Recovery Plan for the Northern Spotted Owl - DRAFT

should play a minor role, compared to efforts based more on understanding thecauses of trends (i.e., population modeling). We believe the latter efforts willprovide a more reliable and cost-effective way to estimate or predict trends.The points discussed previously provide a basis for recommending sample sizeguidelines for the roadside surveys and demographic studies. Obviously, theinitial choice may need revision as new information becomes available. Weassume that 10 years of data will be available, and that both the roadsidesurveys and the demographic analyses should have an 80 percent chance ofdetecting annual changes of 2.5 percent at the statewide level or 3.5 percent atthe province level. The analysis discussed earlier suggests that changes ofsmaller magnitude probably would be expensive to detect and hard to interpretsince such changes may occur commonly in healthy populations. At present,such a guideline would lead to recommending that the roadside survey includevisits to approximately 750 stations per year in each of the three states.Assuming that 15 stations are visited per person-day, the fieldwork wouldrequire 50 person-days per state or 10 to 15 days per province, a modestexpenditure of effort that easily could be continued for many years. As notedearlier, detailed calculations have not been carried out yet for the demographicapproach, but continuation of the density study area programs probably willprovide the required statistical power.Developing sample size guidelines for activity site monitoring is more difficultbecause the data collected during this work will be used for several purposes:demographic analyses, population projections, age ratios and replacementtimes. Objectives for these projects need to be specified before detailed samplesize guidelines are developed for the activity site monitoring, but a few preliminarycalculations may be helpful. Monitoring the sites will take several visitsper season. If a total of 5 person-days were required per site, roughly 40 sitescould be visited per-person year of effort. Thus 5 person-years of effort, forexample, probably would be sufficient to monitor about 200 sites per year.Most of the parameters estimated in the 'activity site monitoring" program areproportions or can be treated as proportions to obtain approximate sample sizerequirements. In most cases, interest probably will center on comparing twoproportions; e.g., productivity or turnover rates in two habitats. The standarderror of a difference between two proportions depends on the proportions and,if data are collected in more than one year, on how much the difference betweenproportions varies among years. For this example, we will assume thatTable A.1 0. Preliminary estimates of the number of stations that must be visited peryear to have an 80 percent chance of detecting trends.Average number of birds recorded per 100 survey stations:8 10Trend 8 years 12 years 8 years 12 years2.0 percent - 1,150 1,400 8502.5 percent 1,400 900 1,200 550\ 3.5 percent 1,000 360 800 250277