Recovery Plan for the Northern Spotted Owl - DRAFT

scribe human actions. Furthermore, results of the "use-availability" analysisprovide no evidence that the features used to categorize habitats had any directinfluence on owls; the features may have been correlates, rather than causes,of the preferences. White and Garrott (1990) provide a good discussion of useavailabilityanalyses.Studies of owl abundance in different habitats usually have employed indexmethods or intensive surveys to estimate relative or absolute abundance in twoor more habitats. Few nonterritorial birds are detected on these surveys, so"abundance" means abundance of territorial birds, not all birds. This issue isdiscussed in more detail in Appendix A. The sample units in these studiesgenerally have been survey stations, circles, or landscapes. Survey stationsare assigned to the habitat category in which the station is located. Circlesand landscapes generally are assigned to habitat categories based on theproportion of the sample unit covered by a particular habitat. For example, thehabitat categories might be defined using percent of the sample unit covered by"older forest" (e.g., stands more than 80 years old). The categories might be 0to 20 percent, 21 to 40 percent, 41 to 60 percent, and more than 60 percent.Alternatively, the habitat type may be regarded as a continuous variable (e.g.,percent older forest). In either case, the investigation would provide informationon the relationship between percent older forest and abundance.Studies of owl demographic rates in different habitats generally have measuredone or more of the following variables: proportion of territories with pairs,number of young fledged per pair-year, turnover rates of territorial adults.survival rates of adults, survival rates of juveniles. Turnover rates are definedas the proportion of adults disappearing from territories each year. Most ofthese analyses have been similar to studies of abundance except that thesample units have been pairs or territories (rather than plots) and demographicrates rather than abundance have been measured for each sample unit. Thedefinition of habitat categories has followed the same process as in studies ofabundance. A few studies (see Appendix C) have used estimates of birth anddeath rates, as well as rates of immigration and emigration, to calculate therate at which population size changed during the study.Studies of the specific resources needed by owls have investigated diet, preybase, nest sites, and home range size and composition. The objective in thesestudies is to identify the resources needed by owls for specific activities and tounderstand the processes that determine how well a particular habitat suppliesthese resources. These studies provide insights about the causes, ratherthan just correlates, of habitat suitability. The studies are reviewed in sectionII.A. of the recovery plan and are therefore not reviewed in this appendix.It may be useful to recognize that the studies mentioned above provide descriptive,correlational, and functional information about habitat relationships ofspotted owls. Measurements of structural features and amounts of habitatprovide descriptive information about the environments where owls are found.Studies of habitat selection, density, and demographic rates provide correlationsbetween habitat features or categories and measures of suitability (e.g.,use, density, or demographic rates). These studies do not necessarily identifythe features of importance to owls, so extrapolation to other environments maynot be warranted. Studies of the specific resources needed by owls, however,help supply this understanding of causal relationships between habitat featuresand owl viability and thus greatly improve the reliability of assessmentsand predictions.287