniles as their

neck muscles were not strong enough to withstand the stress. We noosed several

males while their attention was diverted by a mouse.

For trap-shy females, we did one of two things. Once the young fledged, we studied

the flight pattern of the female when she was near the young. We put up a mist

net (3 by 13 meters) 9-15 meters off the ground between two trees in the path we

thought the female would fly. We secured two eye screws into each of the two trees

where the top and bottom ends of the net would lie. We then raised the net into place

with ropes through the eye screws. The female seemed to see the net during the

day, but sometimes flew into it after dark. She was quickly lowered to the ground by

two ever-present observers and removed from the net. We set the net up just before

dark and stayed close enough to hear the bird or see it with a light when it got

caught.

We used one technique only as a last resort to remove transmitters from females

after the study. We set up two parallel mist nets 2 meters apart near the fledged

young and tethered a live raven between the nets. Females always attacked the

raven and continued to attack it even after becoming tangled in the net. Even though

this technique worked, we do not recommend it because it could condition the female

not to attack ravens in close proximity to their young, which would likely mean their

demise.

Males were more difficult to catch because they were more difficult to find and were

more wary than females. Although males were caught by some of the techniques

described above, most were caught when they came to feed the young. This method

worked only if all the young had fledged and had been captured; otherwise, the male

invariably fed the one young we did not get. This technique worked best if the young

had been out of the nest 2-3 days; before that time, the male often took prey to the

female instead of taking it directly to the young.

We set up four mist nets (2 by 5 meters) in a square near the perched young and

with a 3-meter perch (tree, snag, or fabricated perch) in the middle (fig. 17). This

perch provided the male with a place to land. We collected all the young and tethered two or three of them inside the nets, keeping them at least 2 meters apart to

prevent squabbles. They were tethered securely to the ground with jesses on their

legs. Although they liked to perch on logs they invariably fell off and became tangled.

If there were more than two or three young, the others were put in separate, roomy,

ventilated boxes and kept near the observers. Two observers armed with powerful

flashlights sat within 15-25 meters of the nets so they could see or hear if an owl got

caught. The male typically called when he arrived with prey and the young started

begging. Usually the male landed on the perch inside the nets and then flew down to

the young. After giving the prey to one of the young, he flew into a net as there was

no other way out, and he could not gain enough height to get over the top of the net.
Figure 17-Technique used to trap male great

gray owls shortly after the young have left the

nest. Young are tethered to the ground inside

a square of mist nets, and a perch is provided

for the male to land on.

Once the male was caught, banded, and released, all the juveniles were fed a mouse

and immediately replaced on the perch where we had found them. The mouse was

to offset any delay in feeding as a result of the capture of the male; however, none of

the males quit feeding the young after capture.

We set the nets up before dark and trapped at night. Most males were caught between midnight and 0400 hours, It was essential to have two people trade off the

watching and listening. If the male was not caught the first night, we abandoned the

attempt because of the stress put on the young birds.

Some males already radio-tagged were captured with a net gun. The net gun was

powered by blank .22 shells and shot a 3-meters triangular net with three dumbbell

throwers. We followed a bird around until we found him perched on a branch less

than 3 meters off the ground. If the bird was higher than 3 meters, we did not shoot

at it because climbers were required to get the bird and the net. We found that birds

could outmaneuver a net while flying. We never used the net gun when other techniques would work, as there was considerable risk to the bird if hit with one of the

throwers that carried the net.

The last trap we used was basically a metal plate with a gerbil tethered as bait

(fig. 18). When the owl hits the gerbil, a hoop is released that wraps around the

bird’s legs. The advantages of this trap are that it is safe and easy to use and the

bird gets rewarded with the gerbil even if not caught. All the birds that landed on the

trap were caught, although those taking prey without landing were not always captured. A mist net on the opposite side of the trap often solved that problem.

33
Figure l&--Trap with gerbil used to capture great gray 

owls. 

