Diets of California Spotted Owls in the 

Sierra National Forest1 

Thomas E. Munton,2 Kenneth D. Johnson,2 George 

N. Steger,

2 and Gary P. Eberlein2 

Abstract 

From May 1987 through October 1992 and from July through August 1998, we studied 

diets of California spotted owls (Strix occidentalis occidentalis). Regurgitated pellets were 

collected at roost and nest sites between 1,000 and 7,600 ft elevation in the Sierra National 

Forest and were examined for remnant bones, feathers, and insect exoskeletons. Remains 

of 2,038 individual prey were identified in 1,140 pellets. Woodrats (Neotoma spp.) were the 

predominant prey in low-elevation oak woodlands and riparian-deciduous forests, 

accounting for 74.3 percent of the biomass in diets during the breeding period and 81.9 

percent during the nonbreeding period. In coniferous forests, northern flying squirrels 

(Glaucomys sabrinus) were the major prey, comprising 45.6 percent and 77.3 percent of 

prey biomass during the breeding and nonbreeding periods, respectively. Pocket gophers 

(Thomomys spp.) were the second largest component of owl diets, by biomass, in both 

vegetation types. In the breeding period, birds were a larger part of the owl’s diet in 

coniferous forests (12.9 percent) than in the riparian-deciduous and oak habitats (1.8 

percent). Other small mammals, insects, and lizards were also found in pellets. Diets 

differed among years. 

Concern over the status of the California spotted owl (Strix occidentalis 

occidentalis) increased when the U. S. Fish and Wildlife Service listed the northern 

spotted owl (S. o. caurina) and then the Mexican spotted owl (S. o. lucida) as 

“threatened.” Although not currently federally listed, the California spotted owl 

is classified as a “sensitive species” by the Pacific Southwest Region (Region 5) of 

the USDA Forest Service. 

Several studies have examined diets of California spotted owls in the Sierra 

Nevada (Kadoch 1997, Laymon 1988, Marshall 1942, Thrailkill and Bias 1989, Verner 

and others 1992), but additional information on diets is needed to better understand 

the relations between owl occurrence and breeding and the availability of prey. An 

adequate understanding of diets can then guide studies of the ecological 

requirements of key prey species and help to guide forest management in ways that 

will favor conditions for the prey species and, in turn, for the owls. 

We studied California spotted owl diets in the southern Sierra Nevada to 

describe prey composition and to examine annual and seasonal variations in the 

diets. We also compared diets between owls in coniferous forests and owls in oak 

woodlands and riparian-deciduous forests. 

Study Area 

We studied spotted owl diets in a 265-mi2 study area in the Sierra National 

Forest, Fresno County, California, about 30 mi east of Fresno (Steger and others, 

in this volume). Twenty-three percent of the study area was below 4,000 ft in 

1 	An abbreviated version of this 

paper was presented at the 

Symposium on the Kings River 

Sustainable Forest Ecosystems 

Projects: Progress and Current 

Status, January 26, 1998, Clovis, 

California. 

2	 Wildlife Biologist, Wildlife 

Bioligist, Biological Technician, 

and Statistical Assistant, 

respectively, Pacific Southwest 

Research Station, USDA Forest 

Service, 2081 E. Sierra Ave., 

Fresno, CA 93710. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 99 
Munton, Johnson, Steger, Eberlein 

100 

Diets of California Spotted Owls in the Sierra National Forest 

elevation, in oak-pine woodlands, blue oak (Quercus douglasii) savannas, and 

dense riparian-deciduous forests (Verner and Boss 1980). Coniferous forests 

covered the remainder of the study area, from about 4,000 to 9,600 ft in elevation. 

Ponderosa pine (Pinus ponderosa), sugar pine (Pinus lambertiana), white fir (Abies 

concolor), incense cedar (Calocedrus decurrens), and black oak (Quercus kelloggii) 

were the dominant tree species below 8,000 ft. Inclusions of Jeffrey pine (Pinus 

jeffreyi) and red fir (Abies magnifica) occurred near 8,000 ft. Above that, forests 

were dominated primarily by red fir, lodgepole pine (Pinus contorta), and western 

white pine (Pinus monticola). 

Methods 

Owls regurgitate the less-digestible parts of consumed prey (bones, feathers, 

exoskeletons, fur, and so on) in formed pellets. Prey remains in these pellets have 

long been used to study the diets of owls (Glading and others 1943, Marti 1987). 

