Chapter 14 



A Review of the Effects of Station Placement and Observer 

 Bias in Detections of Marbled Murrelets in Forest Stands 



Brian O'Donnell' 



Abstract: A variety of factors influence the results of surveys 

 conducted for Marbled Murrelets (Brachyromphus marmoratus) 

 in the forest. In this paper we examine observer variability and 

 survey station placement as factors influencing murrelet survey 

 data. A training and evaluation protocol (Ralph and others 1993) 

 was developed to insure high field abilities and comparability of 

 data among and between observers. Site characteristics which may 

 limit the bearing and sighting of murrelets (e.g., wind, road, or 

 stream noise, visual obstructions) can largely be controlled through 

 the sensible placement of survey stations. 



In order to interpret data from inland surveys of Marbled 

 Murrelets (Brachyramphus marmoratus), we must be aware 

 of those factors influencing numbers and types of detections 

 at a site. Topography, stand size and shape, and other factors 

 that may influence murrelet densities and habitat use are 

 examined elsewhere in this volume, as are temporal influences 

 on detections and behavior levels. If two areas, each having 

 equivalent populations of murrelets, were surveyed, why 

 might the survey data differ between the two sites? Variability 

 within and among persons conducting surveys is clearly one 

 factor influencing survey data. Levels of extraneous noise or 

 visibility at survey stations may also differ between sites. 

 This chapter examines the influences of observer and survey 

 station placement on murrelet survey data. 



Observer Variability 



There has been little quantitative analysis of observer 

 effects on murrelet survey results. Rodway and others (1993b) 

 found that the numbers of detections recorded by pairs of 

 observers at the same sites showed significant positive 

 correlation. However, they also found significant differences 

 between observers in the proportion of visual detections. In 

 Ralph and Scott ( 1 98 1 ), there are studies on observer effects 

 on landbird censusing results. Ralph (pers. comm.) compared 

 murrelet surveys by many observers from a high-activity 

 site in northwestern California to find the area around the 

 observer that is effectively surveyed. Kuletz and others 

 ( 1 994c ) found significant observer effects on detection levels 

 at sites in Alaska. 



1 Wildlife Biologist, Pacific Southwest Research Station, USDA Forest 

 Service, Redwood Sciences Laboratory. 1700 Bayview Drive. Arcata. 

 CA 95521 



Site Characteristics 



Visibility at Survey Station 



A high percentage of murrelets remain unseen to the 

 observer during surveys (Paton and Ralph 1988; Nelson 

 1989). The numbers of visual sightings of murrelets are 

 strongly influenced by the location of the observer, yet 

 they are critical for determining murrelet use of a stand 

 (Paton, this volume). Nelson (1989) reported the highest 

 percent of visual detections (55 percent) occurred at a 

 survey station with the greatest view of open sky. Rodway 

 and others (1993b) detected murrelets visually in 19 percent 

 and 26 percent of detections at two sites in British Columbia. 

 They speculate that greater visibility accounted for more 

 visual detections at the latter site. Ralph (pers. comm.) 

 examined the effect of canopy cover upon detection and 

 behavior levels to assess the level of survey effort needed 

 to determine occupancy by breeding murrelets in a stand. 

 Detections indicating probable nesting in a stand are of 

 murrelets below or within the canopy, and require visual 

 sightings of the birds (Paton, this volume). At a dense 

 canopy site, only 3 percent of detections indicated 

 occupancy, while at moderate and open canopy sites, these 

 detections were 14 percent and 30 percent, respectively. I 

 (O'Donnell 1993) found that, in general, sites with greater 

 visibility had more detections of murrelets below the canopy 

 (fig. 1). Visibility at each site was quantified by estimation 

 of the percent of clear view to the horizon in all directions. 

 Including only sites in or near old-growth stands, I found 

 that visibility had a significant positive relationship (r = 

 0.73, P = 0.04, n = 8) with the numbers of below canopy 

 behaviors observed. 



Environmental Noise 



While there are no studies which examine the effects of 

 extraneous noise (e.g., wind, road, or stream) specifically on 

 murrelet survey results, it seems very likely that any noise 

 impairing an observer's ability to hear murrelets will be 

 detrimental to survey goals. The effects of environmental 

 noise on the audio detection of landbirds are discussed in 

 papers in Ralph and Scott (1981). 



Environmental Acoustics 



Acoustical properties in the environment will degrade 

 bird song and calls in a variety of ways (Richards 1981). 

 Attenuation, the decrease in intensity of sound with distance, 

 can be affected by habitat type. Kuletz and others (1994c) 



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



139 



