Recovery Plan for the Northern Spotted Owl - DRAFT

size declines. In the former case, the age ratio would be of little value as anearly warning sign, whereas in the latter case it could be of significant help.Ages of first time breeders should be obtained for a series of populations, andthe behavior of nonterritorial birds should be studied so that computer simulationscan be made more realistic. The age ratio usually are obtained in thecourse of monitoring adult survival rates so this information can be obtainedwith little or no extra cost.The second early warning method is the time that vacancies in the territorialpopulation remain unfilled. Much the same argument as was made earlier canbe applied to this variable. If a large nonterritorial population exists, then onewould expect replacement times to be short, whereas eventually, as the populationdeclines, some vacancies will not be filled at all. Thus, the replacementtime should increase gradually as a population declines. Also, if nonterritorialbirds search widely for vacancies, then the upward trend in replacement timemight not begin until nearly all nonterritorial birds had disappeared, whereas ifthey search only locally, the upward trend might begin much sooner. Thisissue could be investigated to some extent by recording replacement times andtracking the change in their average value as a population changes in size.Empirical information about the behavior of nonterritorial birds, however, alsowould be of great value in understanding this variable and using its behaviorto make inferences about population trends. This method would probably onlywork when some vacancies remain unfilled for at least a year.A Comprehensive Monitoring ProgramThis section contains preliminary recommendations for a comprehensivemonitoring program. The suggestions will need review and refinement, butenough information exists to develop the general outlines for the program.Considerable overlap exists (Table A.8) in the information needed by fourapproaches to estimate trends described earlier. For example, the "Demographicrates" approach and the "Population modeling" approach requireinformation on productivity and adult survival rates, and information on thebehavior of nonterritorial birds will be useful in population modeling and in theearly warning approaches. Study of Table A.8 suggests that the monitoringeffort might be subdivided into five programs (Table A.9).The first program is the roadside surveys for territorial birds. Details of thisapproach have been described previously. The second program, activity sitemonitoring, would provide much of the information needed for demographicanalyses and population projections. The difference in requirements of thesetwo approaches is that demographic analyses require that sites be selectedrandomly and habitat information is not required, whereas population projectionsrequire that sites be selected in different habitats and habitat informationis required. These differences can be accommodated by stratifying the rangeinto habitat types and selecting sites randomly within each stratum. Specifichabitat information about the sites also should be collected. Informationcollected at these sites (age ratios and replacements times) will provide much ofthe data needed for the two early warning methods. The third program is aseries of transmitter studies. Studies are needed of dispersing juveniles andterritorial and nonterritorial adults. As noted earlier, intensive monitoring isneeded in some of these studies, even if this means that the sample size mustbe rather small. The fourth program includes miscellaneous studies that donot fit easily into the categories discussed earlier, and the fifth program iscoordination of the first four efforts.269