Experimental Effects of Hiking on Breeding Mexican Spotted Owls

On the Colorado Plateau, some environments occupied by Mexican Spotted Owls ( Strix occidentalis lucida ) receive a great deal of recreational use. To assess the effects of hikers on breeding owls, we quantified changes in the activity budgets of owls at nests in response to a controlled amount of hiking in canyons of southern Utah in 1997 and 1998. We examined differences in the duration and frequency of eight behaviors as well as in the type and frequency of owl vocalizations between hiking and control treatments. In general, activity budgets of owls did not change markedly when hikers were near nests, although during hiking treatments, females decreased the amount of time they handled prey by 57% and decreased the amount of time they performed daytime maintenance behaviors by 30%. Further, hikers caused both females and males to increase the frequency of contact vocalizations by 58% and 534%, respectively. The order in which we applied treatments at nests (control-hiking or hiking-control) also influenced owl responses to hiking treatments, suggesting that observers near nests may have affected owl behavior. We concluded that the cumulative effects of high levels of short-duration recreational hiking near nests may be detrimental to Mexican Spotted Owls. Given current levels of visitation rates to most remote canyons occupied by owls, however, owl populations on the Colorado Plateau are not likely threatened by hiking. Notable exceptions are those canyons that receive use by 50 hikers per day. We recommend monitoring of owl occupancy, nest success, and hiking intensity in these high-use canyons. Efectos Experimentales del Excursionismo sobre Búhos Manchados Mexicanos Reproductores Resumen: El la Meseta de Colorado, algunos ambientes ocupados por Búhos Manchados Mexicanos ( Strix occidentalis lucida ) reciben altos niveles de actividad recreativa. Para evaluar los efectos de excursionistas sobre búhos reproductores, cuantificamos cambios en presupuestos de actividad de búhos anidantes en respuesta a niveles controlados de excursionistas en cañones del sur de Utah en 1997 y 1998. Examinamos diferencias en la duración y frecuencia de ocho conductas. así como del tipo y frecuencia de vocalizaciones de búhos entre tratamientos de excursionismo y control. En general, los presupuestos de actividad de los búhos no cambiaron marcadamente cuando había excursionistas cerca de los nidos, aunque durante los tratamientos de excursionismo las hembras redujeron el tiempo en que manejan presas en 57% y disminuyeron el tiempo en que presentaron conductas de mantenimiento diurno en 30%. Más aún, los excursionistas causaron que tanto hembras como machos incrementaran la frecuencia de vocalizaciones de contacto en 58% y 534% respectivamente. El orden en que aplicamos los tratamientos en los nidos (control-excursionismo, excursionismo-control) también influyó en las respuestas de los búhos a los tratamientos de excursionismo, lo que sugiere que observadores cercanos a los nidos pudieron afectar el comportamiento de los búhos. Concluimos que los efectos acumulativos de altos niveles de pequeña duración de excursionismo recreativo cerca de nidos pueden ser perjudiciales para Búhos Manchados Mexicanos. Sin embargo, dados los niveles actuales de tasas de visitación a los cañones más remotos ocupados por búhos, es probable que las poblaciones de búhos en la Meseta de Colorado no estén amenazadas por el excursionismo. Notables excepciones son aquellos cañones que son utilizados por 50 excursionistas/día. Recomendamos monitorear la ocupación por búhos, el éxito de anidación y la intensidad de excursionismo en estos cañones de alto uso. * Current address: Cornell Laboratory of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850, email [email protected] Paper submitted August 28, 2001; revised manuscript accepted March 13, 2002. 308 Hiking and Mexican Spotted Owls Swarthout & Steidl Conservation Biology Volume 17, No. 1, February 2003 Introduction The effects of humans on wildlife are pervasive. Although the most wide-ranging effects likely result from long-term physical changes to the environment, effects from short-term human activities are a growing interest (Knight & Cole 1995) because participation in outdoor recreational activities in the United States has increased 10% per year since the 1950s (Walsh 1986). Further, nonconsumptive wildlife-related activities are projected to increase from 63% to 142% during the next 50 years (Flather & Cordell 1995), suggesting that the frequency of interactions between wildlife and humans will increase accordingly. The effects of these encounters on wildlife are complex and can have both shortand longterm consequences for individual organisms, populations, and communities (Knight & Cole 1991). Diurnal raptors respond to disturbance from recreation in a variety of ways, including altered activity budgets (Steidl & Anthony 2000), decreased reproductive success (White & Thurow 1985), decreased nest attendance and prey delivery rates (Fernandez & Azkona 1993; Steidl & Anthony 2000), and altered patterns of habitat use (Knight et al. 1991) and nest-site selection (van der Zande & Verstrael 1985). The responses of nocturnal raptors to recreation have not been well studied, however, because of difficulties in locating owls and observing behavior at night, but there is evidence to suggest that Strigiformes respond adversely to human disturbances. For example, the activity budgets of Burrowing Owls ( Athene cunicularia ) changed in response to vehicular traffic (Plumpton & Lutz 1993), and the prey delivery rates of Mexican Spotted Owls ( Strix occidentalis lucida ) decrease in response to noise disturbances (Delaney et al. 1999). Mexican Spotted Owls are a federally threatened subspecies ( U.S. Fish and Wildlife Service 1993) and the spatially isolated populations on the Colorado Plateau overlap with areas of intense recreational activity (U.S. Fish and Wildlife Service 1995). Owls nest, roost, and forage in narrow, steep-walled, rocky canyons characteristic of the plateau ( Rinkevich & Gutierrez 1996; Willey 1998), where recreational activities such as hiking and off-road vehicle use are concentrated along canyon bottoms. Further, recreational activity on the plateau is highest from March to October ( Zion National Park, unpublished data), which overlaps with the breeding season of the owls ( Willey 1998 ). This spatial and temporal overlap exacerbates interactions between owls and humans and could adversely affect Mexican Spotted Owls (U.S. Fish and Wildlife Service 1995). We designed a manipulative experiment to assess effects of hikers on the behavior of nesting owls. Specifically, we examined activity budgets, nest attendance, and vocalizations of spotted owls in response to hikers passing through nesting territories. Study Area We studied nesting owls in Canyonlands (136,609 ha) and Capitol Reef (97,859 ha) national parks of Utah in 1997 and 1998. Owls in Canyonlands (1160–2160 m) were located in slickrock canyons of Cedar Mesa sandstone in which pinyon pine ( Pinus edulis ) and Utah juniper ( Juniperus osteosperma ) were the dominant vegetation. Owls in Capitol Reef (1150–2500 m) were located in slickrock canyons of Navajo and Kayenta sandstone in which pinyon pine, Utah juniper, ponderosa pine ( Pinus ponderosa ), and boxelder ( Acer negundo ) were the dominant vegetation. During our study, visitation to Canyonlands and Capitol Reef exceeded 400,000 and 650,000 people per year, respectively.