United States Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical Report 

RMRS-GTR-148-WWW

January 2005

Winter Movements and 

Range Use of Radio-marked 

Mexican Spotted Owls:

An Evaluation of Current

Management Recommendations

Joseph L. Ganey and William M. Block
Rocky Mountain Research Station

Natural Resources Research Center

2150 Centre Avenue, Building A

Fort Collins, CO 80526

Ganey, Joseph L.; Block, William M. 2005. Winter movements and range use of radio-marked Mexican 

spotted owls: an evaluation of current management recommendations. Gen. Tech. Rep. RMRS-GTR148-WWW. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station. 9 p.

Abstract

We summarized existing knowledge on winter movements and range and habitat use of radio-marked Mexican 

spotted owls. In light of that information, we evaluated the adequacy of current management guidelines. Seasonal 

movement or “migration” appears to be a regular feature of the winter ecology of Mexican spotted owls. Most 

radio-marked owls studied were resident in and around their breeding areas year-round, but some owls migrated 

in most populations studied. Owls that were year-round residents generally expanded their home range during 

the non-breeding season, and many exhibited spatial shifts in area used. Despite these shifts, however, overlap 

between seasonal ranges was relatively great for most individuals. For these owls, current guidelines aimed at 

conserving nesting habitat also would conserve areas used during the non-breeding season. Additional recovery 

plan guidelines aimed at protecting habitats with structure similar to nesting areas should be useful in protecting 

other areas used by resident owls expanding their range during winter. In contrast, migrating owls typically 

moved to lower elevations and into open habitats not used by breeding owls. Current guidelines do not protect 

these habitats. However, we currently have (1) no evidence that such habitats are limiting, (2) no evidence 

that special protection is necessary in these areas and/or habitats, (3) little information on which to base such 

protective measures if they are necessary, and (4) no objective way to identify important wintering areas used by 

migrating owls. Until better information is available, we see no compelling reason to develop specific guidelines 

for protection of wintering areas.

Key words: Mexican spotted owl, Strix occidentalis lucida, seasonal movements, migration, conservation planning, 

range use, winter

The Authors

Joseph L. Ganey is a Research Wildlife Biologist with the Rocky Mountain Research Station at the Southwest 

Forest Science Complex in Flagstaff, AZ. He holds a B.S. degree in Wildlife Management from Humboldt State 

University, California, and an M.S. degree in Biology and a Ph.D. degree in Zoology, both from Northern Arizona 

University.

William M. Block is a Project Leader and Supervisory Research Wildlife Biologist with the Rocky Mountain 

Research Station at the Southwest Forest Science Complex in Flagstaff, AZ. He holds a B.A. degree in Economics 

from San Diego State University, a B.S. degree in Fish and Wildlife Biology from Michigan State University, an 

M.S. degree in Wildlife Management from Humboldt State University, and a Ph.D.degree in Wildland Resource 

Science from the University of California, Berkeley.

Cover photo: Wintering Mexican spotted owl roosting in a juniper tree. Even in relatively 

open woodlands, owls managed to find roost sites that provided shade and protection 

from predators. All photos by J. L. Ganey
USDA Forest Service RMRS-GTR-148-WWW. 2005 1

Introduction

The Mexican spotted owl (Strix occidentalis 

lucida) occurs widely throughout the mountains and 

canyonlands of the American southwest and well into 

the mountains of Mexico (Gutiérrez and others 1995; 

Ward and others 1995). This owl typically is associated 

with either mature forests or rocky canyonlands (Ganey 

and Dick 1995; Gutiérrez and others 1995; Rinkevich 

and Gutiérrez 1996; Johnson 1997; Willey 1998). It was 

listed as threatened in 1993, primarily because of concerns over the effects of timber harvest and wildfire on 

its forest habitat (USDI 1993).

