Varoujean and Williams 



Chapter31 



Abundance and Distribution in Oregon and Washington 



to be a 1,000 m wide band of coastal water off both the 

 Washington and British Columbia shorelines (area! estimates 

 of 93 km 2 and 65 km 2 , respectively). 



During aerial surveys, Marbled Murrelet location is 

 referenced to the time of sighting, and recorded to the 

 nearest second by the observers. To obtain the location of 

 murrelets. a sighting database file created from the transcribed 

 audio tapes was later merged with the position database file, 

 using interpolation and mapping software. Specifically, 

 CAMRIS (Computer Aided Mapping and Resource Inventory 

 System) was used. The interpolated murrelet database was 

 then used to determine bird density (number of observed 

 murrelets divided by the area of the survey transect). 

 Population estimates are based on projections of the density 

 estimates for 20' latitudinal blocks on the outer coast (except 

 12' near Pt. Saint George, and 24' near Cape Flattery). The 

 projection of density is over the area! extent of a block, 

 which is the length of a block times the width of the study 

 area ( 1 .000 m in Oregon, 4,000 m in Washington). Similarly, 

 abundance data for the Strait of Juan de Fuca are shown in 

 longitudinal blocks of varying length. This broad-scale 

 approach was used to minimize the error of overestimating 

 density, which can occur when block size is small enough to 

 result in the counting of birds more than once as they move 

 between adjacent blocks. 



In 1993. steps were taken to insure that observers 

 maximized the time they actually were looking out the 

 window. The location of all other seabird species was 

 referenced to 5-minute blocks of time as reported by the data 

 recorder over the aircraft's intercom system. This freed the 

 observers from having to look at their watches for every 

 seabird sighting. Before this change, observers could spend 

 20-30 percent of the survey time being spent with an observer 

 looking at their watch, not out the window, when in areas of 

 high seabird abundance. 



When possible, observers noted the number, group size, 

 plumage and age of all seabirds seen, including Marbled 

 Murrelets. When two murrelets were seen <5 m apart they 

 were designated as a pair. Lone birds and birds seen to be >5 

 m apart were designated as singles. Groups larger than 2 

 birds were designated as such, regardless of the distance 

 between individual birds in the group. 



During the 1 993 aerial survey, we noted that a number 

 of adult Marbled Murrelets appeared to be in a transitional 

 molt from alternate plumage to basic plumage (these birds 

 appear mottled gray in color instead of mottled brown). 

 By late August and early September 1993, Ralph and 

 Long [this volume] noted that a number of adult murrelets 

 in northern California had molted into basic (winter) 

 plumage. Similarly, a number of adults in Puget Sound 

 had by early September molted into basic plumage (Stein, 

 pers. comm.). Given these findings, we classified Marbled 

 Murrelets seen during the aerial survey as being in either 

 alternate plumage (presumed to be adults) or black-and- 

 white plumage, a category that comprises adults in basic 

 plumage and hatch-year birds. 



Results 



Abundance 



On-transect observations along the various segments 

 of the Oregon coast resulted in the sighting of 882 Marbled 

 Murrelets, and a projected population estimate of 6,138 

 birds (table 1). Observations along the various segments of 

 the outer Washington coast resulted in the sighting of 226 

 murrelets, and a projected population estimate of 2,907 

 birds (table 2). Flights off the shores of the western Strait 

 of Juan de Fuca resulted in the sighting of 36 Marbled 

 Murrelets on the Washington side, and 18 murrelets on the 

 British Columbia side, with projected population estimates 

 for these two areas of 340 and 306 birds, respectively 

 (tables 3 and 4). The combined population estimate for the 

 Washington outer coast and western Strait on the Washington 

 side is 3,250 birds. 



These projected population estimates are probably 

 underestimates in that, while surveying in aircraft, because 

 foraging murrelets can be readily missed when they are 

 diving. An analysis of dive data obtained from the tracking 

 of radiotagged Marbled Murrelets (table 5) indicates that 

 aerial surveys underestimate abundance by approximately 

 5-10 percent Adjusting the projected population estimates 

 for this source of underestimation yields adjusted population 

 estimates of approximately 6,400-6,800 buds in Oregon and 

 3,400-3,600 birds in Washington. 



Group Size and Plumage 



In both Oregon and Washington (including birds from 

 the British Columbia side of the Strait) approximately one- 

 quarter of the Marbled Murrelets seen were recorded as 

 being in black-and-white plumage (tables I and 2). Groups 

 classified as pairs made up an higher proportion of groups 

 seen in Oregon (45 percent) than in Washington (25 percent). 

 But, if you examine each state separately, the proportional 

 distribution of group size was similar regardless of plumage 

 category (tables 6 and 7). It is unlikely that hatch-year birds 

 would have a group-size distribution similar to those of 

 adults, since many nesting and just post-nesting adults would 

 still be paired in late August and early September. Further, 

 with black-and-white birds making up one-quarter of the 

 murrelets seen, and given our observations of some adults 

 appearing to be in molt from alternate to basic plumage, it is 

 likely that a substantial number of the black-and-white birds 

 seen were adults in basic plumage. 



Distribution 



In Oregon, Marbled Murrelets were found to be most 

 abundant off the central part of the state from Coos Bay 

 north to Cascade Head (table 7). In contrast, murrelets were 

 in general less abundant in the southern and northern thirds 

 of the state. Based on distribution maps presented in 

 Varoujean and Williams (1994a), murrelets appeared to be 

 more abundant near the entrances to major rivers and 

 embayments. Birds also appeared to be more abundant close 



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



329 



