Recovery Plan for the Northern Spotted Owl - DRAFT

__Discussiondata from 41 sites in the Andrews study area. We did not have individualidentification on the birds from the Meyer et al. (1992) study so disappearancesthat were replaced before the start of the next season would not have beendetected. We did have individual identification for the birds on the AndrewsForest and more accurate estimates of turnover rates.We analyzed the two data sets separately and in combination. In all threeanalyses, persistence increased with habitat quality. The increase was significantin the BLM data set, not significant in the Andrews data set, and highlysignificant in the combined data set (Figure B. 1, Table B. 15). Neither theslopes nor the intercepts in the two data sets differed significantly (or close tosignificantly). In the combined data set, there was no evidence of nonlinearity.Heteroscedasticity (i.e., unequal variances) was evident but this causes no biasin the estimate of slope.Simpson Timber Company (1991) reported that 0.72 young per pair wereproduced by 50 owl pairs in 1990 and that 0.63 young per pair were producedby 92 pairs in 1991; 68 percent of the new recruits in 1991 were adults, and:the turnover rate between 1990 and 1991 was 15 percent; three of 19 birdswere found elsewhere and had moved rather than died. These values weresimilar to the values obtained nearby in the Willow Creek study area.The evidence suggesting that adult survivorship varies with amount of olderforest in the home range is important because population viability is highlysensitive to adult survival rates. Obtaining more information of this sort shouldbe a high priority for the future.Results from Simpson Timber Company suggesting that rates of reproductionand adult survival are similar on Simpson's land and at Willow Creek areencouraging. The next few years should provide a good indication of howsimilar long-term reproduction and survival rates are at these two sites andprovide a good basis for developing conservation programs on private land inthis area. Identifying the key habitat features and analyzing whether they willremain present in the future should be a high priority in this study.Table B.1 5. Regression analysis of occupancy data from the Andrews ExperimentalForest and the BLM in Oregon.Regression Equation Standard LevelNumber (h = proportion of Error ofof area covered by of the Signi-Study Nests suitable habitat) Slope ficanceBLM 50 0.58 + 0.40h 0.18 -0.03AndrewsExperimentalForest 41 0.75 + 0.19h 0.18 -0.30Combineddata set 91 0.63 + 0.39h 0.12 -0.005310