From May 1987 through October 1992, and from July through August 1998, we 

studied diets of the owls by examining prey remains in regurgitated pellets 

collected below roost and nest sites. Pellets were classified as egested during 

either the breeding (March 1 through August 31) or nonbreeding (remainder of 

the year) period, and we analyzed only pellets that could be assigned 

unambiguously to either period. Identification to species was impossible for 

some specimens because remains were incomplete. 

Sampling intensity varied considerably among sites, seasons, and years. 

From May 1987 through March 1990, we collected pellets opportunistically 

throughout each year while studying home ranges and habitat use of radiotagged spotted owls. Thereafter, we collected pellets in conjunction with 

demographic studies, when most of the collected pellets were egested during the 

breeding period. Pellets collected at the same site on the same day, or within a 

brief period of a few days, were aggregated into one sample because remains of 

the same individual prey can be egested in more than one pellet (Forsman and 

others 1984). 

As described by Munton and others (1997), pellets were dissected and skeletal 

remains, feathers, and pieces of exoskeleton were identified to species when 

possible, using a reference collection of specimens from the Carnegie Museum of 

Natural History, together with selected references and keys (Borror and DeLong 

1964, Burt and Grossenheider 1976, Dunning 1984, Forsman and others 1984, 

Ingles 1990, Jameson and Peeters 1988, Robbins and others 1983, Swan and Papp 

1972). These same sources were used to assign weights to prey items. Prey 

species with mean weight of at least 3.5 oz were considered to be “large prey.” 

The number of individual prey items in a sample was tallied as the largest 

number of skulls or identical bone parts for each prey type per sample (the 

aggregate of pellets from a single site in one to a few days). The number of insects 

was estimated either from the largest number of heads, paired mandibles, or 

one-sixth the number of legs. The estimated mean weight of each prey type was 

multiplied by its observed frequency in each sample to convert counts to biomass. 

Results 

Coniferous Forests 

In 620 pellets collected in coniferous forests, more than 94 percent from the 

breeding period, we identified remains of 1,344 individual prey, an average of 

2.12 prey items per pellet. Excluding insects—highly variable in number in 

pellets and of little consequence to total biomass—pellets averaged 1.48 prey 

items each. At least 35 species (13 mammals, 11 birds, 10 insects, and 1 lizard) 

were present. Mammals comprised 57.8 percent of all prey and 87.2 percent of 

the biomass; the remainder consisted of birds, lizards, and insects (table 1). Large 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Diets of California Spotted Owls in the Sierra National Forest Munton, Johnson, Steger, Eberlein 

Table 1—Percent frequency and percent biomass of prey in California spotted owl pellets from coniferous forests, Sierra 

National Forest 

Breeding period1 Nonbreeding period2 Mean 

annual 

Wildlife Percent of Percent Percent of Percent percent of 

individuals biomass individuals biomass biomass 

Bats 0.7 0.1 0 0 0.1 

Birds 10.5 12.9 4.0 4.6 8.7 

Diurnal squirrels 1.0 0.9 0 0 0.4 

Flying squirrels 21.1 45.6 46.7 77.3 61.5 

Gophers 11.0 18.4 9.3 11.5 15.0 

Insects 32.3 0.6 24.0 0.3 0.5 

Lizards 0.2 <0.1 0 0 <0.1 

Mice 12.9 5.5 5.3 1.7 3.6 

Moles 1.8 1.5 2.7 1.7 1.7 

Shrews 1.7 0.2 6.7 0.6 0.4 

Unidentified 1.9 1.3 1.3 2.3 1.8 

mammals 

Voles 1.8 1.2 0 0 0.6 

Woodrats 3.0 11.8 0 0 5.9 

1 n = 1,269 individual prey. 

2 n = 75 individual prey. 

prey—flying squirrels (Glaucomys sabrinus), woodrats (Neotoma spp.), pocket 

gophers (Thomomys spp.), Steller’s jays (Cyanocitta stelleri), northern flickers 

(Colaptes auratus), a western screech-owl (Otus kennicottii), an unidentified quail, 

and some unidentified mammals—accounted for 36.5 percent of the individual 

prey items and 78.3 percent of the biomass. Over the course of a year, flying 

squirrels contributed the most biomass to owl diets, followed by pocket gophers, 

birds, and woodrats (table 1). Owls were seldom found above 8,000 ft in 

elevation, and we found no pellets above 7,600 ft, probably because of low 

sampling intensity there. 

We identified enough prey in five breeding periods to evaluate annual 

variations in biomass. The proportion of birds in diets was least variable (CV = 

16.0 percent, table 2), followed by flying squirrels (CV = 22.8 percent). 