As with many threatened species, the Mexican spotted owl and its ecology have received considerable 

research and management attention. Most studies focused on breeding-season ecology, however, so we know 

relatively little about the ecology of this owl during winter. This knowledge gap is important. Winter is a period 

of energetic stress for many birds (e.g., Greenwood and 

others 1992, Newton 1998) and may be a critical period 

for these owls as well. For example, 9 of 11 mortalities documented in studies of radio-marked spotted owls 

in Arizona and New Mexico occurred from November 

through February (J. L. Ganey, unpublished data).

The recovery plan for the Mexican spotted owl (USDI 

1995) focused conservation measures on habitats used 

during the breeding season. It provided no specific recommendations for wintering habitat. This recovery 

plan is now in revision, and numerous individuals have 

recommended including protective measures for wintering habitat in this revision (personal observation). Based 

on these pleas, we re-examined the issue of wintering 

habitat for Mexican spotted owls. Our objectives were to 

(1) summarize information in the literature on seasonal 

movements and wintering habitats of Mexican spotted 

owls, (2) develop new information where appropriate 

and feasible, based on existing data, and (3) evaluate the 

adequacy of recovery plan guidelines in light of that information.

Methods

Data Sources and Analysis

We summarized information from the literature on 

seasonal home-range size, activity-center size, movements, and habitat use of radio-marked Mexican spotted 

owls (Ganey and Balda 1989; Ganey and others 1999; 

Zwank and others 1994; Johnson 1997; Willey 1998). 

These studies were conducted in seven study areas in 

four states and represented a variety of landforms and 

forest and woodland cover types (table 1; additional details on study area characteristics, study duration, and 

specific field methods are contained in the original reports). Some studies provided data on both seasonal 

home ranges and movements (Willey 1998, Ganey and 

others 1999), whereas others provided data only on 

seasonal movements (Ganey and Balda 1989, Johnson 

1997). All studies providing seasonal home-range estimates recognized breeding and non-breeding seasons 

following Ganey and Balda (1989; breeding season = 1 

Mar – 30 Aug).

We supplemented this information with unpublished 

data on radio-marked owls (Skaggs 1990, J. L. Ganey 

and W. M. Block, unpublished data), and both published 

and unpublished accounts of seasonal movements of color-banded owls that were not radio-marked (Gutiérrez 

and others 1996, Duncan and Speich 2002). We also reanalyzed existing data for radio-marked owls from three 

study areas in Arizona and New Mexico (described in 

Ganey and others 1999, 2000) to summarize those aspects of winter range use that were not addressed in the 

original papers but that were relevant to evaluating adequacy of recovery plan guidelines for habitat protection 

(see below).

Previous studies documented winter movements or 

“migration” of some radio-marked owls, whereas other 

owls remained in or near their breeding areas throughout the winter. Migration typically is defined as a regular 

seasonal movement from a breeding to a wintering area 

(e.g., Welty 1975: 463). Movements documented in 

Mexican spotted owls were highly variable in terms of 

Winter movements and range use of radio-marked 

Mexican spotted owls: an evaluation of current 

management recommendations

Joseph L. Ganey and William M. Block
2 USDA Forest Service RMRS-GTR-148-WWW. 2005

both distance moved and duration, however. In some 

cases, these movements did not appear to fit the classic 

definition of migration, because they were of relatively 

short distance or duration. For this discussion, we operationally defined all owls that moved <2 km from their 

breeding-season home-range center as residents, and 

all other owls as “migrants.” This distance criterion approximated mean nearest-neighbor distances reported 

in studies of Mexican spotted owls in Arizona (2.4 km, 

n = 42 pairs; May and Gutiérrez 2002) and New Mexico 

(2.1 km, n = 31 pairs; Peery and others 1999). We recognize that a gradient exists in terms of both distance and 

duration of winter movements of these owls, however.

For migrating owls, we asked:

1. How common are winter movements outside the home 

range?

2. What habitats are used by migrating owls?

3. Can important wintering areas and/or habitats be identified objectively?

4. Are specific protective measures needed for such areas and/or habitats?

5. If so, do recovery plan guidelines provide the necessary protection?

For resident owls, we evaluated changes in range use 

and habitats used between the breeding and non-breeding season, and spatial overlap between seasonal ranges. 

