Center for Conservation Biology

Impacts of vehicle exposure on northern spotted owl

Conservation endocrinology

Two owls huddled on a tree branch in daylight

Mating pair of Strix occidentalis (northern spotted owl).Photo by Zachary Folk

The northern spotted owl, Strix occidentalis caurina, is the flagship threatened species of the Pacific Northwest. Federally listed under the Endangered Species Act in 1990, the northern spotted owl continues to decline at a rate of about 4% throughout its range. Despite the fact that the northern spotted owl is one of the best-studied wild vertebrate species in the world, the relative importance of the threats that it faces remain controversial.

Our center developed non-invasive fecal hormone measures of physiological stress (glucocorticoids), nutritional state (thyroid hormones) and reproductive activity (sex steroids) in northern spotted owl. Combined, these measures provide a comprehensive profile of northern spotted owl physiological health that can serve as a powerful diagnostic tool in studies of disturbance impacts. We applied these tools to document impacts of off-highway vehicle and road exposure on the owl.

Research overview

A man rides a dirt bike

A volunteer shows us his moves during a simulated enduro event in Mendocino National Forest.Photo by Steve Sabo

Off-highway vehicle (OHV) use has more than tripled in the last decade and continues to gain in popularity. Finding ways to manage recreation that better protect threatened and endangered species is a high priority for USDA Forest Service (USFS) and US Fish and Wildlife Service (USFWS). We measured the effects of OHV use on the northern spotted owl in the Shasta-Trinity and Mendocino National Forests of northern California, in collaboration with USFS, USFWS, Hubbs-Sea World, Blue Ribbon Coalition and other motorcycle non-profit groups.

We combined correlational and experimental approaches and measured sound level, annual reproductive success and fecal hormones including stress steroids (glucocorticoids) and metabolic hormones (thyroid hormones). Correlational analysis was used to quantify the effects of road noise on northern spotted owl hormone levels and reproductive success. Non-invasive fecal measures of glucocorticoids (GCs) and thyroid hormone (T3) were used to assess disturbance. GCs reflect psychological disturbance and nutritional status, whereas T3 reflects nutritional status only. The combination allowed partitioning of these two stress effects. We also experimentally applied an hour of motorcycle exposure to spotted owl territories in May and July to assess whether and how acute effects of traffic exposure on GC level varied by month, sex and breeding status.

Don Amador poses for camera next to a research van

Don Amador, Western Representative of the Blue Ribbon Coalition and an integral member of the research team.

For each territory that received experimental OHV exposure we simultaneously sampled an untreated control site using identical methods. Treatment and control sites were assigned at random. In total, 105 experiments were conducted in Shasta-Trinity and Mendocino National Forest between the years of 2005-2008. Final analysis included physiological samples from 165 individual spotted owl.

Our results suggest that OHV exposure has negative effects on northern spotted owl, increasing levels of GCs in the short term and contributing to noise impacts that decrease spotted owl reproductive success over the long term. Proximity to road increased northern spotted owl reproductive success on sites with low ambient noise, but had the opposite effect on sites with high ambient noise. On both quiet and loud sites, levels of T3 increased with proximity to road, suggesting that the positive effects of proximity to roads were mediated through increased prey abundance or foraging efficiency. However, increased GCs associated with noise suggest the positive effects of roads on northern spotted owl reproductive success were reversed when noise level (i.e. traffic) was high. Combined, these results suggest that the negative effects of traffic noise on northern spotted owl may be amplified by the draw that roads provide for better feeding.

Experimental application of OHV exposure showed that males are most sensitive to disturbance from traffic in May when they are solely responsible for feeding themselves, their mates and their nestlings. In contrast, response to motorcycle exposure among females depended on their T3 levels (nutritional status) and number of young. GCs were highest relative to controls post treatment among females with two young and high T3 (good nutrition). Females with two young and low T3 (compromised nutrition) showed a strong treatment effect but with GC levels actually lower in treated females than in controls. This extreme response indicates that females may be in a state of chronic stress wherein the hypothalamic-pituitary-adrenal (HPA) axis actually shut down and ability to respond to further challenge is severely compromised.

Researcher collecting owl scat from below where a spotted owl sits high in a tree

Jennifer Hartman collects scat from a spotted owl during her field work for the center.Photo by Keegan Ramey

Experimental approach

Twice during the breeding season, once in May and again in July, northern spotted owl in both high and low OHV use areas were experimentally exposed to an hour of motorcycle use between 8:00 and 12:00 in the morning. All scat from all northern spotted owl on the territory were collected throughout the day. Identical data collection was conducted on randomly assigned control pairs in high and low OHV use areas that did not receive the experimental OHV exposure.

The intensity of the motorcycle exposure simulated what occurs during enduros, popular events in which large numbers of motorcyclists ride hard for long distances on National Forest trails, sometimes within meters of spotted owl nests. Our simulated enduros were conducted by volunteers from the local OHV community. Volunteer interns from the Student Conservation Association put in long hours to assist with data collection.


During May male spotted owls are responsible for feeding themselves, their mates and their chicks, while females incubate almost continuously until chicks are ten days of age. In July females and males share the job of feeding their fledglings.

Data chart of research with text in image. Title: Simulated enduros increased NSO glucocorticoids. Text in chart: Description: Spotted owls exposed to one hour of experimentally applied motorcycle exposure in showed elevated fecal glucocorticoids relative to controls. Overall, males and juveniles were more responsive than females. Sample sizes are shown in parenthesis.

Data chart with text in image. Title: Noisy roads decreased NSO reproductive success. Description: Figure 4: Among the NSO in our population, proximity to a quiet road (i.e. one with no traffic) was strongly associated with increased reproductive success. Because T3 increased with proximity to road (regardless of noise on that road), this is probably due to more prey near roads for easier hunting. However when road noise (i.e. traffic) was high, NSO reproductive success dropped off steeply on sites close to that road.

Combined these results suggest that:

Northern spotted owl show a physiological response to traffic exposure that varies with sex, season, breeding status and nutritional condition.

In our Northern Californian study population, spotted owls close to roads were in better nutritional condition. In California, woodrats comprise a majority of the spotted owl diet. Similar results would not be expected further north where northern spotted owl eat primarily flying squirrels (old growth specialist).

Reproductive success is higher close to quiet roads (likely due to better feeding). However, proximity to roads with high noise (i.e. traffic) decreases spotted owl reproductive success.

These results represent the first evidence to date that Off-Highway Vehicle use is having a strong negative impact on northern spotted owl population viability. Our report to USDA Forest Service and US Fish and Wildlife Service will be used to help manage recreation so as to minimize impacts on this federally threatened species.


Hayward, LS, AE Bowles, JC Ha, SK Wasser. 2011. Impacts of acute and long-term vehicle exposure on physiology and reproductive success of the northern spotted owl. Ecosphere 2(6): 1-20.