Interestingly, flying squirrels were most prevalent (54.1 percent of the biomass) 

in diets in 1992, when 87 percent of owl pairs produced young. No clear pattern 

emerged, however, when comparing percent biomass of flying squirrels in the 

diet and overall reproductive success of owls in the study. For example, flying 

squirrels were only slightly less prevalent (52.6 percent of the biomass) in 1988 

diets, a year when pellets were collected from nonreproductive owls; and flying 

squirrels represented only 38.3 percent of the diet biomass in 1990, when 72 

percent of pairs fledged young. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 101 
Munton, Johnson, Steger, Eberlein Diets of California Spotted Owls in the Sierra National Forest 

Table 2—Annual variation in percent biomass of prey in pellets of California spotted owls during the breeding 

period in conifer habitats of the southern Sierra Nevada; sample sizes (numbers of individual prey in parentheses 

beneath each year) 

1988 1989 1990 1992 1998 Mean ± SD Percent 

Wildlife (83) (295) (227) (362) (278) CV 

Bats 0.1 0.1 0.2 0.1 0.1 0.1 ± 0.05 41.1 

Birds 10.1 14.5 13.1 11.4 15.0 12.8 ± 2.05 16.0 

Diurnal squirrels 1.7 0.3 1.0 0.2 2.1 1.1 ± 0.83 77.3 

Flying squirrels 52.6 47.7 38.3 54.1 30.1 44.5 ± 10.16 22.8 

Gophers 21.1 19.3 23.8 12.3 23.5 20.0 ± 4.70 23.5 

Insects 0.6 0.8 0.5 0.4 0.8 0.6 ± 0.20 33.1 

Lizards 0.0 0.0 0.1 0.0 0.0 <0.1 ± 0.03 223.6 

Mice 1.7 3.1 3.5 6.0 11.7 5.2 ± 3.95 75.9 

Moles 3.3 2.8 1.3 0.7 0.8 1.8 ± 1.18 66.4 

Shrews 0.1 0.4 0.1 0.1 0.3 0.2 ± 0.13 66.6 

Unidentified mammals 0.0 1.3 0.4 0.5 4.9 1.4 ± 2.02 141.8 

Voles 0.7 0.8 0.3 0.6 3.2 1.1 ± 1.20 106.3 

Woodrats 8.0 9.0 17.4 13.6 7.5 11.1 ± 4.28 38.6 

Oak Woodlands and Riparian-Hardwood Forests 

Data from low elevations in oak woodlands and riparian-deciduous forests are 

summarized from Munton and others (1997). In 520 pellets collected primarily 

during the nonbreeding period in 1988-1992, we identified remains of 664 

individual prey, an average of 1.28 per pellet. Excluding insects, pellets averaged 

only 0.96 prey items each, 35 percent fewer than identified in pellets from the 

conifer forests. At least 20 species—nine mammals (70.0 percent of prey by 

frequency, 96.5 percent by biomass), six birds, and five insects—were identified 

(table 3). Large prey accounted for 48.5 percent of the total by frequency and 91.8 

percent by biomass. Species included were woodrats, pocket gophers, western 

screech-owls, and unidentified mammals and owls. Woodrats were consistently 

the dietary staple—74.3 percent of prey biomass during the breeding period and 

81.9 percent during the nonbreeding period. Pocket gophers comprised only 12.9 

percent of the prey biomass over the full year; other mammal species, each 

contributing less than 5 percent of the total biomass, included mice (mainly 

Peromyscus spp.), voles (Microtus spp.), broad-footed moles (Scapanus latimanus), 

and shrews (Sorex spp.). Western scrub-jays (Aphelocoma californica) were the 

most numerous birds identified, followed by other species of owls. 

Discussion 

Diets of spotted owls at higher elevations (coniferous forests) and at lower 

elevations (oak woodlands and riparian-deciduous forests) were similar in 

dominance by a single prey species (flying squirrels at high elevations, woodrats 

102 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Diets of California Spotted Owls in the Sierra National Forest Munton, Johnson, Steger, Eberlein 

Table 3—Percent frequency and percent biomass of prey in California spotted owl pellets from low-elevation oak 

savannas, oak/foothill pine forests, and riparian-deciduous forests, Sierra National Forest (after Munton and others 

1997) 

Breeding period1 Nonbreeding period2 Mean 

annual 

Wildlife Percent of Percent Percent of Percent percent of 

individuals biomass individuals biomass biomass 

Birds 

Gophers 

Insects 

Mice 

Moles 

Shrews 

Unidentified 

mammals 

Voles 

Woodrats 

2.2 1.8 6.7 4.2 3.0 

11.9 18.5 7.3 7.3 12.9 

44.6 0.1 11.1 0.1 0.1 

7.6 2.2 24.1 4.7 3.5 

0.0 0.0 0.5 0.2 0.1 

0.4 <0.1 0.3 <0.1 <0.1 

1.1 0.2 1.6 0.7 0.5 

5.0 2.7 2.6 0.9 1.8 

27.3 74.3 45.9 81.9 78.1 

1 n = 278 prey items. 

