Recovery Plan for the Northern Spotted Owl - DRAFT

Table A.1. General approaches for estimating or predicting population trends.1. Counts of territorial birdsa. Trends in abundanceb. Absolute abundance2. Measurement of demographic ratesa. Territorial birdsb. Nonterritorial birds3. Population projections4. Early warning methodsa. Age ratios among first-time territory holdersb. Replacement times for territorial birdsRoadside Surveysgraphic rates and population trends. Development of this approach couldhelp researchers predict the effects of particular management strategies andmight reduce the cost of monitoring populations by using habitat variables as apartial substitute for owl population variables. Fourth, two miscellaneous'early waming" methods for detecting a change in the number of territorialbirds are described in this appendix. Neither method permits estimation oftrends, but the data required could be collected relatively easily and wouldprovide advance warning of imminent population declines in areas not wellsurveyed by the more sophisticated methods.In the later sections, we describe each approach including its strengths andweaknesses and key research or design topics that should be investigated indeveloping the approach. We then identify overlap in the data needed for eachapproach and use this analysis as a basis for suggesting that the monitoringeffort be divided into five programs. In a final section we provide preliminarysample size guidelines and estimate the amount of time needed before delistingshould be considered.We appreciate the help of the FWS Breeding Bird Survey program in providingdata on short notice. Earlier drafts of this appendix were reviewed by RobertAnthony, R. J. Gutierrez, Jesefa O'Malley, Edward Starkey, and Jerry Verner.One way to monitor owl populations is by counting all the owls in an area, orby using some of type of index method that, while not revealing actual abundance,provides a reliable estimate of trends in abundance. We refer to theseas roadside surveys, although counts might actually be made from trails rathrthan roads. Owl surveys routinely are made by nighttime or daytime calls.Unfortunately, nonterritorial birds seldom respond to the calls, thus thismethod only monitors the territorial population. Concern exists, that theoverall population might be changing, and in particular that it might bedeclining, but territorial birds that died might be quickly replaced bynonterritorial birds. As a result, there might be little or no change in the252