Chapter 13 



Interannual Differences in Detections of Marbled Murrelets 

 in Some Inland California Stands 



C. John Ralph 1 



Abstract: I compared the mean level of detections of Marbled 

 Murrelets by month over five years at three inland sites in northern 

 California. These areas all have relatively high levels of detec- 

 tions. There were no significant differences in mean detection 

 levels year to year at any site, and for any month with the excep- 

 tion of April at one site. This lack of evidence for significant 

 interannual variation in the number of detections of birds suggests 

 that data from any one of the years would have been sufficient to 

 detect occupancy of these stands by Marbled Murrelets. Caution 

 must be used in applying this result, as interannual variation in 

 detection rates may be greater at sites with relatively few birds, 

 and only three sites were investigated in this study. 



Most species of birds vary in the proportion of birds 

 breeding among years, with profound effects upon the 

 demography of the species. In the case of the Marbled 

 Murrelet, it would be useful to know the proportion of the 

 population breeding. This knowledge would help determine 

 if surveys taken in different years are comparable for purposes 

 of determining the occupancy status of stands proposed for 

 timber harvest. Changes in the number of murrelets detected 

 in a stand during the breeding season are assumed to be 

 related to changes in the number actually breeding in the 

 stand. In this study, I compared the detection rates of murrelets 

 at three sites for evidence of year-to-year variation. Finding 

 a significant difference would indicate that surveys in any 

 one year might not detect birds in a stand that would have 

 had birds in another year, especially a stand with a relatively 

 low detection rate. Although detection rates are not equivalent 

 to numbers of birds actually breeding in a stand (Paton, this 

 volume), I make the assumption that they are analogous. 



Methods 



I examined the among-year variations for three areas 

 with moderate and high detection levels (table 1) in northern 

 California: Lost Man Creek, in Redwood National Park, 

 Humboldt County; James Irvine Trail, in Prairie Creek 

 Redwoods State Park, Humboldt County; and Redwood 

 Experimental Forest, near Klamath, Del Norte County. These 

 three survey sites all are located within large contiguous 

 stands of old-growth redwood in a natural reserve and parks. 



Data used in this analysis were total number of detections 

 (both audio and visual) per survey for each study site for the 



1 Research Wildlife Biologist, Pacific Southwest Research Station, 

 USDA Forest Service, Redwood Sciences Laboratory, 1 700 Bayview Drive, 

 Arcata, CA 95521 



years 1989-1993. Surveys at these sites were conducted 

 according to Marbled Murrelet survey protocol (Ralph and 

 others 1993). Only data from April through August of each 

 year were used, the recommended murrelet survey period in 

 the protocol. 



Data were analyzed using one-way ANOVAs (a < 

 0.05), for each month and year with surveys (table 1). The 

 number of birds detected in a morning's survey were log 

 (count + 1) transformed to approximate normality of the 

 distribution of detections. 



Results 



The detection rate was highest at Lost Man Creek with 

 monthly means ranging up to 240 detections in July 1990 

 (table 1). James Irvine Trail had fewer detections with a 

 maximum average of 146 in July 1990. Experimental Forest 

 had the lowest rate, with a maximum average detection rate 

 of 111 in July 1993. 



I first compared each site separately by month. An 

 inspection of the average number of detections of murrelets 

 (table 1) shows that months in a given year, even with only a 

 few samples, were generally very similar to the averages for 

 that month in the other years with more robust samples. 

 Monthly means were not significantly different at any site, 

 with the exception of April at Lost Man Creek (P = 0.004). 

 This month had a larger range of mean detections than in 

 other months or at other sites. Comparing among years at 

 Lost Man Creek in April, I found that 1990 and 1991 were 

 similar, but that 1989 and 1992 were both different from 

 each other, as well as from other years (Ryan-Einot-Gabriel- 

 Welsch multiple comparison test). 



Discussion 



Though only one month was significantly different over 

 a five-year period at three sites, it is quite likely that further 

 data would show that detections are lower in certain years at 

 specific sites. 



Particularly unseasonable weather during the breeding 

 season could impact numbers of inland detections at specific 

 sites. Fluctuations of prey fish populations may also be a 

 factor in inland murrelet detection levels. Warmer ocean 

 temperatures associated with an El Nino event are 

 responsible for changing local and global weather cycles 

 that affect many species of marine animals, including 

 nesting seabirds and their food (Ainley and Sanger 1979). 

 The ocean temperature events may also affect Marbled 

 Murrelet prey (Burkett, this volume), although this has not 

 been documented. The effects of warmer offshore water 



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



135 