2 n = 386 prey items. 

at low elevations), increased reliance on the dominant prey species during the 

nonbreeding period, pocket gophers as the second greatest contributor to diet 

biomass and a higher proportion in diets during the breeding than the 

nonbreeding period, and higher frequency of insects during the breeding period. 

Main differences were the absence of flying squirrels from low-elevation diets 

(generally absent from those vegetation types); a greater reliance on the main 

prey species (woodrats) at low elevations; greater consumption of birds in 

coniferous forests, especially during the breeding period; greater consumption 

of birds and mice in coniferous forests in the breeding period than in the 

nonbreeding period, while the opposite trend occurred in oak woodlands and 

riparian-deciduous forests; and 1.55 times as many vertebrate prey per pellet in 

samples from the conifer forest than in samples from low-elevation sites. 

The different numbers of prey items per pellet between high- and lowelevation sites most likely reflected the prevalence of woodrats in diets of owls in 

the low-elevation sites. Because woodrats outweigh flying squirrels by a factor of 

1.82, one flying squirrel would provide only about 55 percent of the biomass 

available in one woodrat. Interestingly, note the close correspondence between 

the factor of 1.82 for the difference between weights of woodrats and flying 

squirrels and that of 1.70 for the difference between the numbers of prey items 

per pellet in the two areas. 

Other studies (Kadoch 1997, Marshall 1942, Verner and others 1992) also found 

that flying squirrels dominate the diets of California spotted owls in coniferous 

forests of the southern Sierra Nevada during the breeding period. In addition, 

pocket gophers were found to be important prey in the Sequoia and Kings Canyon 

National Parks (Verner and others 1992) and in the Sierra National Forest by Kadoch 

(1997). Birds averaged 13.3 percent of the diet biomass during the breeding period on 

the Sierra National Forest in 1995 and 1996 (Kadoch 1997). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 103 
Munton, Johnson, Steger, Eberlein Diets of California Spotted Owls in the Sierra National Forest 

Diets of spotted owls in the Eldorado and Stanislaus National Forests from 

1982 to 1984 were dominated by dusky-footed woodrats (Neotoma fuscipes) (31 

percent) and flying squirrels (25 percent), followed by diurnal squirrels (14 

percent) (Laymon 1988). Thrailkill and Bias (1989) also found that woodrats (37.4 

percent) and flying squirrels (31.2 percent) dominated spotted owl diets in the 

Eldorado National Forest, with birds making up another 12.4 percent of the 

biomass during studies in 1986 and 1987. The higher proportion of woodrats and 

lower proportion of flying squirrels found in their studies, in comparison with 

results of the present study in the southern Sierra Nevada, probably reflect the 

fact that the study areas in the central Sierra Nevada included considerable 

landbase at lower elevations, probably in the transition zone where flying 

squirrels are less abundant and woodrats more abundant than at the higher 

elevations corresponding to those in the coniferous forests of our study area. 

The studies by Laymon (1988) and Thrailkill and Bias (1989) both found a 

markedly higher percentage of diurnal squirrels (14.0 and 6.7 percent, 

respectively) than we found (0.9 percent) in the conifer zone of the Sierra National 

Forest. Pellets examined by Thrailkill and Bias (1989) included golden-mantled 

ground squirrels (Spermophilus lateralis) and a western gray squirrel (Sciurus 

griseus). Laymon’s (1988) pellets included chipmunks (Eutamias spp.), California 

ground squirrels (Spermophilus beecheyi), western gray squirrels, and Douglas’ 

squirrels (Tamiasciurus douglasii). Kadoch (1997) found no diurnal squirrels in 

spotted owl diets in the Sierra National Forest in 1995 and 1996. 

Other authors have observed annual (Kadoch 1997, Laymon 1988) and 

seasonal (Kadoch 1997, Munton and others 1997) variations in diets of the 

California spotted owl, although this study was the first to examine diets in the 

nonbreeding period in coniferous forests of the Sierra Nevada. Verner and others 

(1992) suggested that we need to manage for thriving populations of northern 

flying squirrel populations in Sierra coniferous forests and woodrats in other 

areas occupied by California spotted owls. The greater dominance of flying 

squirrels in coniferous forests and woodrats in riparian-deciduous and oak 

vegetation types in diets during the nonbreeding period emphasizes this need in 

the southern Sierra Nevada. 

