The Northern spotted owl (Strix occidentalis caurina) is federally threatened throughout its range in the Pacific NW, experiencing an eight percent annual decline.  However, the causes of the species decline remain ambiguous. Measures are needed that reliably reflect the impact of human and other disturbances (e.g., barred owls) on this species. Our Center developed and implemented noninvasive fecal hormone measures of physiological stress and reproductive activity in the Northern Spotted Owl to help quantify such disturbance impacts.

We are currently conducting two related studies on impacts of disturbance on the Northern Spotted Owl (NSO), both tied to impacts of roads. 

Previous studies in our lab demonstrated a relation between roads as well as timber harvest practices on the NSO (Wasser et al 1997).  We replicated this study in a cross-sectional investigation of the effects of roads on NSO across WA, OR, and CA, and also compared results to impacts of barred owls on the NSO. 

Fecal samples were collected during the 2004 and 2005 field seasons by biologists involved in spotted owl demography studies across WA, OR and CA. Results showed a pattern of increased corticosterone (an adrenal stress hormone) with proximity to roads in a dose-dependent manner (Fig.1).  Standard error bars were large.  However, much of this variation could be attributed to differences based on sex, breeding stage and year, as indicated by the strong significance of each variable and their interactions (Table 1).

Figure 1:  Levels of corticosterone in Northern spotted owls increase with proximity to roads.  On the Y-axis are residuals from a regression of corticosterone with sample mass.  On the x-axis is distance of the pair from the closest road.  Sample sizes for groups from left to right were 75, 31, 25 and 7.  Error bars represent one SE.

Table 1:  Effects of sex, breeding stage (julian date), year and their interationc on corticosterone levels in Nothern Spotted Owls.  When proximity to nearest road is treated as a continuous variable and sex, date and year are added to the analyses, the main effect of nearest road is highly significant, as are all other factors and their interactions.

The invasive barred owl has also been suggested to be a significant contributor to the NSO decline.  In fact, some managers have implied that the impact of barred owls is so severe that modifying timber practices may no longer matter. We addressed this latter issue by comparing levels of corticosterone in NSOs breeding in Rainier and Olympic National Parks with their adjacent National Forests. Human disturbances (e.g., roads and timber harvests) were assumed to be higher in the national forest compared to its adjacent national park, whereas barred owl densities were assumed to be comparable in these adjacent land classification areas.   NSO on national forest land had higher levels of corticosterone than those in adjacent national parks, suggesting that human disturbance impacts are not being overshadowed by barred owl impacts.

Figure 2:  Levels of fecal glucocorticoid metabolites were higher in owls breeding in national forests than in their adjacent national parks, after controlling for year, sex and breeding stage (F = 17.53; df = 1, 81; p < 0.0001).  Error bars represent one standard error.   

Moreover, a comparison of corticosterone levels in Northern spotted owls in relation to their proximity to barred owls found no pattern whatsoever (Figure 3).  Although our barred owl sample size was smaller than for the comparison of proximity to roads, proximity to barred owls did not seem to have the same consistent effect in elevating corticosterone in the NSO as does nearness to a road.

Figure 3.  Levels of corticosterone in the Northern Spotted Owl did not vary with proximity to barred owl.  Sample sizes from left to right are 9, 6, 24 and 57.  Error bars represent one standard error.

In a related study, we are collaborating with the USDA Forest Service, U.S. Fish and Wildlife Service, and Blue Ribbon Coalition to understand the impacts of off-highway vehicle (OHV) use on Northern Spotted Owl (NSO) in northern California .  OHV users increased from less than 3 million to over 8 million in the last ten years.  It is essential to know whether these trends are impacting the NSO.  We are currently using behavior, physiology and reproductive measures to assess OHV impacts on these owls throughout Mendocino and Shasta-Trinity National Forests .

Each spring and summer, NSO in this area were exposed to large enduro motorcycle races where up to 500 riders pass within a quarter mile of owl territories over a one hour period.  We examined the effects of off-highway vehicle use on the NSO through controlled experimental treatment involving a simulated enduro event; 2-5 volunteer motorcycle riders passed the owl’s territory on the closest road up to 45 times in an hour.  Acute vehicle exposure significantly increased corticosterone in male NSO relative to controls that did not receive the simulated enduro treatment (F = 5.09, df = 1, 16; p = 0.04).  Interestingly, we found no effect of treatment on female NSO (F = 2.98; df = 1,8; p = 0.13).

Figure 4:  Male Northern spotted owls show a significant increase in fecal glucocorticoid metabolites after (post) experimental exposure to an hour-long simulated motorcycle enduro event.  On the Y-axis are the residuals from a regression of fecal glucocorticoids with sample mass.  Sample sizes for each treatment group are given in parenthesis.  Error bars represent one SE.

This work is still ongoing.  We are now comparing stress measures in owls inhabiting areas with high or low OHV use throughout the year.  Half of the owl pairs in these high and low exposure habitats are exposed to the simulated enduro races, using the unexposed owl pairs in their respective habitat type as controls.  These data will show how chronic exposure impacts response to this acute stressor. 

Results are being used by state and federal wildlife authorities to establish future guidelines for this rapidly growing activity in California .  Our collaboration with motorcycle riders in the area also serves to keep them informed and more willing to conform to concessions that may be required by the new guidelines in the future.

Over 15 years have passed since the first alarms sounded on the status of the Northern Spotted Owl.  Yet, its recovery remains imperiled.  Adaptive management could provide some relief to this urgent situation.  Fecal hormone measures could prove quite useful in this regard.  Fecal stress hormone measures can help identify those pressures most in need of mitigation, as well as track success of these mitigation by indicating stress relief.

In these ways, we hope that our studies will help revise management practices to help assure the Northern Spotted Owl’s recovery.