Because recovery plan guidelines were based on activity-center size, we present estimates for activity-center 

size (based on the 75% adaptive kernel contour; USDI 

Figure 1—Biologist John Stephenson radio tracking Mexican spotted 

owls during winter, San Francisco Peaks, Arizona.

Table 1—Characteristics of study areas where movements of radio-marked Mexican spotted owls were monitored.

Study area State General landform Primary cover types1 Source2

San Francisco Peaks Arizona Montane slopes Mixed-conifer, ponderosa pine 1

Walnut Canyon Arizona Incised canyon Mixed-conifer, ponderosa pine, riparian, 1

ponderosa pine / pinyon-juniper / oak

Sacramento Mountains2 New Mexico Montane slopes and canyons Mixed-conifer, ponderosa pine, 2

pinyon-juniper

Sacramento Mtns – mesic3 New Mexico Montane slopes and canyons Mixed-conifer

Sacramento Mtns. – xeric3 New Mexico Montane slopes and canyons Mixed-conifer, ponderosa pine, 3

pinyon-juniper

Bar-M Canyon Arizona Rolling hills, cinder cones Ponderosa pine-Gambel oak, 4

ponderosa pine

Colorado Colorado Incised canyons Mixed-conifer, pinyon-juniper, 5

ponderosa pine

Southern Utah Utah Incised canyons Pinyon-juniper, mixed-conifer 6

1 Dominant species: Mixed-conifer = Douglas-fir (Pseudotsuga menziesii) and/or white fir (Abies concolor), with ponderosa pine (Pinus ponderosa) and southwestern white pine (P. 

strobiformis) common. Ponderosa pine = ponderosa pine. Riparian = box-elder (Acer negundo), narrowleaf cottonwood (Populus angustifolia), and quaking aspen (P. tremuloides). 

Ponderosa pine / pinyon-juniper / oak = ponderosa pine, pinyon pine (P. edulis), various juniper species (Juniperus spp.), and Gambel oak (Quercus gambelii). Pinyon-juniper = pinyon pine, 

various species of junipers. Ponderosa pine-Gambel oak = ponderosa pine, Gambel oak.

2

 Sources: 1 = Ganey and Balda (1989); 2 = Skaggs (1990), Zwank and others (1994): 3 = J. L. Ganey and W. M. Block (unpublished data); 4 = Ganey and others (1999); 5 = Johnson (1997); 

6 = Willey (1998).

3

 J. L. Ganey and W. M. Block (unpublished data) recognized two distinct study areas in the Sacramento Mountains, whereas Skaggs (1990) and Zwank and others (1994) pooled owls across 

these areas when estimating size of seasonal home ranges. The mesic area was dominated by mixed-conifer forest, the xeric area by drier forest and woodland types. See Ganey and 

others (2000) for more details on study areas.
USDA Forest Service RMRS-GTR-148-WWW. 2005 3

1995) where possible. We supplemented that information with estimates of home-range size where estimates 

were not available for activity-center size. We also summarized shifts in spatial use patterns of resident owls in 

terms of proportional overlap between seasonal activity centers of individual owls. We used two measures of 

seasonal overlap. One measured the proportion of the 

breeding-season activity center contained within the 

non-breeding-season activity center; the other measured 

the proportion of the non-breeding-season activity center contained within the breeding-season activity center.

We asked the following questions about resident 

owls:

1. Do owls use larger ranges during the non-breeding 

season than during the breeding season?

2. To what extent do seasonal ranges overlap spatially?

3. Do owls use the same types of habitat during the 

breeding and non-breeding seasons?

4. In light of the above, do recovery plan guidelines offer 

sufficient protection to wintering areas and habitats?

Recovery Plan Guidelines Evaluated

The recovery plan for the Mexican spotted owl recognized three primary habitat categories: protected 

habitat, restricted habitat, and other forest and woodland 

types (USDI 1995:84-96). Protected habitat included 

Protected Activity Centers (PACs) and mixed-conifer 

and pine-oak forests on steep slopes with no history of 

timber harvest within the past 20 yrs. PACs were areas 

of ≥243 ha established around nesting areas of resident 

owls, to protect those nesting areas (USDI 1995:84-89). 

