Recovery Plan for the Northern Spotted Owl - DRAFT

3. Life Historycannot reject the hypothesis that prey availability explains spotted owl selectionfor older age forests. In addition, if they feed in open areas, spotted owlsmay be killed by great homed owls (Forsman et al. 1984).Reproductive BiologyNesting phenology: Spotted owls begin their annual breeding cycle in latewinter (February or March) when the pair begins to roost together. Copulationoccurs during this nuptial phase (Forsman et al. 1984). Some owl pairs usethe same nest site repeatedly, some use new ones each year, and othersalternate nest sites from year to year. Once a clutch of eggs is laid, the femaleincubates the eggs for approximately 30 days (Forsman et al. 1984). After theeggs hatch, the owlets remain in the nest and usually are fed by the pair untilthey leave the nest. Juvenile owls leave the nest 3 to 5 weeks after hatching.Many abandon the nest site well before they are able to fly. They jump into thebranches of surrounding trees or fall to the ground only to clamber up aleaning tree to a safe perch. The adaptive significance of this behavior isunknown, but Forsman et al. (1984) suggest that it serves to avoid increasingparasite loads in the nest as the season progresses. Once out of the nest theyoung owls are fed by the male and the female. They grow rapidly under goodfood conditions, reaching their parents' body mass prior to dispersal (Gutierrezet al. pers. obser.). Although juvenile owls are dependent on their parents,they begin to hunt by late summer. Dispersal begins in the early fall, signallingthe end of the annual reproductive cycle (Gutierrez et al. 1985, Miller andMeslow 1985, Miller 1989). Therefore, spotted owls are considered to be "birthpulse" breeders (i.e., they have distinct annual breeding periods)(Caughley1977). This knowledge is important when choosing an appropriate model withwhich to derive population projections.Variation in clutch size and nesting success: Spotted owls have one of thelowest clutch sizes among North American owls (Johnsgard 1988). Normally,spotted owls lay one or two eggs (Forsman et al. 1984). A small proportion ofthe population will lay three-egg clutches. Records of four-egg clutches arerare (Bendire 1892, Dunn 1901). Because clutch size is small there is relativelylittle variation in the number of eggs laid by a female. However, there islarge variation in nesting success and in the proportion of the population thatbreeds over time and among regions (Forsman et al. 1984, Gutierrez et al.1984, Thomas et al. 1990, Lutz 1992, LaHaye et al. In Press). Nesting successwithin a population can range from 0 to 100 percent (Forsman pers. comm.,Gutierrez et al. 1984, Gutierrez 1991 pers. comm.). Interestingly, Franklin etal. (1 990a) reported little variation in nesting during a 6-year study in northwesternCalifornia.Fecundity: Technically speaking, fecundity is the number of female youngproduced per female (Caughley 1977). However, the term has been used in avariety of ways in wildlife literature. Fecundity usually is defined relative tofemales because it is the female segment of the population that is mathematicallymodeled to project population trends. Because reproductive activityvaries greatly, fecundity also varies. Since biologists assume there is a 50:50sex ratio (Noon and Biles 1990, Thomas et al. 1990, USDI 1990) in a spottedowl population, fecundity in owl populations is almost always between 0.1 and1.5 (Thomas et al. 1990, Forsman 1988a, Franklin et al. 1990a, Lutz 1992,LaHaye et al. In Press).Age atfirst reproduction: Spotted owls can breed as early as 1 year old(Barrows 1985, Miller et al. 1985). Yet most birds probably do not breed before26