Ralph and Long 



Chapter 35 



Productivity in California-Observations At Sea 



major factor, with under 40 meters considered good under 

 most conditions. Distances as far as 50 m, with a longer 

 viewing time and high light levels, were also considered 

 good observations. Beyond 50 m, observations were usually 

 of poorer quality and were usually qualified as probable or 

 unknown. Observations from shore with a telescope were 

 used if the bird was less than 400 m from the observer and 

 other criteria above were met. 



Plumage was the primary criterion used for determining 

 the age category, since no egg teeth were seen. Any bird in 

 breeding plumage or in the process of molting out of breeding 

 plumage was a definite adult. Black-and-white birds with 

 missing flight feathers were also categorized as adult. Black- 

 and-white birds with no information on flight feathers were 

 categorized in part by the date of observation, as during the 

 molt period it was difficult to distinguish adults in winter 

 plumage from juveniles. In this regard, Carter found all 

 birds to be in alternate plumage from early May to late July 

 (Carter and Stein, this volume). However, by late July, some 

 adults might begin to molt if they were failed breeders, and 

 take possibly as little as 6 weeks to complete enough of the 

 molt to appear black and white. Therefore, we considered 

 any black-and-white birds seen before 15 August as juveniles. 

 After that date, birds were not considered juveniles unless 

 other criteria were noted. Other potential criteria for identifying 

 juveniles were the presence of the fine breast markings, 

 relative size, and behavior. Black-and-white birds 

 accompanied by an adult and less than 90 percent of the size 

 of the adult were also categorized as juveniles. There were 

 no observations of what we would have considered juvenile 

 behaviors, such as begging from an adult. After 1 5 August, 

 all winter-plumaged birds were categorized as unknown in 

 the absence of other identifying criteria. 



Results 



We attempted to determine the age of 1,174 murrelets 

 (table 1). We successfully aged by the above criteria 1,084 

 birds and had only 103 birds of unknown age. Only 23 birds 

 (2.2 percent overall) were juveniles, when the probable and 

 definite categories were combined. If we excluded the probable 

 observations, then the estimate of juveniles was much smaller 

 at only 0.6 percent. 



We found that juveniles occurred equally as often alone 

 (n = 12) as in groups with 1 or 2 other murrelets of either 

 adult or unknown plumages (n = 11) (table 2). We did not 

 find juveniles in groups with other known juveniles. 



We analyzed the distribution of adults versus juveniles 

 relative to the distance from shore, based on boat surveys 

 alone to eliminate the bias from on-shore surveys. We found 

 no significant difference (jf, P > 0.05) in distribution out to 

 1600 m (table 3). 



The percentage of juveniles by area was: Crescent City 

 0.6 percent (n = 2 juveniles), Trinidad 4.0 percent (n = 12), 

 Eureka 1.1 percent (n = 3), and Santa Cruz 3.4 percent (n = 

 6). With so few birds in juvenal plumage, we did not consider 

 the differences between areas to be biologically significant. 



We divided the 1993 study period into 10-day periods 

 (table 1). In June and early July, nearly 6 percent of the 

 known-aged birds observed were juveniles. This percent 

 varied through early September with 2.9 percent juveniles 

 recorded. No juveniles were identified after mid-September. 

 If only the data before 9 September were included (which 

 excluded the time when juveniles were difficult to identify), 

 the overall proportion of juveniles did not change. 



The first juvenile was seen on the second survey of the 

 study on 26 June in Crescent City. The first juveniles for 



Table 1 Classification of plumages of Marbled Murrelets seen off the California coast by 10-day periods in 1993. All birds with identifiable plumages are 

 categorized as definite or probable adult, definite or probable juvenile, or unknown age bird in basic plumage. Percentage of adults andjuvenUes are calculated 

 based on the total number of known ages, while percentage of unknown ages is calculated as a percentage of all birds with identified plumages 



Period 



Adult 



Juvenile 



Known age Unknown age Total 



Definite Probable Total Percent 



Definite Probable Total Percent total Total Percent 



6/21-7/30 



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



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