PAC size was based on the median size of activity centers 

(as defined by the 75% adaptive kernel estimate) used 

throughout the year by 14 pairs of radio-marked owls. 

Steep-slope mixed-conifer and pine-oak forests without 

recent timber harvest were included to protect habitats 

similar in structure to nesting areas (USDI 1995:89-90). 

As the name implies, PACs and protected habitat were 

“protected” from many management treatments.

Restricted habitat included the remainder of the 

mixed-conifer and pine-oak cover types (i.e., not on 

steep slopes or included in PACs), as well as riparian 

habitats. Management was more flexible in restricted 

habitat than in protected areas. Guidelines called for 

managing a specified portion of the overall landscape 

as replacement owl habitat, with the remainder open to 

most forms of management (USDI 1995:90-95).

Cover types not specifically classified as protected or 

restricted habitats fell in the “other forest and woodland” 

category. The recovery plan included no specific recommendations for these cover types (USDI 1995:95-96).

Results and Discussion

Seasonal Migration

Seasonal movements or migration occurred in most 

areas where movements of radio-marked owls were 

monitored (table 2). Seasonal migration generally 

involved a subset of the population, with the size of that 

subset varying both among study areas and among years 

within study areas. For example, seasonal migration was 

observed in the Sacramento Mountains, New Mexico, 

during the winter of 1989 – 1990, but not in two subsequent studies in the same mountain range (table 2). 

Similarly, movements of one radio-marked female from 

the Bar-M Canyon study area were monitored over three 

winters. This owl was resident during the first winter but 

migrated in each of the following two winters (table 3).

Migrating owls typically left study areas in November 

or December and returned from January to April 

(table 3). Distance moved ranged from 5 km to 50 km for 

owls whose wintering areas were located. Wintering areas of two owls from the San Francisco Peaks could not 

be located, despite an aerial search covering thousands 

of square kilometers. This suggests that some owls may 

Table 2—Numbers of radio-marked Mexican spotted owls observed to migrate during the winter in various studies.

Number of owls Number of owls 

Study area Years radio-marked migrating1 Source

San Francisco Peaks 1986-1987 4 2 Ganey and Balda (1989)

Walnut Canyon 1986-1987 2 2 Ganey and Balda (1989)

Sacramento Mountains 1989-1990 8 4 Skaggs (1990)

1990-1991 9 0 Zwank and others (1994)

1992-1995 15 0 J. L. Ganey and W. M. Block (unpublished data)

Bar-M Canyon 1990-1993 13 3 Ganey and others (1999)

Southern Utah 1991-1995 15 2 Willey (1998)

Colorado 1992-1996 5 3 Johnson (1997)

1

 Migration was defined as movement >2 km from the center of the breeding-season home range. See table 3 for information on movements of individual owl.
4 USDA Forest Service RMRS-GTR-148-WWW. 2005

Table 3—Distance moved, movement duration, elevation change, and habitats used by migrating Mexican spotted owls.

