contained 7 species and was predominantly composed of 

 wedge-tailed shearwaters (8695 ) and unidentified noddies. It 

 was encountered on 13 October adjacent to Pohnpei. Three 

 environmental factors undoubtedly contributed to the size and 

 diversity of this flock: 



/. The flock was near a high island at 07°N. within the 

 Equatorial Countercurrent boundaries (ca. 4°N and 9°N ). where 

 particularly rich upwellings provide feeding grounds for both 

 fish and birds (Ashmole & Ashmole, 1967: Gould, 1974a); 



2. Flying fish, considerably more abundant compared to 

 the previous 3 days, indicated increased productivity; and 



3. The flock was attracted to masses of predatory fish 

 (primarily tunas) vigorously leaping from the water, causing 

 the water to "boil." 



Flock Composition 



Five families (Laridae. Procellariidae. Fregatidae, Sulidae, 

 Phaethontidae ), 1 2 species, and 3 unidentifiable groupcategories 

 were represented in feeding flocks. Sooty terns dominated 

 the feeding flocks, followed numerically by noddies and wedge- 

 tailed shearwaters (Fig. 7; Tables 5,7). These three species 

 groups accounted for 95% of all birds seen in flocks. 



Sooty terns participated in 71.4% of all flocks (Table 7). a 

 similar proportion to that found by POBSP (76.0%) over a 

 longer time frame (King, 1970). White terns were found in 

 57% of the flocks, noddies (primarily brown noddy) in 38%, 

 and wedge-tailed shearwaters in 29%. 



White terns were also regular, though minor, members of 

 pelagic feeding flocks, found in 57% of all flocks and 

 representing 2% of total numbers (Table 7, Fig. 7). King 

 ( 1970) found that white terns participated in 9% of flocks and 

 noted that the relatively high flocking tendency of white terns 



lie. 7. Relative abundance >>i spe*. us ..i -,|x\ io groups loniul in .ill feeding 

 flocks (N = 14). Total numbei ol birds was -.(172 (\ = I4S). 



was of interest since this species had been thought to be a 

 solitary feeder. Our data also suggest that the white tern 

 commonly joins mixed-species feeding flocks. 



White terns are generally more solitary on land and at sea 

 and have not been found to exhibit such a high rate of flock 

 participation (57.1%) as we observed (King, 1970). This 

 species, however, joined flocks only in small numbers. Overall. 

 five white terns on our cruise joined over half the flocks, but 

 their total numbers within flocks accounted for only 2.1% 

 (Fig. 7; Table 7). 



Geographic Distribution 



We encountered feeding flocks in Regions I through IV 

 (Table 5). viz.. in the Pacific Ocean from 150°W to 158°E 

 longitude. We saw no flocks from Pohnpei westward to 

 Singapore. 



We found only one flock, composed of 85 sooty terns, in 

 the Line Islands. More than half the flocking birds were within 

 the Phoenix Islands (Table 5). where the highest seabird 

 populations and greatest species diversity were found. Great 

 frigatebirds joined flocks only in the Phoenix Islands. 



In Region III (Gilbert Islands), sooty terns, brown noddies, 

 and white terns were the only species present in feeding flocks. 

 Sooty terns and brown noddies accounted for 92% of flocking 

 birds in this area (Table 5). 



Region IV (Micronesia) was relatively rich in feeding 

 flocks that contained several species not encountered elsewhere: 

 stercorariids, Kermadec and Bulwer"s petrel. Audubon's 

 shearwaters, and white-tailed tropicbirds. 



The feeding flocks we observed were found in areas 

 known to be nutrient-rich. In broad terms, latitudes north of 

 10°N and south of 10°S are poor in nutrients. The zone in- 

 between, especially from 04 or 05°N to 09 or 1 0°N ( Equatorial 

 Countercurrent) and from 00 to 09 or 10°S. is considerably 

 richer in plankton and schools of small fish and tuna (Ashmole 

 & Ashmole. 1967). Superimposed on this general pattern are 

 local upwellings of plankton or "fronts" that occur close to 

 islands and arc particularly evident between 1 °30'S and 05°N 

 (King & Hida. 1957). 



The geographic distribution of our feeding flocks fits these 

 general patterns (Fig. 8). Their latitudinal limits were07°N and 

 07°S. At 02°S. within the plankton bloom on either side of the 

 equator, we encountered a feeding flock of 153 birds. Rocks 

 were more frequent around island clusters. We saw four flocks 

 each close to the Phoenix and Gilbert Groups, and the day our 

 ship arrived at 05°N (13 October, south of Pohnpei! we 

 immediately observed a substantial increase in flying fish. 

 From 04°N to 07°N, we encountered 3 feeding flocks of 223. 

 22. and 408 birds, respectively. The dearth of fish. and. hence, 

 seabirds. north of 10°N was particularly evident. Not only 

 were there no feeding flocks, there was a substantial decrease 

 m (he number of birds compared to all other Pacific areas 

 (Table 1 ). 



We saw no flocks in southeast Asia, where total numbers 

 of birds were low. Here we-encountered complicating factors 

 due to heavy pressures from commercial and subsistence 

 fisheries and other human population factors, which override 

 changes in oceanography that are associated with the continental 



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