Recovery Plan for the Northern Spotted Owl - DRAFT

Food Habitsto any particular area. These birds often are referred to as "floaters." Theecology of floaters is critical to understanding the dynamics of spotted owlpopulations, but we know the least about them (Franklin In Press).Diet: Although spotted owls take prey from a broad array of taxa (e.g., mammals,birds, insects), they primarily eat small mammals (Marshall 1942,Barrows 1980, 1985, 1987, Solis 1983, Forsman et al. 1984, Laymon 1988,Richards 1989, Thrailkill and Bias 1989, Ward 1990). Three mammal species,woodrats (Neotomafuscipes and N. ctnerec) and flying squirrels (Glaucomyssabrinus), compose the majority of the prey biomass eaten by these owls (Solis1983, Forsman et al. 1984). One of these species usually dominates the diet inan area, and this regional variation in diet is related to habitat and the distributionallimits of the prey species (Forsman et al. 1984, Thomas et al. 1990).Barrows (1985, 1987), Laymon (1988), and Thrailkill and Bias (1989) reportedthat the diet of breeding owls was dominated by larger prey (i.e., woodrats)whereas nonbreeding owl diets were characterized by smaller prey species.This suggested a strong ecological or evolutionary relationship between spottedowls and these larger small mammal species. Unfortunately, the small sampleof owls among these studies precludes strong inference about these relationships.Thomas et al. (1990) also pointed out that large prey may be transportedat a higher rate to nest sites than smaller prey. In addition, Ward(1990) and Forsman et al. (pers. comm.) were unable to document this relationship.Spotted owl prey: Strong functional responses between prey and a variety ofowl species have been demonstrated in both North America and Europe(Southern 1970, Rusch et al. 1972, Adamcik and Keith 1978, Sonerud et al.1988, Saurola 1989). It is surprising, therefore, that until recently, littleresearch effort has been devoted to understanding spotted owl prey andecological responses of spotted owls to their prey. Most prey studies have beenconcerned with patterns of abundance and distribution of small mammalswithin the range of the owl (summarized by Thomas et al. 1990). One study,(Ward 1990), has related variation in prey abundance and distribution to owlreproductive success and hunting behavior, although several studies linkingprey and spotted owls have been undertaken (Thomas et al. 1990). In Ward's(1990) study, woodrats were the primary prey. Spotted owls foraged in areaswhere the abundance of woodrats was less variable. This suggests that theowls may have been optimizing their search effort. That is, they were foragingin stands that did not necessarily contain the most abundant woodrats, butthey hunted in areas where the occurrence of the animals was more predictable.Ward (1990) also showed that not only was prey abundance low but alsothat prey populations were variable across the landscape. These limited observationshelp explain the large home range sizes observed among spotted owls.Availability of spotted owl prey has been advanced as an explanation for theoccurrence of spotted owls in old-growth/mature forests (Forsman 1980,Forsman et al. 1984; also see Carey 1985, Gutierrez 1985). Northern flyingsquirrels clearly depend on forest communities, but woodrats do not.Woodrats are more abundant in early seral stage vegetation (e.g., brushy areas)than they are in old-growth forests (Thomas et al. 1990). Yet spotted owlsspend little time hunting in clear-cuts (Forsman et al. 1984, Solis 1983). Thisunpredicted foraging behavior may be related to the relative availability ofwoodrats to predation in the two habitats. That is, in the dense vegetation ofearly shrub dominated seral stages, spotted owls may not be able to capturewoodrats effectively. In the more open older forests, spotted owls may be moreeffective predators even though the woodrats are less abundant. Thus, we25