Noninvasive Physiological Measures of Disturbance in the Northern Spotted Owl

Abstract

The Northern Spotted Owl ( Strix occidentalis caurina ) continues to decline across its entire range at a rate exceeding 4% per year, with a 1% annual acceleration (Burnham et al. 1994). One possible factor impeding its recovery is the lack of adequate measures that reflect the impact of habitat disturbance on this species (Hanson et al. 1993). Measures of reproductive success between individual owl sites and areas are critical to evaluating long-term effects of management in species such as the Northern Spotted Owl. The considerable time-span from nesting to fledging, however, makes it difficult to pinpoint individual factors associated with nest failure among the myriad sources of disturbance over the course of the breeding season. At the least, large numbers of breeding pairs and considerable time are required to use reproductive success to index such disturbance impacts. More immediate measures of disturbance could complement measures of reproductive success, providing a proactive tool to guide adaptive management in a manner that does not compromise owl survivorship and reproduction. We demonstrate that noninvasive hormone measurements of physiological stress in feces could provide an objective, readily accessible and cost effective means of quantifying impacts of disturbance in Northern Spotted Owls that can directly address pivotal conservation questions surrounding this and other listed species. It is well-documented that physiological stress occurs in response to environmental pressure and affects resistance to disease, survival, and reproduction negatively in a variety of species (Moberg 1985; Munck et al. 1984; Wingfield & Farner 1993). Until recently, however, the application of physiological stress measures to free-ranging threatened or endangered species has been severely limited by the invasiveness of sample collection procedures such as repeated capture and blood withdrawal. Fecal steroid measures provide an ideal alternative to serum measures because they are relatively easy to collect longitudinally, without disturbing study subjects. Results illustrate that fecal stress hormone disturbance measures can be used as a proactive management tool, enabling the industry and wildlife managers to optimize the tradeoffs between resource utilization and proliferation of species at risk.