 Distance Elevation Cover

Study area Years Sex moved (km) Duration change (m)1 type2 Source3

San Francisco Peaks 1986-87 F Unknown4 Nov – Apr Unknown Unknown 1

San Francisco Peaks 1986-87 M Unknown4 Nov – Apr Unknown Unknown 1

Walnut Canyon 1986-87 F 10 Dec-Jan 100 MC, PP, P/O/J, R 1

Walnut Canyon 1986-87 M 10 Dec-Jan 100 MC, PP, P/O/J, R 1

Sacramento Mtns. 1989-90 F 10-24 Unknown Unknown PJW, SDS 2

Sacramento Mtns. 1989-90 M 10-24 Unknown Unknown PJW, SDS 2

Sacramento Mtns. 1989-90 M ~5 Unknown Unknown MC 2

Sacramento Mtns. 1989-90 M ~11 Unknown Unknown MC 2

Bar-M Canyon 1990-91 M 10 Dec-Jan 0 PO 3

Bar-M Canyon 1990-91 M 50 Dec-Apr 920 PJW 3

Bar-M Canyon5 1991-92 F 50 Dec-Apr 920 PJW 3

Bar-M Canyon5 1992-93 F 50 Dec-Apr 920 PJW 3

Colorado 1992 M 7 Nov-Apr 407 PJW, PP 4

Colorado 1995 F 17 Nov-Feb 335 PJW, PP 4

Colorado 1995 M 7 Dec-Jan 182 PJW, PP 4

Utah Unknown F 25 Unknown +913 SF 5

Utah Unknown M 20 Unknown 685 MS 5

1

 Elevation changes are negative unless otherwise indicated.

2

 Cover types: MC = mixed-conifer forest, MS = mountain shrub, PJW = pinyon-juniper woodland, PO = ponderosa pine-Gambel oak forest, PP = ponderosa pine forest, P/O/J = ponderosa 

pine/oak/juniper, R = riparian, SDS = semi-desert scrub; SF = spruce-fir forest.

3

 Sources: 1 = Ganey and Balda (1989); 2 = Skaggs (1990); 3 = J. L. Ganey and W. M. Block (unpublished data); 4 = Johnson (1997: table 5); 5 = Willey (1998:54-55).

4

 Wintering areas not located despite an aerial search covering thousands of square kilometers.

5

 These two records represent one female owl that migrated to the same area in two consecutive winters. This owl did not migrate in the winter of 1990-1991 (but her mate did).

Figure 2—Overview of relatively open pinyon-juniper woodland used during winter by radio-marked Mexican spotted owls 

from the Bar-M study area, Arizona. This type of habitat was used for nocturnal foraging.
USDA Forest Service RMRS-GTR-148-WWW. 2005 5

move long distances. Gutiérrez and others (1996) provided another possible example of long-distance winter 

movement. They banded an adult female owl in 1994 as 

a member of a territorial pair in the Tularosa Mountains, 

New Mexico. This owl was found dead in January 1995 

in the Chihuahuan desert near Deming, New Mexico, 

187 km from the banding location. Gutiérrez and others 

(1996) described this as an example of adult dispersal, 

but given movements of radio-marked owls to lowelevation habitats during winter, we cannot rule out seasonal migration as an alternative explanation.

Some non-resident owls moved only short distances and remained in the same forest types represented in 

their breeding-season home ranges (table 3). In contrast, 

other owls moved down in elevation, and most of these 

down-slope migrants wintered in habitats more open in 

structure than typical breeding habitat, such as semidesert scrub, pinyon (Pinus spp.) – juniper (Juniperus

spp.) woodland, or mountain shrub (table 3). Duncan 

and Speich (2002) provided additional evidence for 

such down-slope migration. They documented a subadult owl overwintering in Sonoran riparian deciduous 

woodland at 838 m in the foothills of the Santa Catalina 

Mountains, Arizona. They relocated this owl five years 

later as a member of a territorial pair near the summit of 

this range, in mixed-conifer forest at an elevation of 2682 

m. We also are aware of numerous anecdotal observations of Mexican spotted owls in woodland, semi-desert, 

and desert cover types during the winter months. In most 

cases, it is impossible to tell whether these represent migrating territorial owls or dispersing juveniles, which 

use similar habitats (Arsenault and others 1997; Ganey 

and others 1998; Willey and van Riper 2000; Duncan 

and Speich 2002).

Thus, available information suggests that seasonal 

migration of some individuals occurs in many or most 

populations of Mexican spotted owls, and that such migration occurs in both sexes (table 3). Partial migration 

also occurs in California spotted owls (S. o. occidentalis; Laymon 1989; Verner and others 1992). In contrast, 

migration appears to be rare in Northern spotted owls (S. 

o. caurina; Gutiérrez and others 1995).

Reasons why only some owls migrate are unknown. 

In addition, some individual Mexican spotted owls migrate in some years but not others (table 3). Migration 

generally entails a change in elevation for both Mexican 

(table 3, Duncan and Speich 2002) and California spotted owls (Laymon 1989, Verner and others 1992), with 

most owls moving down slope (but see Willey 1998). 

