Environmental Factors Associated with 

Spotted Owl Reproduction1 

Malcolm P. North2 

Abstract 

Although research on spotted owls (Strix occidentalis) has increased dramatically in the 

last decade, factors influencing owl reproduction still are poorly known. This ongoing 

study uses 9 years of demographic data to analyze associations between owl reproduction 

and weather, cone crop abundance, and nest-site structure. Initial results indicate no 

correlation between cone crop abundance and owl reproduction, but significant 

associations between owl reproductive success and spring weather conditions and nestsite structure. 

The management of many western forests has been directly influenced by 

legal and biological requirements for maintaining viable populations of the 

spotted owl (Strix occidentalis). Although most of the controversy has focused on 

protecting the old-growth habitat associated with the owl, the long-term viability 

of the species will also depend on its reproductive success. It has been difficult to 

assess owl population trends because owls disperse over large areas, and 

demographic studies rarely have a closed population in which a thorough, longterm census has been done. In the absence of better demographic data, identifying 

factors that may influence reproduction can be an important tool for inferring 

population response to changing environmental conditions. 

Several studies of the spotted owl have inferred that habitat quality, prey 

abundance, and weather are likely influences on reproduction (Franklin and 

others 2000, Thomas and others 1990, Zabel and others 1996). Reproduction by 

the northern spotted owl (S. occidentalis caurina) may be negatively correlated 

with current winter precipitation (Wagner and others 1996, Zabel et al 1996). In 

the San Bernardino Mountains of southern California, however, reproduction by 

the California spotted owl (S. occidentalis occidentalis) was shown to be positively 

correlated with rainfall during the preceding winter (LaHaye and others 1997). 

Spotted owls in the San Bernardino Mountains are part of a southern California 

subpopulation, geographically isolated from a larger subpopulation in the Sierra 

Nevada of California, and with a diet largely dependent on woodrats (Neotema 

sp.). This contrasts with most other populations of spotted owls, for which 

northern flying squirrels (Glaucomys sabrinus) dominate diets, especially in 

conifer forests. 

Our objectives in this study are to assess the possible relations between 

weather conditions and California spotted owl reproduction, to look for nest-site 

attributes correlated with reproductive success, and to characterize nest-site 

structure for owls in the southern Sierra Nevada (North and others 2000). 

1 	 An abbreviated version of this 

paper was presented at the 

Symposium on the Kings River 

Sustainable Forest Ecosystems 

Project: Progress and Current 

Status, January 26, 1998, Clovis, 

California. 

2 Research Ecologist, Forestry 

Sciences Laboratory, Pacific 

Southwest Research Station, 

USDA Forest Service, 2081 E. Sierra 

Ave., Fresno, CA 93710. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 117 
North Environmental Factors Associated with Spotted Owl Reproduction 

Preliminary Results and Discussion 

On the basis of a 9-year demographic study in the Sierra National Forest and in 

Sequoia/Kings Canyon National Parks, two observed trends are being explored 

in this study. First, reproductive success of all surveyed owls tended to be 

synchronous throughout both study areas within any given year. The common 

reproductive response produces distinct annual fluctuations, with good years in 

which each owl pair averaged from 0.6 to 1.7 fledged young, and bad years with 

pairs averaging fewer than 0.4 fledged young. This pattern suggested the 

operation of a factor with an extensive regional influence on owl reproductive 

success. 

Weather and the abundance of prey are two possible explanations for this 

regional effect. The possible role of weather in the reproductive performance of 

the owls is under investigation (North and others 2000), but region-wide data on 

the abundance of prey are not available. An alternative is to infer prey availability 

indirectly from studies of the two food sources most widely consumed by small 

mammals—truffles and conifer seeds. Truffle abundance did not vary 

significantly over a 2-year study, and the available standing crop always 

exceeded a hypothesized consumption rate of 0.6 kg/ha (North and others 1997). 

By using data from the USDA Forest Service’s tree nursery at Placerville, 

California, the annual quantity of dried seed per bushel of cones of conifer 

species was compared to the annual reproductive rates of owls in the study area 

(table 1). Although some of the correlation coefficients were above 0.50, owl 

reproduction was not significantly correlated with any of the previous year’s 

conifer seed abundance. 

The second trend noted over the 9-year study was that some nests had 

consistently higher reproductive success than others. Even during several “bad” 

years, some nests with repeated use continued to produce young. To rank nests 

in terms of their reproductive performance, I weighted reproductive success of 

each nest in the study area by dividing the number of fledglings produced in 

each nest in a given year by the mean, annual reproductive rate for all surveyed 

owl pairs in the study (table 2). This increased the ranking scores of nests that 

produced young in bad years and gave less weight to nests that were productive 

in years when most pairs nested. On this weighted scale, the 117 nests found 

during the study ranked from 0 to 34 in weighted reproductive success. Most of 

the nests with higher reproduction have been used year after year, but commonly 

the nests with low ranking scores were used only once. This pattern suggested 

that, in addition to regional influences that affect all owl pairs, local nest-site 

factors must also influence reproduction 

Table 1—Correlation coefficients (P-values) for seed crop abundance and spotted owl reproduction, 1989-1998 

Species Correlation coefficient 

White fir (Abies concolor) 0.61 (0.15) 

Red fir (Abies magnifica) 0.57 (0.32) 

Jeffrey pine (Pinus jeffreyi) 0.01 (0.97) 

Sugar pine (Pinus lambertiana) -0.25 (0.51) 

Ponderosa pine (Pinus ponderosa) 0.42 (0.27) 

Mean seed abundance 0.57 (0.11) 

118 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Environmental Factors Associated with Spotted Owl Reproduction North 

Table 2—Mean weighted number of young per owl pair and number of nests, classed by years of nest use 

Number of years Number Mean weighted number 

a nest was used of nests of young per owl pair1 

a 1 76 1.61

2 30 2.54ab 

≥ 3 11 3.29b 

1 Values with different superscripts are significantly different (P < 0.05, posthoc ANOVA). 

Acknowledgments 

This work has been a collaboration with George Steger, Gary Eberlein, Renee 

Denton, Tom Munton, and Ken Johnson, all from the Forestry Sciences Laboratory, 

Fresno, California, and the many field assistants who have collected spotted owl 

information for 9 years. Bill Laudenslayer and George Steger provided helpful 

comments on the manuscript. 

References 

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LaHaye, Willian S.; Gutiérrez, R. J.; Call, Douglas R. 1997. Nest-site selection and reproductive 

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Association of weather and nest-site structure with reproductive success in California spotted 

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North, Malcolm; Trappe, James; Franklin, Jerry. 1997. Standing crop and animal consumption of 

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