BIRDS IN FLOCKS 



JULY 1964 



SEPTEMBER 1964 



Figure 35b. — Distribution of birds in flocks, July-November 1964. 



live advantage of flock feeding, e.g., many eyes 

 to spot food and a larger area under surveil- 

 lance, is partially counterbalanced by the 

 necessity for sharing the food supply, once 

 found, because of intraflock competition. 



A low flocking tendency index may also be 

 the result of direct migration through the area, 

 e.g., Sooty Shearwater, so that even though 

 this species feeds in large flocks in its breed- 

 ing and nonbreeding ranges, the marginal at- 

 traction of an unaccustomed food supply is 

 usually more than outweighed by the migration 

 tendency. 



Thus, flocking tendency provides a measure 

 of species utilization of the area. The fact that 

 83.3 percent of Juan Fernandez Petrels and 

 only 1.4 percent of Sooty Shearwaters were in 

 flocks suggest that Juan Fernandez Petrels 

 were resident in the area for part of the year, 

 and that Sooty Shearwaters were not. 



No account can be taken of nocturnal feeding 

 behavior, since all observations were made be- 

 tween sunrise and sunset. It is presumed that 

 nocturnal feeding by some, if not all, species 

 is considerable, and that it may exceed diurnal 

 feeding (see Gould, 1967). 



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