Migration to lower elevations allows owls to winter in 

Figure 3—Relatively dense pinyon-juniper woodland used during winter by a radio marked-Mexican spotted owl in Arizona. 

This area was frequently used for diurnal roosting. 
6 USDA Forest Service RMRS-GTR-148-WWW. 2005

areas that are warmer than their breeding areas during the winter and that lack persistent snow. This may 

facilitate hunting for small mammals, which comprise 

the bulk of the diet (Ward and Block 1995). It also may 

allow the owls to move to areas with more concentrated prey resources, as populations of small mammals 

reach their nadir in owl breeding areas during the winter months (Ward and Block 1995). From a conservation 

perspective, some migrating owls occupy cover types 

that fall in the “other forest and woodland types” category, and thus have no protective measures under the 

recovery plan (USDI 1995). These cover types also are 

used by dispersing juvenile owls during the fall and winter (Arsenault and others 1997; Ganey and others 1998; 

Willey and van Riper 2000; Duncan and Speich 2002).

Also presently unknown is how and why migrating 

owls select particular wintering areas. For example, the 

two migrating owls from the Bar-M Canyon study area 

moved 50 km to winter in pinyon-juniper woodlands. 

These owls were members of a mated pair but did not 

migrate together or even in the same year. They occupied adjacent but non-overlapping winter ranges. Much 

of the area between these wintering areas and their 

breeding area consisted of pinyon-juniper woodland that 

was, at least superficially, similar to the wintering areas 

occupied.

At present, we have little information on specific habitat features that migrating spotted owls use in wintering 

areas (but see Peterson 2003). Further, owls use these 

areas at a time of year when they are unlikely to vocalize (Ganey 1990), making it difficult to locate such areas 

through calling surveys. This leaves us with no objective means to identify and protect such areas. The types 

of lowland areas in which wintering owls have been observed cover vast areas, and we have no evidence that 

suitable wintering areas are limiting. Thus, we see little 

evidence that specific protective measures for wintering 

areas or habitats used by migrating spotted owls would 

be useful or appropriate at this time.

Range Use by Resident Owls

Resident owls expanded their home range during the 

non-breeding season in all areas where seasonal range 

estimates were available, although the magnitude of 

this seasonal expansion varied among areas (table 4). 

Clearly, owl home ranges were larger than the areas 

protected in PACs. PAC size was based on the size of 

owl activity centers, however, not home ranges. Size of 

owl activity centers was more comparable to PAC size 

in the three study areas where estimates were available (table 4). Further, spatial overlap between seasonal 

Table 4—Size (ha) of home ranges or activity centers (where available) of radio-marked Mexican spotted owls during the breeding and nonbreeding seasons, as estimated in various studies. Seasons followed Ganey and Balda (1989) in all studies (breeding season = 1 Mar – 30 

Aug). N = number of owls included in estimates. Home range estimates based on the minimum convex polygon estimator (Zwank and others 

1994) or 95% adaptive kernel estimator (Ganey and others 1999; Willey 1998; J. L. Ganey and W. M. Block unpublished data). Activity 

centers based on the 75% adaptive kernel estimator.

Breeding season Non-breeding season

Study area N Mean SE N Mean SE Source

Home-range size

Sacramento Mountains1 9 278 75.32 9 365 100.32 Zwank and others (1994)

Sacramento Mtns. – mesic1 6 228.1 37.3 6 328.0 59.3 J. L. Ganey and W. M. Block

 (unpublished data)

Sacramento Mtns. – xeric1 6 458.9 83.4 6 895.5 304.8 J. L. Ganey and W. M. Block

 (unpublished data)

Bar-M Canyon 8 392.5 70.0 143 948.9 146.8 Ganey and others (1999)

Southern Utah 13 506 143.14 13 1028 174.74 Willey 1998

Activity-center size

Sacramento Mtns. – mesic1 6 69.9 9.5 6 89.3 14.4 J. L. Ganey and W. M. Block

 (unpublished data)

Sacramento Mtns. – xeric1 6 156.2 22.0 6 352.1 131.1 J. L. Ganey and W. M. Block

 (unpublished data)

Bar-M Canyon 8 121.7 21.8 143 326.5 53.8 Ganey and others (1999)

1

 Zwank and others (1994) pooled owls within the Sacramento Mountains when estimating size of seasonal home ranges, whereas J. L. Ganey and W. M. Block (unpublished data) recognized 

two distinct study areas in the Sacramento Mountains. The mesic area was dominated by mixed-conifer forest, the xeric area by drier forest and woodland types.

