Recovery Plan for the Northern Spotted Owl - DRAFT

the estimate (using the simple equation above and ignoring sampling error)would be 1.002. No one knows the true value for P but 0.7 is reasonableand serves to indicate the potential seriousness of t~his source of error.2. Underestimation of 9YSome birds born in year t, leave the study area but survive until the start ofyear t+ 1. They usually are counted as having died which causes sj, to beunderestimated. The significance of this problem depends on how manyjuveniles emigrate (and survive), which in turn depends on the size of thestudy area, intensity of searches outside the study area, and dispersaldistances. If a proportion of the juveniles, pie, emigrates from the study areabut survives at the same rate as those juveniles that remain within thestudy area, then s is underestimated (aside from sampling error) by pje, andX is underestimated by fSpA.. For example, if 20 percent of the juvenilesleave the study area (Pie = 0.2) and f = -§J = 0.3, then the error is 0.018. Ifthe true X was 1.00, the estimate (aside from sampling error) would be0.982. Detailed studies that would permit estimation of p,, for real studyareas have not yet been carried out.3. Temporary emigration.Population ProjectionsBecause nonterritorial birds are seldom detected, young birds typically"disappear" for a few years and then reappear when they gain territories.The capture probabilities vary in exactly the way they would if the birdstemporarily left the study area and then returned to it as adults. Suchbehavior violates the assumptions of capture-recapture models, and isknown in capture-recapture literature as the problem of "temporary emigration."The problem is complex because its effects on the capture-recaptureestimates are difficult to assess. Balser (1981) showed that emigration bysome juveniles, who return to the study area as adults, causes over-estimationof the juvenile survival rate. The process with spotted owls is somewhatdifferent because the period of temporary emigration (i.e., period duringwhich the birds are nonterritorial) may last a few years into the adult ageclasses. Presumably, however, the behavior causes survival rates, particularlyof juveniles, to be overestimated. As a result, X would also be overestimated.Despite these possible problems, measurement of demographic rates andcalculation of trends from them, offer the only way at present to estimatetrends in the whole population (i.e., both territorial and nonterritorial individuals)within a study area.Use of these methods is illustrated in Appendix C which describes analyses ofdata from five demographic study areas. The studies provided data from 5 to 7years and from approximately 300 to 700 birds. The confidence intervals for Xvaried from approximately 0.04 to 0.09. These levels of precision were sufficientto show that the populations studied were declining, however considerablymore data would be required to detect trends of 2 to 4 percent per year.Various difficulties with the direct counts and demographic analyses as ameans of estimating population trends have been identified earlier in thisappendix. One additional problem with these methods is that they apply onlyto the area and period of time actually monitored. No rigorous basis is providedfor predicting trends in the future or trends that would occur in hypotheticalsituations (e.g., under a proposed management plan). The simplest266