Recovery Plan for the Northern Spotted Owl - DRAFT

Northern goshawks (Accipiter gentilis) also prey on adult and juvenile spottedowls (Forsman et al. 1984, Gutierrez et al. 1985, Miller 1989, Johnson 1991pers. comm.). Nevertheless, spotted owls will nest within a goshawk territory(Forsman et al. 1984) and will defend their young against attacks by goshawks(Gutierrez unpub. data). Thus, goshawks probably are not serious threats tospotted owl populations.Until recently, people rarely have encountered spotted owls and there has beenno historic persecution by humans of this docile creature. The recent, conspicuousrise in spotted owl deaths at the hands of humans is a potentialthreat to local owl populations.Diseases and ParasitesHabitatDisease and parasite infections represent another form of interspecific interactionbecause it is the relationship (in the broad biological meaning) of oneorganism with another. However, the topic of pathogens is treated separatelyhere because it is treated separately in status analyses by the FWS whenlisting a species as threatened or endangered.Relatively little is known about the diseases and parasites of spotted owls.Gutierrez (1989) conducted an extensive survey of hematozoan parasites (thosethat live in the blood) among all three subspecies of the spotted owl. Of the sixhematozoan species found, all but one species occurred in the northernspotted owl. The infection rate was 100 percent, which was one of the highestrates of infection by these parasites recorded amongbirds (Greiner et al. 1975).However, spotted owls must be adapted to carry these high parasite loadsbecause their survival rates are very high where infection rates are high (e.g.,northwestern California, see following text). Hoberg et al. (1989) examined 20northern spotted owls for helminth (worm) parasites and found eight species,representing nematodes (round worms), cestodes (flat worms), and acanthocephalans(spiny-headed worms). More than 80 percent of the birds wereinfected with at least one species; and multiple infections were common.Young et al. (In Review) reported two hippoboscid fly (louse) species fromspotted owls in northwestern California. One species of fly was recorded onlyonce among the 382 owls examined, but approximately 17 percent of the owlsthey examined were infested by the other species. Fly densities on owls werehigher in years of higher summer and fall temperatures and lower winterprecipitation. The authors speculated that low temperatures may have depressedsurvival of fly pupae. Finally, Forsman (pers. comm.) observed twonests where owlets had such high infestations of hippoboscids that the fliescaused severe trauma to the young birds.Habitat selection and its context: Perhaps the most controversial aspect ofthe natural history of the spotted owl concerns its habitat requirements.Thomas et al. (1990:143-144) discussed the complex habitat needs of thenorthern spotted owl.Most species exhibit variation in habitat selection (i.e., most species are notstrict habitat specialists). Spotted owls are known to use many habitats.Empirical observations of spotted owls in different habitats can provide understandingof the birds' habitat requirements at three different levels (Peek 1986).Habitat use is the simple observation of an animal in a habitat without understandingthe context of the observation. Habitat selection is the choice of ahabitat or habitats among those that are directly available to the animal.Habitat preference is the selection of habitat that would be made by an animal18