2 SE recomputed from data in Zwank and others (1994)

3

 Fourteen range estimates computed for 13 individual owls. One radio-marked female dispersed to a new territory during the study, so two separate range estimates were computed for this 

owl, in different years.

4 SE recomputed from data in Willey (1998).
USDA Forest Service RMRS-GTR-148-WWW. 2005 7

activity centers was considerable (table 5). Most of the 

breeding-season activity center was contained in the 

non-breeding-season activity center. The mean proportion of the non-breeding-season activity center contained 

in the breeding-season activity center was lower, but it 

still indicated considerable spatial overlap. Further, the 

smaller size of breeding-season activity centers relative 

to non-breeding-season activity centers effectively limits the possible range for this measure of overlap. Thus, 

protection of nesting areas provides protection to areas 

and habitats used throughout the year, not only during 

the breeding season.

Habitat Use by Resident Owls

Little detailed information is available on habitat use 

by resident owls during the non-breeding season. Zwank 

and others (1994) reported that owls in the Sacramento 

Mountains roosted in “shorter trees with less dense foliage” during the winter. Willey (1998:73) reported that 

“During winter... spotted owls were observed roosting 

in more open habitats...”. He also reported that some 

owls moved out of steep slickrock canyon terrain and 

into forested uplands during winter. Johnson (1997:49) 

noted that wintering owls used “... canyons with a 

north-south orientation dominated by pinyon/juniper 

woodlands with scattered patches of ponderosa pine.” 

This latter description included owls classified as both 

residents and migrants here.

Relative areas of cover types did not differ between 

seasonal ranges of radio-marked owls in the Bar-M 

Canyon area, but relative area in canopy-cover classes 

did (Ganey and others 1999). Relative to non-breedingseason ranges, breeding-season ranges contained more 

area with canopy cover ≥60% and less area with 20-

39% canopy cover. Structural features of forest stands 

used by foraging owls did not differ between seasons, 

but structure of stands used by roosting owls did. Stands 

used for roosting during the breeding season had greater live-tree basal area, oak basal area, and canopy cover 

than stands used during the non-breeding season (Ganey 

and others 1999: table 5). In an analysis focused on a 

finer spatial scale, Ganey and others (2000) concluded 

that canopy cover surrounding roost “microsites” also 

was greater during the breeding than the non-breeding 

season in this area.

Ganey and others (2000, 2003) reported on aspects 

of habitat use by radio-marked owls in the Sacramento 

Mountains at these same spatial scales (stand and 

roost microsite). They found little evidence for differences in seasonal habitat use at either scale within the 

Sacramento Mountains, for either foraging or roosting 

use.

Thus, there is some evidence for shifts in habitats 

and areas used in some areas, but not in others. In general, evidence for seasonal differences in habitat use 

appears strongest where owls occupy rocky canyons 

in the northern portion of their range. In mixed-conifer forests farther south, few seasonal differences were 

observed, and patterns were intermediate in an area 

where owls occupied pine-oak forest. Where owls do 

use different habitat during the winter, available evidence suggests that those habitats generally are more 

open in structure. As most recovery plan guidelines are 

aimed at retaining structural features typical of closedcanopy forests, special protections may not be as 

necessary where owls are using more open forest (or 

other cover) types.

Table 5—Spatial overlap between seasonal activity centers (estimated as the 75% adaptive kernel contour) of radio-marked Mexican spotted 

owls in three study areas1. Only owls with valid range estimates during both the breeding and non-breeding seasons were included in 

estimates.

