Recovery Plan for the Northern Spotted Owl - DRAFT

SurvivorshipDeensitythey are 3 years old (Franklin et al. 1990a, Thomas et al. 1990). In addition,subadult owls have lower fecundity than do adults (Franklin et al. 1990a).Age-specific fecundity also is an important demographic parameter, but thereare no age-specific estimates of fecundity for the adult age classes. Therefore,for purposes of demographic modeling, adult fecundity is assumed to be equalacross age classes.Adult rates: Adult northern spotted owls' annual survival rates are very high.Thus, they must be long-lived birds. Based on banding and radio telemetry,the chance of an owl living from one year to the next is 81 to 96 percent(Barrowclough and Coats 1985, Lande 1985, Franklin et al. 1990a, Thomas etal. 1990). In short-term studies, survival rates may vary as a response bybirds to varying environmental conditions (Gutierrez and Pritchard 1990,LaHaye et al. In Press). Thus, in long-lived species, studies must be of longduration to achieve reliable estimates of age-specific survival rates. The mostrecent estimates of survival are in Appendix C.Subadult survival: Subadult owls have a lower survival rate than adult owls(Franklin et al. 1990a, Thomas et al. 1990)(Appendix C). Since subadults alsohave lower reproductive rates and fecundity, it may be possible that the sameenvironmental or behavioral factors influence all aspects of the demography ofthe subadults in the same way. Nevertheless, the subadult segment of thepopulation is relatively small and makes only a modest contribution to thedynamics of the population (Noon and Biles 1990, Thomas et al. 1990).Juvenile survival: Juvenile survival rates have been measured from bandedbirds and radio-marked birds (Barrowclough and Coats 1985, Gutierrez et al.In Prep.). Survival rates for this age class are low (the chance of a juvenileliving from one year to the next is 15 to 29 percent) relative to adult survivalrates (Thomas et al. 1990). It is well known that first-year birds, in general,have low survival. The rigors of dispersal and the consequences of inexperience(e.g., poor hunting skills, lack of familiarity with a territory) lead to highermortality rates.Mortality: Spotted owls die from a variety of causes. The most frequent causeof mortality recorded among radio-marked birds is predation by other animals(Johnson pers. comm.). They also die from accidents (e.g., flying into objects,automobiles, and drowning) (Gutierrez et al. 1985, Franklin, LaHaye, Gutierrezpers. comm., Johnson pers. comm.). Accidents are considered to be densityindependent,whereas predation usually is density-dependent in most prey,although predation may be density-independent in spotted owls. Anothersource of mortality is starvation. Starvation is common among spotted owls(Gutierrez et al. 1985, Miller 1989, Johnson pers. comm.), but occurs lessfrequently among adult spotted owls (Sisco 1990). Starvation could be aconsequence of low prey abundance, low prey availability (i.e., poor huntinghabitat), or inexperience (inability to hunt successfully).The number of northern spotted owls is a topic of much debate (Thomas et al.1990). With increasing survey and monitoring of populations, the number ofknown owls has increased greatly since the first estimates of total abundancewere made. These observations of higher owl abundance reflect greater knowledgeand effort expended by biologists to count owls; these observations shouldnot be interpreted as evidence that the owl population is increasing. The totalabundance of owls appears to be declining gradually over time (Forsman et al.27