Recovery Plan for the Northern Spotted Owl - DRAFT

complete count of the territorial birds within the study area. The results mightbe thought of as "plot-type data" because within each study area all, or nearlyall birds, are detected. Alternatively, "index methods" might be used in whichno attempt was made to count all birds in any area. Results would be expressedas birds recorded per station or some other measure of effort, andinvestigators would assume that this measure had an approximately constantrelationship to true density in the surveyed area. Under this assumption,changes in the index reflect changes in the population being surveyed. Thoroughsurveys give data less susceptible to substantial bias, but cost muchmore to collect. Index data are far less expensive to collect but may give biasedestimates of temporal trends if detection rates change through time. Manyareas, several of considerable size, probably will be searched thoroughly forterritorial birds each year regardless of whether index or plot-type methods areused in the monitoring program. These data provides a basis for "calibrating"the index data by using a technique known as double-sampling. We describethis method below.A double-sampling approach for monitoring territorial spotted owls mightproceed as follows. First, areas to be searched thoroughly would be selectedand delineated on maps. The main criterion for selecting these areas would beavailability of surveyors willing to search the areas thoroughly (defined, forexample, as searching to protocol). The areas could be large (e.g., demographicstudy areas) or small (e.g., single patches of old-growth) and would not have tobe selected randomly, though random selection might be advantageous insome cases. Presence of a spotted owl within the area would be defined asoccurring when the bird's activity center was within the thoroughly searchedarea. The assumption would be made that such searches constituted censusesof the territorial birds in the areas. The results would provide "truedensities" for each area. Of course, in reality some owls would be missed, butsuch errors would have little effect on the estimate of trend if fewer than 10percent of the birds were missed and this figure did not vary greatly amongyears. The thoroughly searched areas would be regarded as one stratum.The next step in developing the program would be the delineation of additionalstrata. Strata could be defined to include DCAs and other high-interest areas,areas in which density is anticipated to be high, areas in which density isanticipated to be low, etc. The strata would not have to be contiguous; thusone stratum might consist of all the areas dominated by old-growth, anotherstratum might consist of all the areas with moderate amounts of old-growth,and so on.Survey routes then would be selected using a well-defined random selectionplan. For example, starting points for routes could be selected randomly andthen the route could be laid out following predetermined rules. Efficiency (e.g.,precision for a given amount of labor) would be maximized by (a) maximizingthe number of different stations visited, and within this constraint, (bM maximizingthe number of owls recorded. These principles suggest that eachstation should be visited at most once a year and that surveys should beconducted at a time and in a way that maximizes the number of owls recordedper hour of surveyor effort. Sampling intensity could vary among these strataso that more stations were located within areas of high interest, easy access, orhigh density. Sampling intensity in these strata could be detennined subjectively,or they could be determined with formulas for maximizing statisticalefficiency.Each year, some or all of the randomly selected routes in these aditional stratawould be visited and a random selection of stations (or routes) would be visiteda single time in the censused stratum. Subsequent work would reveal theactual densities in the thoroughly searched areas, and these densities would254