Chapter 35 



Productivity of Marbled Murrelets in California from 

 I Observations of Young at Sea 



C. John Ralph 



Linda L. Long 1 



Abstract: We designed and tested an intensive survey method in 

 1993 to identify juvenile Marbled Murrelets (Brachyromphus 

 marmoratus) at sea. From this, we used the percent of juveniles 

 seen in the sample as an index of productivity of murrelets along 

 the California coast. We found 2.2 percent of the population sampled 

 were juveniles, similar to our estimates in 1989 1992 of 3 percent 

 from less stringent survey methods in this area. Percent of juveniles 

 in the 1993 sample ranged from almost 6 percent in late June to 

 none in mid-August and September. Juveniles were as often alone 

 as in a group with 1 or 2 adults, and showed a similar distribution 

 in distance from shore as adults. We found that some adults were 

 molting into basic plumage as early as 21 June, with three-fourths 

 in molt by mid-August. Therefore, during September, most birds 

 were unidentifiable to age since most appeared to be in basic 

 plumage unless they flapped their wings to expose molting primaries 

 or markings on the lower breast or belly. 



One of the vital components of making a demographic 

 model of any species is a measure of that species' productivity. 

 In the case of the Marbled Murrelet (Brachyramphus 

 marmoratus), virtually all measures that would go into a 

 demographic model (Beissinger, this volume) are conjecture, 

 based upon studies of other species. Many of these species 

 have only fleeting similarities to the life history of the Marbled 

 Murrelet. The percent of young birds found at sea in the 

 summer is one of many potential estimators of productivity, 

 and is the one pan of the demographic life history that could 

 be based on actual numbers. Since murrelets are often difficult 

 to observe closely, this quantity has previously only been 

 estimated. We describe our efforts to put this vital parameter 

 on a firm, quantitative foundation. 



We have conducted offshore population surveys during 

 1 989- 1 992 in California. During these surveys, we determined 

 the proportion of murrelets in juvenal plumage as they 

 occurred in late July and early August. However, the 

 proportion of birds in juvenal plumage was exceedingly 

 small, usually less than 3 percent. During these offshore 

 surveys ( Ralph and Miller, this volume), we made a conceited 

 attempt to determine the age of all birds not in obvious 

 breeding plumage. 



Other investigators have found similar low proportions 

 in recent years. During the 1992 offshore Oregon surveys, 

 Strong and others (1993) found the proportion of juveniles to 

 be 2.7 percent. At three points on the Oregon coast over a four 



' Research Wildlife Biologist and Wildlife Biologist, respectively, 

 Pacific Southwest Research Station. USDA Forest Service, Redwood Sci- 

 ences Laboratory. 1700 Bayview Drive, Arcata, CA 95521 



year span (1988-91), Nelson and Hardin (in Beissinger, this 

 volume) found that juveniles made up 2, 4, 2, and 5 percent of 

 the population, respectively. If these estimates of reproduction 

 are accurate, this low rate of recruitment indicates one of 

 three possibilities: a markedly declining population, one whose 

 low reproduction must be offset by years of much higher 

 production; or the species has to be extremely long-lived. 



These low figures prompted a reevaluation of our methods 

 for the 1993 breeding season. We felt that it was possible that 

 our measure of productivity might be misidentifying some 

 juveniles. Therefore, we designed an intensive survey method 

 to identify juvenile birds at sea and report here on the surveys 

 used to test the new method and to assess its accuracy. 



Molt Sequence 



The molt sequence has been investigated by Carter and 

 Stein (this volume), and the information below is largely 

 taken from their paper. The breeding plumage is dark overall, 

 with the entire breast, belly, and sides covered with blotches 

 of dark color, each blotch taking up about half of each 

 feather. During the fall pre-basic molt, the back color changes 

 from the rich brownish black to a duller grey black, but this 

 is difficult to see in the field. Mated pairs may often stay 

 together and molt fairly synchronously. In adults, the timing 

 of the change into winter plumage is poorly documented. It 

 seems generally to be underway by late July, and probably 

 takes 6 to 8 weeks. Failed or non-breeders may molt much 

 earlier. The pre-basic molt begins in the throat area, as the 

 dark feathers are replaced by white, then spreads to the 

 breast, belly, sides, and lower belly. At approximately mid- 

 molt, the first six primaries are lost almost simultaneously. 

 This leaves the bird flightless, with a conspicuous gap in 

 the wings, and thus distinguishable from young when the 

 birds flap their wings. The remaining primaries are lost 

 shortly thereafter. As the bird molts to winter plumage, the 

 dark blotches gradually become fewer in number, but 

 generally remain as identifiable blotches until the breast 

 and belly become white. 



Fledglings are seen on the water as early as the second 

 week in June, but the majority will not appear until July. 

 Fledging of young from North American nests begins in 

 early June, reaching a plateau from early July through late 

 August (Hamer and Nelson, this volume a). When first 

 fledged, they resemble winter adults, dark above, and light 

 below. However, in contrast to the clean, white breast and 

 belly of the winter-plumaged adult, the neck and breast of 

 the young will have a highly variable pattern of fine markings 

 or tiny dots on the outer edge of some of the feathers. This 



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



371 



