Recovery Plan for the Northern Spotted Owl - DRAFT

Home Range SizeHome range is defined generally as the area used by an animal and to whichthe animal exhibits fidelity. The size of home ranges of spotted owls is a focalpoint of controversy because of their large size (Table 2.1; Thomas et al. 1990).Forsman (1980) was the first to critically estimate spotted owl home range sizeby using radio telemetry, although Marshall (1957) guessed at the nightlyranges of Mexican spotted owls in Arizona and Mexico. Radio telemetry is theonly method through which scientists reasonably can estimate the size ofspotted owl home ranges. There has been some concern expressed about theeffect of radio transmitters on survival and reproduction (Paton et al. 1991).Foster et al. (1992) found no significant differences in survival or body massbetween radio-marked and unmarked spotted owls, although some owls diddie as a result of improper transmitter attachment. But they did record asignificant negative effect on reproductive output of radio-marked owls.Because of Forsman's (1980) initial observations that spotted owl home rangeswere very large (more than 2,000 acres on the average) a great deal of scientificeffort has been devoted to verifying his original observations as well as estimatingthe geographic and inherent variation in spotted owl home ranges(Forsman 1981, Solis 1983, Forsman et al. 1984, Gutierrez et al. 1984, Siscoand Gutierrez 1984, Forsman and Meslow 1985, Allen et al. 1989, Hamer et al.1989, Hays et al. 1989, Carey et al. 1990, Paton et al. 1990, Sisco 1990,Thrailkill and Meslow 1990). In addition, Thomas et al. (1990) summarizedthis information as well as other unpublished estimates of home range size (seeTable 2.1).Interpreting the variation in home range size and habitat use has been asignificant challenge to spotted owl ecologists. Variation (i.e., the distributionof observations of a trait) in observed home range size has formed the basisupon which scientific inference and generalization were based about spottedowl home range requirements. From the studies cited earlier, some generalizationscan be made about home range characteristics. First, all studies of homerange size are consistent with Forsman's (1980) original observations of largespotted owl home ranges (see Table 2.1). Second, there is a large degree ofoverlap in home range areas between members of the same pair (Forsman etal. 1984, Solis and Gutierrez 1990) and lesser overlap among adjacent pairs(Forsman et al. 1984). Third, there is considerable geographic variation inhome range size, with owls occupying Washington's Olympic Peninsula havingthe largest home ranges (Thomas et al. 1990). Fourth, home range size increasesas the amount of old forest within the home range decreases (i.e., lossof habitat from logging; Carey 1985, Forsman et al. 1984, Thrailkill andMeslow 1990). It is unknown if this geographic variation is related to latitude,habitat, individual, temporal, or prey-base variation.The size of an owl's home range probably is dependent on many factors (e.g.,food availability, interspecific competition, amount and arrangement of suitablehabitat). For example, spotted owl home range size may be a reflection of anadaptive response to low prey abundance and variation in abundance anddistribution of prey (Ward 1990). Further, estimates of owl home range sizecan be influenced by the sampling design of the home range study and thehome range estimator used in the analyses (Carey et al. 1989, Call 1989).Although these factors may influence the estimation of owl home range size,predictions of home range sizes of birds of the size and trophic level of spottedowls based on allometric equations are similar to empirical estimates of spottedowl home ranges (Schoener 1969). Predictions of spotted owl home range size,based on allometric analysis of mammals, underestimate direct observations of22