Strachan and others 



Chapter 23 



At-Sea and Foraging Behavior 



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>3/GROUP 



PAIRS 



JFMAMJJASOND 

 n = 5 12 21 158 427 424 801 694 702 451 115 15 



Figure 2 Group size of Marbled Murrelets by month on Ano Nuevo Bay during 1 990, detailing 

 percentage of groups observed consisting of single birds, pairs, or groups of >3 birds, n = number 

 of groups observed. 



they dive almost together". Carter and Sealy (1990) reported 

 that pairs were often seen swimming towards each other 

 before diving, and that three or more birds never dove 

 together in a coordinated fashion. They also stated that 

 foraging by singles and pairs may prevent foraging 

 interference, competition, and kleptoparasitism that would 

 be more likely in foraging flocks. 



Flock size and frequency Carter and Sealy (1990) sug- 

 gested that murrelets are most aggregated during the nesting 

 period. Aggregations of large numbers have been reported in 

 the northern range (Carter 1984; Carter and Sealy 1990; 

 Hunt, pers. comm.; McAllister, unpubl. data). Foraging 

 aggregations were probably related to concentrations of prey. 

 McAllister (unpubl. data) observed an aggregation of 4,000 

 to 6.000 individuals at Point Adolphus on Icy Strait, in 

 southeast Alaska, on 3 May 1991. 



Observers have noted great variation in size of flocks 

 (defined as three or more birds in close proximity and 

 maintaining that formation when moving). In southeast Alaska, 

 Quinlan and Hughes (1984) reported flock sizes up to 50 

 birds in Kelp Bay. Kuletz (199 la) found in another Alaskan 

 population that flock sizes greater than three birds made up 

 about 8 percent of the birds, 7 percent of the birds were 

 found in groups of four birds, 3 percent of the birds in groups 

 of five, and 1 percent were found in groups larger than five. 

 The largest number in a concentrated flock was 22 birds. In 

 British Columbia, Carter (1984) found larger, non-feeding 

 flocks of up to 55 birds. The larger flocks usually occur 

 during the later part of the breeding season, and may be 

 made up of juveniles and subadults. Sealy (1975c) found 

 that flocks would feed together at Langara Island, British 

 Columbia, with the mean flock size of eight. 



Flock sizes in the southern populations of California, 

 Oregon, and Washington, rarely number more than 10, 



according to our and others' observations. Nelson (pers. 

 comm.) recorded groups greater than 3 as very uncommon in 

 Oregon, with a maximum of 10 birds in a flock. Also in 

 Oregon, Strong (pers. comm.), found similar flock sizes 

 during his 1992 study. The largest flock that he observed 

 was 15 birds. In California, Ralph and Long (this volume) 

 found two was the most frequent group size (63 percent), 

 while less than 10 percent of flocks contained more than 

 three birds. The largest flock seen was 12 birds at Santa 

 Cruz. At Ano Nuevo Bay, in central California, flocks are 

 similar in size (fig. 2). Here, at the southern end of the 

 species' range, during late summer and early fall, flocks of 

 over three would often contain juvenile birds. Groups of 

 three or more were found during the summer, when the 

 population is highest (fig. 1), and may be a function of 

 density, rather than flocking. 



Behavior in flocks Sealy (1975c) observed that the 

 flocks would tend to dive against the current, and soon 

 become spaced in a linear fashion with the main axis of the 

 flocks paralleling the direction of the current. Carter and 

 Sealy (1990) observed that larger flocks do not appear to be 

 foraging. Sealy (1975a) stated that birds foraging during the 

 breeding season "invariably occur in pairs or as single 

 individuals." Early in spring adults feed in pairs while the 

 subadults feed singly, but in early July, when pairs are still 

 feeding young at the nest, mixed flocks of adults and subadults 

 begin to form. 



Foraging of Juveniles 



When the first juveniles reach the water during the 

 breeding season, usually by early July (Hamer and Nelson, 

 this volume a), they are distinctive in plumage from adults, 

 making identification of individuals in a small flock possible 

 (Carter and Stein, this volume). From this we can learn 



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



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