Table 6-Weights and measurements of 30 female and 18 male great 

gray owls in northeastern Oregon 

Female Male 

Measurement Mean Range Mean Range 

Weight (g) 1149 1030-1310 894 763-l 080 

Body length (mm) 612 540-670 575 470-620 

Wing length (mm) 463 430-485 433 41 o-455 

Tail length (mm) 311 215-330 295 230-310 

Bill length (mm) 40.6 38.2-46.1 40.4 32.7-43.1 

Tarsus length (mm) 69.2 64.1-75.5 63.5 52.0-70.8 

Tarsus width (mm) 9.8 8.9-l 1.9 8.9 7.6-l 1.8 

Talon length (mm) 28.0 27.1-31.8 24.5 21.9-26.5 

We always tried the safest trapping technique first, and two people were always 

present when trapping or handling young, because female owls frequently attacked 

us. Anyone climbing a nest tree or adjacent tree wore a padded jacket and a helmet 

with face protection because the owls often attacked the eyes. We handled the birds 

as quickly as possible. If we were attaching a transmitter, we wrapped each foot with 

a nonadhesive elastic wrap to prevent injury from the talons and covered the bird’s 

head and body with a stretch sock with a hole cut out for the beak. The hood typically calmed the birds. We held the bird upright or placed it on its back while holding 

the legs. Placing the bird on its breast could cause suffocation and should never be 

done. As we handled the owls, we recorded weight and other body measurements 

listed in table 6. 

34 
Attaching Radio

Transmitters

Management

Practices

Figure 19-Backpack harness used to attach

transmitter to great gray owl.

We used a 22-gram transmitter attached with a backpack harness made from 6-

millimeter tubular Teflon (fig. 19). Antennas were 30 centimeters long and made of

heavy-duty material because some of the birds broke antennas of lighter material.

We had several birds carry transmitters for 3 years (replaced each year) and two

birds carried the same transmitter 2 years; none showed any signs of wear or

abrasions.

We attached transmitters to juveniles, and because they were only 50-70 percent of

their adult weight, we fitted the harnesses loosely and glued the transmitters to the

feathers on the backs of the birds to hold them in place. The top loop of the harness

was 21 centimeters, and the bottom loop was 31 centimeters. This harness was

large enough to fit an adult female. On the juveniles retrapped as l-year-old birds,

none of the harnesses were too tight.

It is essential to remove transmitters when they are no longer needed. One female

we could not catch was located 2 years later with the radio dangling around her neck.

We then trapped her and removed the transmitter; however, the potential existed for

the loose harness to catch on a branch and strangle the bird.

Several things can be done to preserve, enhance, or create habitat for great gray

owls. Because these owls seem to occur in local concentrations, habitat management should be done where great gray owls have been seen in the past or in prime

areas. Prime areas are those with a high density of small mammals in deep-soiled

open forests or deep-soiled meadows with forest edges (Forsman and Bryan 1987).

35
The easiest management practice is to put up nest platforms, because they are

readily used and the birds often successfully raise young on them. These platforms

should be open (design shown in fig. 6), placed at 9-15 meters above the ground,

and located in a forested stand next to foraging areas. The platform should be positioned so that canopy cover over it provides shade and concealment from avian

predators. The area immediately around and above the nest should be open so the

birds can easily fly into the nest.

To provide natural nest sites, large-diameter (more than 50 centimeters d.b.h.) dead

trees with broken tops should be left standing and marked so they will not be cut.

Large-diameter live larch with dwarf-mistletoe provide natural stick nest platforms.

Any tree with a stick platform, particularly old raptor nests, should be left standing

with cover around the nest. Managing habitat for northern goshawks will provide nest

sites over time for great gray owls, because the owls used old goshawk nests more

than any other type of nest.

The areas used most often for nesting were unlogged, mature or older stands containing grand fir, Douglas-fir, and western larch. These stands usually contained a

fairly open understory facilitating easy flight, yet a fairly dense overstory (more than

60 percent canopy closure) providing shade and concealment from above.

At all nests, dead and downed material and leaning trees are important so that the

juveniles can climb them to avoid mammalian predators. In addition, a dense stand

of trees around or near the nest tree provides critical cover for the juveniles for several months. It does no good to provide nest sites if the young owls are immediately

killed by predators for lack of cover.

Great gray owls are efficient predators and can make an impact on a population of

small mammals. A single bird can consume more than 1,400 voles in a year, and a

family group can consume many more. For open areas such as clearcuts to be used

by great gray owls for foraging, perches must be available for the owls to hunt from.

Perches can be as short as 3 meters and should occur every 20 meters. The owls

do more foraging near the edges of openings so will use a greater percentage of

area in a small clearcut than in a large one.

Partial cuts are generally suitable foraging habitat because the stand is open enough

for maneuvering and adequate perches are available. Dead and downed material

should be left for cover for voles. Burning that destroys this downed material should

be avoided.

