Recovery Plan for the Northern Spotted Owl - DRAFT

AbstractIntroductionThis appendix discusses methods for estimating or predicting populationtrends. Four approaches are discussed: roadside surveys of territorial birds,demographic analyses, population projections, and two "early warning" methods.Strengths, weaknesses, and requirements of each approach are discussed.The overall monitoring effort could be divided into five programs: 1)roadside surveys, 2) monitoring of activity sites, 3) transmitter studies, 4) otherstudies, and 5) coordination. Computer simulations and analyses of data fromthe Fish and Wildlife Service Breeding Bird Survey both suggest that a minimumof 8 years, and probably at least 10 years, of survey data probably will berequired to obtain reliable estimates of long-term population trends. Delistingtherefore should not be considered until at least 8 years of survey data havebeen collected. Preliminary estimates of sample size requirements suggest thatan adequate sample of the roadside surveys could be obtained with less than 1person-year of effort per state, and that approximately 5 person-years per statemight be an appropriate level of effort for the program to monitor activity sites.These estimates do not include the costs of monitoring remote areas in wildernessesand national parks or continuation of the density studies, althoughthese efforts are regarded as essential if the monitoring effort is to succeed.The program to monitor activity sites probably can be reduced or terminatedafter several years. The estimates in this appendix will need review andrefinement, and are intended mainly as an incentive for further discussion andto provide reassurance that the monitoring program can be carried out at areasonable cost.The sections in the body of the recovery plan on delisting criteria (section III.A.)and research and monitoring (section III.C.6.) identify many questions whichshould be addressed during the next several years. For example, more shouldbe learned about what habitats, and how much of each, should be maintainedin DCAs; proposals for modifying the recovery plan should be formulated andevaluated; and, at some time in the future, consideration should be given todelisting the northern spotted owl. In most of these issues, a need exists either1) to estimate population trends, 2) to compare population trends in differentenvironments, or, 3) to understand the factors that determine populationtrends. For some objectives, primary attention may be given to a single determinantof population trend (e.g., predation rates, productivity), but the overallgoal of the recovery effort is to identify conditions under which owls will have ahigh probability of persisting, and predictions about persistence are difficult tomake when only one factor that determines trends has been studied. Thus,monitoring or predicting population trends is one of the most fundamentalobjectives in the recovery plan.This appendix discusses ways to estimate or predict population trends. Fourgeneral approaches are distinguished (Table A. 1). First, roadside surveysinvolve counts of territorial birds present in an area. An index (e.g., call count)method may be used to estimate trends, or surveyors may attempt to find allterritorial birds in a study area in which case absolute densities and trends ofterritorial birds can be estimated. Second, measurement of demographic ratesinvolves estimating birth, death, and movement rates. Losses (deaths, emigrants)and gains (births, immigrants) are then estimated and compared toinfer how the number of birds changed during the study period. This methodpermits inferences about trends in the total population, including nonterritorialindividuals. Third, population projections involve finding secondary factors,such as habitat parameters, that can be measured and used to predict demo-251