% of breeding-season % of non-breeding-season 

activity center contained activity center contained 

in non-breeding-season in breeding-season 

activity center activity center

Observed Theoretical maximum2

Study area N Mean SE Mean SE Mean SE

Sacramento Mountains –mesic 6 85.7 4.6 66.9 5.6 82.6 8.4

Sacramento Mountains – xeric 6 78.0 7.1 48.8 6.7 65.4 14.0

Bar-M Canyon 8 91.4 3.1 35.5 9.0 42.6 13.0

1

 Based on data from a sample of radio-marked owls included in Ganey and others (1999, 2000, 2003). The parameters estimated here were not included in previous papers.

2

 Theoretical maximum proportion of the nonbreeding-season activity center that could be contained in a breeding-season activity center given observed size of those breeding-season activity 

centers, assuming maximum spatial overlap.
8 USDA Forest Service RMRS-GTR-148-WWW. 2005

Management Implications

Seasonal migration of some individuals appears to be 

a regular feature of Mexican spotted owl populations. 

Although some owls moved short distances and remained 

within the same general cover type, others moved considerable distances down slope into more open habitats 

than those used by breeding owls. These habitats are not 

protected under current recovery plan guidelines, nor do 

guidelines exist for protecting specific wintering areas. 

We have no clear understanding of how and why owls 

select wintering areas, however, and no clear evidence 

that wintering areas or habitat are limiting. Further, we 

currently have no objective means to identify appropriate areas or habitats for protection. Given this inability 

to focus recommendations on the appropriate areas and 

lacking clear evidence that specific protections are necessary for such areas or habitats, we recommend against 

including specific guidelines to protect lower-elevation 

wintering areas or habitats in the recovery plan.

Clearly, PACs would not protect all of the large home 

ranges used by resident owls during the non-breeding 

season, but they could potentially protect a good portion 

of non-breeding-season activity centers (range = 69% 

- 272%, based on size of non-breeding-season activity 

center; table 4). Overlap between seasonal ranges was 

substantial in all study areas (table 5), suggesting that 

most resident owls used much of the same area throughout the year. Available evidence also suggested that 

resident owls used similar habitats during the breeding 

and non-breeding seasons. This appeared to be truer in 

forested area than where owls occurred in canyon situations (e.g., Johnson 1997, Willey 1998). Thus, forested 

areas managed to protect nesting habitat should provide habitat useful to these owls throughout the year. In 

addition, recovery plan guidelines aimed at providing additional habitat similar in structure to nesting areas (see 

USDI 1995:89 – 92) should aid in protecting additional 

habitat near PACs, including areas used by owls expanding their home range during the non-breeding season.

Like movements of longer distance, the shortdistance movements undertaken by some owls also present a potential problem. Reasons for these movements 

are unknown, but the areas visited may provide important resources and thus be worthy of special management 

consideration. Such areas are essentially impossible to 

locate where owls are not radio-marked, but most owls 

undertaking these movements used habitats similar to 

those in their breeding areas. Consequently, we again 

argue that recovery plan recommendations focused 

on providing additional habitat similar in structure to 

nesting areas (see USDI 1995: 89 – 92) should provide 

considerable protection to these areas.

We recognize that winter likely is an important period for Mexican spotted owls, and that we still know 

relatively little about their ecology during this period. 

Addressing that knowledge gap is an obvious research 

priority. Until better information is forthcoming, however, 

we suspect that current recovery plan recommendations 

are sufficient to protect appropriate wintering habitats 

near nesting areas. The recovery plan currently contains 

no guidelines to protect remote wintering areas of migrating owls. Given uncertainty over the location of such 

areas, as well as the lack of evidence that special protective measures are needed in these areas, we see no 

compelling reason to change that situation.

Acknowledgments

We thank the many dedicated field-crew members 

who collected the radio telemetry data summarized here. 

W. S. LaHaye, T. Theimer, D. Willey, J. C. Bednarz, and 

two anonymous reviewers commented on earlier drafts 

of this paper.

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