Recovery Plan for the Northern Spotted Owl - DRAFT

number of territorial birds even though the overall population was declining.Eventually, of course, the nonterritorial population would disappear and theterritorial population would begin declining, but the true population declinecould be masked for many years if only territorial birds were being surveyed(Franklin In Press).Upper limits exist on the proportion of all birds that can be nonterritorial. Theproportion is higher with high fecundity and juvenile survivorship. To investigatethis issue, we prepared a computer simulation in which the demographicrates could be varied, and the nonterritorial population was allowed to reachequilibrium levels. With productivity (defined as number of female fledglingsproduced per territorial female per year) of 0.4, and survival rates of 70 percentin the first year and 90 percent thereafter, the model predicts populationstability when about two-thirds of the individuals are floaters. With lowersurvival or productivity rates, the proportion is smaller. For example, withfirst-year productivity of 0.25, subsequent productivity of 0.35, first-yearsurvival of 50 percent, second-year survival of 80 percent, and subsequentsurvivorship of 90 percent, the model predicts stability when about one-third ofthe individuals are nonterritorial.Suppose that 60 percent of the owls in a population were floaters (i.e.,nonterritorial), and the overall population began declining. If floaters immediatelyfilled all vacancies in territories, then the population would decline by 60percent before any decline in the territorial population occurred. Even withrather sharp, and perhaps irreversible, declines, an overall decline in populationsize of 60 percent would take many years. For example, if the populationdeclined 5 percent per year, then 18 years would be required before the territorialpopulation began declining. This example, while extreme, indicates thepitfalls of basing conclusions about population trends solely on programs thatmonitor the territorial population.Despite these problems, surveying territorial birds is an important componentof a monitoring program for several reasons. First, if declines in the territorialpopulation occur, then we can be reasonably certain that the overall populationis declining. Thus, decline of the territorial population does provide evidenceof an overall decline even though the obverse is not true. Second, whileit is possible that replacement of vacancies in territories by floaters might maska population decline, this would only occur if floaters search for vacancies overa rather wide area. For example, if each nonterritorial bird fills any vacancy ina three-territory area, then if the population declines, some vacancies soon willbegin to occur in areas where no nonterritorial birds of the same sex arepresent. Thus, the territorial population will decline, though at a slower ratethan the overall decline. Third, this hypothetical problem caused by floatersonly occurs if a substantial floater population exists: many areas may haveonly a few floaters, in which case surveys of territorial birds might faithfullyreflect population trends. For these reasons, surveying territorial birds shouldbe an important component of the monitoring program for spotted owls, eventhough it should not be the only method used to estimate trends.Design of the sampling programSeveral issues must be considered in designing the roadside survey program.We discuss some of these in this section. Throughout, we assume the purposeis to estimate trends in the territorial population, roughly defined as thosebirds that respond (or might respond) to owl calls.An initial issue is whether the survey should be designed to provide census orindex data. One possible design involves saturating each of several areas withsurvey stations and visiting each one many times, thereby obtaining a nearly253