Strachan and others 



Chapter 23 



At-Sea and Foraging Behavior 



180 n 



160- 



co 140 

 Cfl 



o 



w 100 



Q 



i 80 



LL 



O 60 



cc 



LJJ 



m 40 



z 



20- 



0- 



n = 



221 1254899 11 594 



7 9 



4 2 



M 



M 



N 



Figure 1- 



the mean 



- Average number of Marbled Murrelets/census by biweekly periods on Ano Nuevo Bay during 1 990. Figure shows 

 standard error, and minimum and maximum values of from-shore censuses, n = number of censuses/time period. 



Foraging Behavior 



Pairing and Group Size 



Frequency of pairs Murrelets forage mostly in pairs 

 throughout the year. This is an important aspect of their life 

 at sea, as we have often observed murrelets vocalizing on the 

 water while foraging, apparently attempting to locate the 

 other member of a pair when coming to the surface, or after 

 a disturbance. The call usually used is the typical "keer" in 

 rapid succession or singly. The percentage of birds in an area 

 that are foraging or loafing in pairs varies, but not greatly. 

 Mean group size from Oregon was 1 .8 birds, with about 70 

 to 80 percent of the birds observed in pairs (Nelson 1990). 

 Along the central Oregon coast, Strong and others (1993) 

 observed that murrelets almost always occurred as single 

 birds or pairs. In Alaska, pairs made up 45 percent of the 

 population (Kuletz, pers. comm.). Carter and Sealy (1990) 

 found in Trevor Channel, Alaska, that pairs were 40 percent 

 of the birds seen. During the summer of 1993, Ralph and 

 Long (this volume) reported 63 percent of groups were pairs 

 and 27 percent were single birds in northern California. 



In central California, 75 to 80 percent of birds foraged 

 as pairs during the breeding season (fig. 2). Single birds are 

 more common in the winter, when the populations are low at 

 this location (fig. 2). Sealy (1975c) suggested that, during 

 the incubation period, a daily pairing of birds occurred as 

 birds flew around in the forested nesting area after an 

 incubation exchange. We have observed many single birds 

 circling and calling at inland sites until joined by a second 

 bird, when both headed west to the ocean. We have also 

 observed at times many hundreds of birds arriving at the 

 ocean in the morning from inland nesting sites, usually in 



pairs, threes, or fours. Observations at the nest would suggest 

 that the birds should arrive singly (Naslund 1993a; Nelson 

 and Hamer, this volume a), as pair members are rarely at the 

 nest simultaneously, which might suggest that the birds pair 

 with non-mates enroute to the sea. 



Composition of pairs In British Columbia, Sealy 

 (1975c) found that 1 1 out of 13 pairs collected in late April 

 were composed of an adult male and adult female. After egg 

 laying occurred, more single "off duty" birds were encountered 

 at sea. He surmised that both adults stay together during the 

 day and returned to the nest site at night to feed their chick. 

 The subadults (birds one or two years old who have not yet 

 bred, as determined by collecting) also returned in late April, 

 but were encountered only as single individuals until late 

 June and early July when mixed groups of "off duty" adults 

 and subadults, were observed. During late July newly fledged 

 young were frequently seen in these groups. 



Reason for foraging in pairs Sealy (1975c) stated "I 

 believe that the occurrence of these pairs can be adequately 

 explained on the basis of pair bond maintenance and that an 

 advantage to feeding need not be involved." Possible 

 evidence of pair bonding is found in observations of pairs 

 separated by boats. Ralph (unpubl. data) and Miller (pers. 

 comm.) have noted that about two-thirds of these pairs call 

 and attempt to reunite, while the remaining birds simply 

 disperse. However, we feel that foraging plays the major 

 role in pairing, and probably involves some sort of cooperative 

 foraging technique. Evidence of this includes the observation 

 that the vast majority of actively foraging paired murrelets 

 consistently dive together (Carter and Sealy 1990). Laing 

 (1925) stated that the "birds of this genus work in winter 

 and summer in pairs, but not as a defensive measure, for 



248 



USDA Forest Service Gen. Tech. Rep. PS W- 152. 1995. 



