Recovery Plan for the Northern Spotted Owl - DRAFT

owls. Consequently, the issue of whether a study has used good measures ofsuitability is important, even though we usually cannot provide a precise,quantitative definition of suitability.Information from studies of habitat suitability will be useful in many parts ofthe recovery program. For example, habitats supporting self-sustaining owlpopulations should be identified and described, proposed conservation programsmust be evaluated, models of owl population dynamics must haverealistic descriptions of habitat relationships, and the structure of habitats thatmanagers will attempt to develop and maintain must be described. In thissection, we discuss the types of information needed for these assessments andpredictions, and provide a brief evaluation of the ways in which each kind ofinformation will be most useful in the recovery program.Types of information collectedStudies of habitat suitability for owls may be divided into six broad categories:1. Structural features of utilized habitats2. Amount of suitable habitat in home ranges3. Habitat selection for roosting and foraging4. Abundance of owls in different habitats5. Demographic rates of owls in different habitats6. Studies of specific resources needed by owlsStudies describing structural features of utilized habitats present quantitativemeasurements of the habitats used by spotted owls for roosting and foraging.Variables reported frequently include dbh, canopy cover, and tree speciescomposition. These variables, and others, may be reported for all trees or foroverstory and understory. Separate analyses may be carried out for differentactivities such as nesting, roosting, and foraging, or for different periods suchas breeding and non-breeding. Measures of central tendency (e.g., mean,median), variability (e.g., range, standard deviation), and precision (e.g.,standard error, coefficient of variation) usually are presented for each variablemeasured.Studies estimating the amount of habitat in home ranges generally haveemployed telemetry methods and the "minimum convex polygon" approach todelineate home ranges (see Thomas et al. 1990:193). A few studies havecalculated the amount of habitat within some distance (e.g., 1.3 or 2.1 miles) ofthe nest. Lehmkuhl and Raphael (1991) investigated the use of this method onthe Olympic Peninsula and found that it gave generally satisfactory results forhabitat assessment. The habitats usually have been defined using size classes(e.g., old-growth, mature, pole, etc.) or age classes (e.g., more than 200 years,120 to 200 years, etc.). Combinations of variables (e.g., mean dbh more than21 inches and canopy cover more than 70 percent) have been used to definecategories in a few studies.Habitat selection for roosting and foraging has been assessed using telemetrymethods to define home range borders and identify specific sites used byforaging or roosting birds. Investigators then compare the proportion of thelocations in a habitat with the proportion of the home range covered by thehabitat. Statistical analyses usually have been employed to determine whichhabitats (if any) were used significantly more often than would be expected iflocations were distributed randomly with respect to the habitat types. Habitatsused significantly more often than expected often are referred to as 'preferred"and habitats used significantly less often than expected under the randomdistribution hypothesis often are referred to as 'avoided." There is no implicationof a state of mind, as is usually true when these words are used to de-286