The best way to increase populations of great gray owls is to provide the best habitat

possible, meaning solid nest sites in suitable areas with adequate cover. Many juveniles are killed by avian predators; this number might be reduced by providing dense

stands around nests to furnish more cover. In addition, any practice enhancing populations of voles and pocket gophers will be beneficial to these owls.
Acknowledgments We are grateful to R.G. Anderson, H.A. Akenson, J. Akenson, H.D. Cooper,

S.P. Feltis, R. Goggans, R.A. Grove, J.S. Henderson, J.E. Hohmann, M. Hunter,

M.D. Snider, M.E. Walker, and W.G. Williams for their assistance with field work.

D.R. Johnson, C. Maser, and R.E. Williams assisted with preparation of pellets and

identification of skulls. Funding was provided by the USDA Forest Service, Pacific

Northwest Research Station, and the Oregon Department of Fish and Wildlife Nongame Fund. W.I. Haight was instrumental in obtaining funds. Additional assistance

was provided by Wallowa Valley, La Grande, and North Fork John Day Ranger

Districts.

Literature Cited Bohm, Robert T. 1985. Use of artificial nests by great gray owls, great horned owls,

and red-tailed hawks in northeastern Minnesota. Loon. 57: 150-151.

Bull, Evelyn L. 1987. Capture techniques for owls. In: Proceedings of biology and

conservation of northern owls; 1987 February 3-7; Winnipeg, MB. Gen. Tech. Rep.

RM-142. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky

Mountain Forest and Range Experiment Station: 291-293.

Bull, Evelyn L.; Henjum, Mark G. 1987. The neighborly great gray owl. Natural

History. 9: 32-41; 1987.

Bull, Evelyn L.; Henjum, Mark G; Anderson, Ralph G. 1987. Nest platforms for

great gray owls. In: Proceedings of biology and conservation of northern owls;

1987 February 3-7; Winnipeg, MB. ‘Gen. Tech. Rep. RM-142. Fort Collins, CO:

U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range

Experiment Station: 87-90.

Bull, Evelyn L.; Henjum, Mark G.; Rohweder, Ron S. 1988a. Home range and

dispersal of great gray owls in northeastern Oregon. Journal of Raptor Research.

22: 101-106.

Bull, Evelyn L.; Henjum, Mark G.; Rohweder, Ron S. 1988b. Nesting and foraging

habitat of great gray owls. Journal of Raptor Research. 22: 107-115.

Bull, Evelyn L.; Henjum, Mark G.; Rohweder, Ron S. 1989a. Diet and optimal

foraging of great gray owls. Journal of Wildlife Management. 53: 47-50.

Bull, Evelyn L.; Henjum, Mark G.; Rohweder, Ron S. 1989b. Reproduction and

mortality of great gray owls in Oregon. Northwest Science. 63:38-43.

Collins, K. Michael. 1980. Aspects of the biology of the great gray owl, Strix nebulosa Forester. Winnipeg, MB: University of Manitoba. 219 p. M.S. thesis.

Forsman, Eric D.; Bryan, Terry. 1987. Distribution, abundance, and habitat of great

gray owls in southcentral Oregon. Murrelet. 68: 45-49.

Franklin, Alan B. 1988. Breeding biology of the great gray owl in southeastern Idaho

and northwestern Wyoming. Condor. 90: 689-696.

37
Hilden, Olavi; Helo, Pekka. 1981. The great grey owl Strix nebulosea bird of the

northern taiga. Ornis Fennica. 58: 159-166.

Hoglund, Nils. H; Lansgren, Erik. 1968. The great grey owl and its prey in Sweden.

Viltrevy. 5: 360-421.

Mayfield, Harold F. 1975. Suggestions for calculating nest success. Wilson Bulletin.

87: 456-466.

Mikkola, Heimo. 1972. Food of great grey owls in the Lapland Reserve, USSR.

British Birds. 65: 31-32.

Mikkola, Heimo. 1981. Der Bartkauz strix nebulosa. Die Neue Brehm-Bucherei 538.

Wittenberg-Lutherstadt, Germany: A. Ziemsen Verlag. 124 p.

Mikkola, Heimo. 1983. Owls of Europe. Vermillion, SD: Buteo Books. 397 p.

