Miller and Ralph 



Chapter 20 



Inland Habitat Relationships in California 



volume), and observer (O'Donnell, this volume). To help 

 eliminate the effects of observer bias, all stands were surveyed 

 by two or more observers. The influence of weather on 

 numbers of detections appears to be highly variable (Naslund 

 and O'Donnell, this volume). The effect of weather is 

 probably stochastic with respect to survey days, and we 

 assumed it did not have an overall impact at a site because 

 surveys were distributed throughout the breeding season. 

 The seasonal variation in detection levels, however, has 

 been well documented and quantified at several sites in 

 California (O'Donnell and Naslund, this volume). To identify 

 differences in murrelet use (detection levels) of stands in 

 our study, we first accounted for the effect of season on 

 detection levels. 



Morning surveys were conducted throughout the breeding 

 season in multiple years at three sites in Humboldt County. 

 The sites at Lost Man Creek (Redwood National Park) and 

 James Irvine Trail (Prairie Creek Redwoods State Park) 

 were surveyed from 1989-1993. The Experimental Forest 

 site was surveyed in 1989, 1990, 1992, and 1993. We 

 attempted to monitor each site weekly. Data from these three 

 sites was used to calculate standardization factors. 



Standardization 



The following method was used to calculate a factor to 

 standardize the number of detections for seasonal differences. 



1 . We examined the distribution of detections (fig. 4) 

 over all years for the three sites and used a Kruskal- 

 Wallis test to determine that the distributions by 

 season were similar for the three sites (P < 0.0001). 

 Surveys from all sites and years then were pooled. 



2. We calculated the mean number of detections per 

 survey for the period 15 April to 12 August, that we 

 refer to as the summer mean. 



3. We then calculated the mean numbers of detections 

 per survey for each 10-day interval, the interval mean. 

 Detection levels for periods longer than 10 days 

 began to show the effects of seasonal variation. 



4. The ratio of each of the 12 interval means and the 

 summer mean was calculated (interval mean/summer 

 mean = standardization factor). 



The 10-day intervals and corresponding standardization 

 factors calculated for the data from the three sites are presented 

 in table 1. 



250 -r 



200 - 



O 150 



& 



6 



g 100 



50 



All sites combined 

 Experimental Forest 

 James Irvine Trail 

 Lost Man Creek 



Q 



10 

 CM 



(0 



> 



CD 



CO 



10 

 CM 



- 

 r 



10 Day Period 



3 

 CO 



Figure 4 Mean Marbled Murrelet detections from forest surveys at three sites in northern California: James Irvine 

 Trail, Prairie Creek Redwoods State Park; Lost Man Creek, Redwood National Park; and the USDA Forest Service 

 Experimental Forest, Klamath. Means for the three sites combined by 10 day intervals also are presented. Surveys 

 were conducted 3-4 times per month most years from 1 989-1 993 and points represent the means for 1 0-day intervals. 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



209 



