Recovery Plan for the Northern Spotted Owl - DRAFT

Dispersal1984, Thomas et al. 1990, USDI 1990). One important step toward estimatingthe number of owls is to estimate their density. The density of an animalpopulation is the number of individuals per unit area. Once an estimate ofdensity is derived, the estimate can be used to compute an estimate of abundancefor all of the area containing similar habitats or environmental conditions.Census and monitoring of northern spotted owls have been a high priority withland management agencies and research scientists (O'Halloran 1989, Simon-Jackson 1989, Azuma et al. 1990, Max et al. 1990, Franklin et al. 1990b,Thomas et al. 1990, Ward et al. 1991). Franklin et al. (1990b) estimated thedensity of spotted owls in northwestern California to be 0.65 owls per squaremile. They also estimated the density of owls within suitable habitat; thisvalue was 1.51 to 1.83 owls per square mile, depending on the method used toestimate the density. Using their estimates of density, they projected declinesin spotted owl populations between 60.0 percent and 82.5 percent in their areaif proposed management scenarios (USDA 1988) were implemented.Densities of spotted owls vary across their range as a function of habitatquality, geographic province, and current environmental conditions (Forsmanet al. 1977, Franklin et al. 1990b, Gutierrez and Pritchard 1990, Lutz 1992,Ward et al. 1991, LaHaye et al. In Press). Although much emphasis is placedon density, high densities can be mistaken as evidence of high quality habitat(Van Home 1983). Density estimates are useful for relative comparisons andfor evaluating management objectives, but they must be used in conjunctionwith knowledge of corresponding survival and fecundity values for the populations(see Habitat about spotted owl use of young, managed timberlands).Dispersal among vertebrates is the process of an animal leaving one area toestablish a new home range in another area. Dispersal can be undertaken byjuvenile or adult spotted owls. Further, dispersal is often characterized aseffective (i.e., successful breeding occurs at the end of the dispersal event) orgross (i.e., breeding may or may not be successful following dispersal) (Greenwood1980). Scientists know a great deal more about the process and patternof juvenile spotted owl dispersal than of adult dispersal, despite the difficulty ofclosely following large numbers of dispersing juvenile owls (Gutierrez et al.1985, Miller and Meslow 1985, Miller 1989, Gutierrez et al. In Prep.).Gutierrez et al. (1985), Miller (1989), and Gutierrez et al. (In Prep.) used radiotelemetry to estimate patterns of gross juvenile dispersal. Juvenile spottedowls dispersed from their natal areas in September and October after they hadreached adult body mass (Gutierrez et al. 1985, Miller 1989). They apparentlyleft their natal areas in random directions (Gutierrez et al. 1985, Gutierrez etal. In Prep.), and traveled moderate distances (approximately 9 to 30 miles onaverage) during their first autumn (Gutierrez et al. 1985, Miller 1989, Gutierrezet al. In Prep.). The pattern of dispersal varied among cohorts in a variety ofways including differences in direction, distance, and survival (Gutierrez et al.In Prep.). Effective dispersal distance, estimated from returning banded birdsaveraged about 4 miles for juvenile male spotted owls and about 12 miles forfemale juveniles (Gutierrez et al. In Prep.). Dispersal distances from bandedbirds were slightly higher for Oregon owls (Johnson pers. comm.). Estimates ofdispersal distance based on studies with finite area size have been shown to beunderestimates of true dispersal distance (Barrowelough 1980).Adult spotted owls will leave mates or move from territories, but the causes ofthese adult dispersal events are unknown. Spotted owls normally form long,28