Mikkola, Heimo; Sulkava, S. 1970. Food of great grey owls in Fenna-Scandia.

British Birds. 62: 23-27.

Nero, Robert W. 1980. The great gray owl-phantom of the northern forest.

Washington, DC.: Smithsonian Institution Press. 167 p.

Nero, Robert W. 1982. Building nests for great gray owls. Sialia. 4: 41-48.

Nero, Robert W.; Copland, Herbert W.R. 1981. High mortality of great gray owls in

Manitoba-winter 1980-81. Blue Jay. 39: 158-165.

Oeming, A.F. 1964. Banding recovery of great gray owl. Blue Jay. 22: 10.

Pulliainen, Erkki; Loisa, Kalevi. 1977. Breeding biology and food of the great grey

owl, Strix neblosa, in northeastern Finnish forest Lapland. Aquilo. Ser Zoologica.

17: 23-33.

Reynolds, Richard T.; Meslow, E. Charles; Wight, Howard M. 1982. Nesting

habitat of coexisting accipiters in Oregon. Journal of Wildlife Management. 46:

124-138.

Servos, Maria C. 1986. Summer habitat use by the great gray owl (Strix nebulosa)

in southeastern Manitoba. Winnipeg, MB: University of Manitoba. 63 p. M.S. thesis.

Winter, Jon. 1986. Status, distribution and ecology of the great gray owl (Strix nebulosa) in California. San Francisco, CA: San Francisco State University. 121 p. M.S.

thesis.
Appendix Scientific names of plants, birds, and mammals mentioned in this paper.

Common name Scientific name

Plants:

Dwarf-mistletoe

Grand fir

Western larch

Lodgepole pine

Ponderosa pine

Douglas-fir

Birds:

Northern goshawk

Red-tailed hawk

Great gray owl

Great horned owl

Northern flicker

Common raven

American robin

Mammals:

Shrew

Vagrant shrew

Pacific mole

Columbian ground squirrel

Golden-mantled squirrel

Yellow pine chipmunk

Red squirrel

Northern flying squirrel

Northern pocket gopher

Deer mouse

Bushytail woodrat

Heather vole

Redback vole

Mountain vole

Longtail vole

Richardson vole

Domestic mouse

Gerbil

Coyote

Arceuthobium spp.

Abies grandis (Dougl. ex D. Don) Lindl.

Larix occidentalis Nutt.

Pinus contorta Dougl. ex Loud.

P. ponderosa Dougl. ex Laws.

Pseudotsuga menziesii (Mirb.) Franco

Accipiter gentilis

Buteo jamaicensis

Strix nebulosa

Bubo virginianus

Colaptes auratus

Corvus corax

Turdus migratorius

Sorex spp.

S. vagrans

Scapanus orarius

Citellus columbianus

C. lateralis

Eutamius amoenus

Tamiasciurus hudsonicus

Glaucomys sabrinus

Thomomys talpoides

Peromyscus maniculatus

Neotoma cinerea

Phenacomys intermedius

Clethrionomys gapperi

Microtus montanus

M. longicaudus

M. richardsoni

Mus musculus

Gerbilus sp.

Canis latrans
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Bull, Evelyn L.; Henjum, Mark G. 1990. Ecology of the great gray owl. Gen. Tech. Rep.

PNW-GTR-265. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific

Northwest Research Station. 39 p.

Information is needed on the great gray owl to understand its ecology and to consider

this species in land management decisions. From 1982 to 1988, we studied 24 pairs and

107 juvenile great gray owls in northeastern Oregon. Forty-nine nests were located; 16

were used more than once, so we observed 71 nesting attempts. Seventy-eight percent

of these nesting attempts were successful in raising 1 to 5 young (mean = 2.2). The

nests were on stick platforms, on top of broken-off dead trees, and on artificial wooden

platforms. Nest trees occurred in a variety of habitats, although most were in mature or

older, unlogged stands of mixed conifer. Diet by biomass consisted mainly of northern

pocket gophers (67 percent) and voles (27 percent). The size of the home range for

16 adult owls and 19 juvenile owls averaged 67 square kilometers and 157 square

kilometers, respectively. Management practices enhancing habitat for great gray owls

include providing artificial nest platforms, protecting existing nest platforms and largediameter dead trees, providing dense tree cover around or adjacent to the nest, and

providing perches for recently fledged young.

Keywords: Owl, great gray owl, management, conifer forest, Oregon.

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