Differences in diets of this owl have been observed during the same season 

and year between pairs in sites in similar habitats (Kadoch 1997, Laymon 1988) 

and between breeding and nonbreeding pairs (Barrows 1987, Smith and others 

1999, Thrailkill and Bias 1989, White 1996). Although we found annual and 

seasonal shifts in spotted owl diets, sampling effort was not consistent from site 

to site, period to period, or year to year, so our results must be viewed with 

caution. We did not factor the breeding status of pairs into our analysis, primarily 

because pellets were collected only incidental to home-range and demographic 

studies. Nonetheless, we believe that results reported here are indicative of the 

overall trends of the owls’ diets in our study area. Additional studies are needed 

that focus on diets in relation to prey populations to determine whether the 

relative importance of different prey species in their diets reflects changing owl 

preference, prey availability, or a mix of these factors. 

Acknowledgments 

Numerous members of the owl field crew assisted with the collection of pellets 

over the years. Joyce Kadoch Morrill and Jared Verner provided constructive 

comments on the draft manuscript. To all we express our sincere appreciation. 

104 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Diets of California Spotted Owls in the Sierra National Forest 

References 

Barrows, Cameron W. 1987. Diet shifts in breeding and nonbreeding spotted owls. Journal of 

Raptor Research 21: 95-97. 

Borror, Donald J.; DeLong, Dwight M. 1964. An introduction to the study of insects. New York: 

Holt, Rinehart and Winston; 819 p. 

Burt, William H.; Grossenheider, Richard P. 1976. A field guide to the mammals. Boston, MA: 

Houghton Mifflin Company; 289 p. 

Dunning, John B., Jr. 1984. Body weights of 686 species of North American birds. Western Bird 

Banding Association, Monograph No. 1; 38 p. 

Forsman, Eric D.; Meslow, E. Charles; Wight, Howard M. 1984. Distribution and biology of the 

spotted owl in Oregon. Wildlife Monographs 87; 64 p. 

Glading, Ben; Tillotson, Daniel F.; Selleck, David M. 1943. Raptor pellets as indicators of food 

habits. California Fish and Game 29: 92-121. 

Ingles, Lloyd G. 1990. Mammals of the Pacific states. Stanford, CA: Stanford University Press; 

506 p. 

Jameson, E. W., Jr.; Peeters, Hans J. 1988. California mammals. Berkeley, CA: University of 

California Press; 403 p. 

Kadoch, Joyce Lisa. 1997. Diet and behavior of the California spotted owl (Strix occidentalis 

occidentalis) in the southern Sierra Nevada. Davis, CA: University of California; 38 p. 

M.S. thesis. 

Laymon, Stephen A. 1988. The ecology of the spotted owl in the central Sierra Nevada, California. 

Berkeley: University of California; 285 p. Ph.D. dissertation. 

Marshall, Joe T. 1942. Food habits of the spotted owl. Condor 44: 66-67. 

Marti, Carl D. 1987. Raptor food habits studies. In: Pendelton, Beth A. Giron; Millsap, Brian A.; 

Cline, K. W.; Bird, D. M., eds. Raptor management techniques manual. Washington, DC: National 

Wildlife Federation; 67-80. 

Munton, Thomas E.; Johnson, Kenneth D.; Steger, George N.; Eberlein, Gary P. 1997. The diet of 

California spotted owls in riparian deciduous and oak habitats of the southern Sierra Nevada. 

In: Pillsbury, Norman H.; Verner, Jared; Tietje, William D., technical coordinators. Proceedings 

of a symposium on oak woodlands: ecology, management, and urban interface issues; 19-22 

March 1996; San Luis Obispo, CA. Gen. Tech. Rep. PSW-GTR-160. Berkeley, CA: Pacific 

Southwest Research Station, Forest Service, U.S. Department of Agriculture; 683-687. 

Robbins, Chandler S.; Bruun, Bertel; Zim, Herbert S. 1983. Birds of North America. New York: 

Golden Press; 360 p. 

Smith, Richard B.; Peery, M. Zachariah; Gutiérrez, R. J.; LaHaye, William S. 1999. The relationship 

between spotted owl diet and reproductive success in the San Bernardino Mountains, 

California. Wilson Bulletin 111: 22-29. 

Swan, Lester A.; Papp, Charles S. 1972. The common insects of North America. New York: Harper 

and Row; 750 p. 

Munton, Johnson, Steger, Eberlein 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 105 
106