Hunt 



Chapter 24 



Foraging Associations 



the 127 mixed species foraging flocks, Marbled Murrelets 

 and Glaucous-winged Gulls (Larus glaucescens) were the 

 only participants. Other species that accompanied murrelets 

 included Bonaparte's Gulls (Larus Philadelphia), Common 

 Mergansers (Mergus merganser), Pigeon Guillemots (Ceppus 

 columba), and Pelagic Cormorants (Phalacrocorax pelagicus). 

 Mahon and others (1992) observed the initiation of 27 flocks. 

 In each case, the flocks began after Marbled Murrelets drove 

 a school of sand lance (Ammodytes hexapterus) to the surface, 

 where the fish "thrashed briefly in a tightly packed 'boil.'" 

 Gulls and, if the boil of small fish jumping from the water's 

 surface lasted sufficient time, other birds were then attracted 

 to this food resource. The feeding flocks observed by Mahon 

 and others (1992) had on average 7.7 murrelets and 5.9 

 Glaucous-winged Gulls, with a positive correlation between 

 the numbers of murrelets and gulls in the flocks. Flock 

 duration varied between 1 and 79 minutes and was also 

 positively correlated with the number of murrelets present. I 

 have seen Marbled Murrelets in mixed species foraging 

 flocks in the San Juan Islands of Puget Sound. There, murrelets 

 foraged on young of the year herring (Clupea harengus), 

 and started foraging flocks when they forced dense schools 

 (balls) of herring to the surface. Thus, these mixed-species 

 feeding flocks are similar to the Type I feeding flocks 

 described by Hoffmann and others (1981). Within these 

 flocks, the murrelets acted as catalysts, even though their 

 foraging did not appear to be as conspicuous as that of 

 species considered catalysts by Hoffmann and others. In the 

 San Juan Islands, I have seen Marbled Murrelets join mixed 

 species foraging flocks that contained not only gulls 

 (Glaucous- winged, Mew [L. canus], and Heermann's [L. 

 hermanni}) but also harbor seals (Phoca vituillina), Rhinoceros 

 Auklets (Cerorhinca monocerata), and Common Murres 

 (Uria aalge). Both in the cases where murrelets were catalysts, 

 and when they joined flocks, they foraged beneath the surface 

 and probably served to drive small, highly clumped schools 

 of fish to the surface. 



The variability in associations between foraging Marbled 

 Murrelets and other seabirds noted above and elsewhere (Carter 

 and Sealy 1987a, Hoffman and others 1981) demonstrates the 

 plasticity in both the behavior of murrelets and in the frequency 



with which other species take advantage of the foraging 

 behavior of the murrelets. Reasons for the differences in the 

 frequency that foraging murrelets are associated with other 

 bird species on exposed outer coast waters, when compared 

 with more sheltered inside waters, is not known. It may be 

 that interspecific competition with other alcids is more intense 

 along the outer coast of Vancouver Island than in sheltered 

 waters (see Chilton and Sealy 1987; Mahon and others 1992; 

 Piatt 1990), but this cannot explain the lack of mixed species 

 flocking farther south where large alcids are scarce. 

 Alternatively, differences in the roughness, clarity, or depth 

 of water may influence the ability of surface foragers to take 

 advantage of the murrelet foraging. Additionally, the size, 

 distribution or behavior of fish aggregations may differ in the 

 two habitats. It would be useful to explore why these mixed 

 species foraging flocks occur in one habitat, but not in the 

 other, and their importance to murrelet foraging success. 



Although it would seem reasonable to assume that 

 murrelets benefit from joining mixed species foraging flocks 

 in those instances when they choose to join, it is not known 

 whether murrelets benefit from being joined by other species. 

 Gulls sometimes attempt to steal fish from murrelets when 

 they surface with a fish in their bill (G. Hunt pers. obs.) and 

 larger alcids may interfere with their foraging (Chilton and 

 Sealy 1987, Piatt 1990). Alternatively, the presence of surface- 

 foraging gulls may aid the murrelets by driving fish from 

 their protective balls where they may be less vulnerable to 

 underwater predators (Girsa and Danilov 1976, Gotmark and 

 others 1986, Grover and Olla 1983). Clearly the gulls benefit 

 from the activities of the murrelets in driving fish to the 

 surface and holding them there (Grover and Olla 1983; Hoffman 

 and others 1981). 



From the point of view of murrelet conservation, it 

 would be useful to know the costs and benefits to Marbled 

 Murrelets of being joined by gulls. Gulls apparently forage 

 with murrelets in those areas where murrelet populations are 

 most dense. If gull populations are artificially increased by 

 the wasteful habits of people, and the gulls suppress murrelet 

 capture or retention of fish, then it would be useful to 

 investigate how this indirect anthropogenically caused 

 pressure on murrelets could be relieved. 



256 



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



