Advantages of Flocking in Some Wintering Shorebirds.

In this study, I assess the relative importance of three major factors as influences on the flocking behavior of twelve species of shorebirds (Charadrii) wintering on Bolinas Lagoon, California. These factors are localization of resources, increased feeding efficiency, and decreased predation. The social tendencies of the Bolinas shorebirds can be categorized as feeding solitarily, feeding in loose aggregations with no cohesiveness, feeding in cohesive and mobile flocks, and roostering at high tide in tight and cohesive groups. Localization of resources probably plays a major role in the build-up of aggregations. If a sufficiently large number of birds occupies a small enough area, birds will appear to be grouped. Feeding sites are used day after day for most of the season. Some sites such as rain pools formed in pastures attract several birds for short periods. The speed with which these groups build up suggests local enhancement--individuals using others as indicators of food patches. Localization of resources cannot explain the cohesiveness of flocks which move together throughout an area, nor the tightness of roosts. Feeding efficiency may be increased by sharing food finds, avoiding duplication of feeding sites, and using roosts as information centers. No evidence was found to support any of these hypotheses. Data from American Avocets Recurvirostra americana and marbled Godwits Limosa fedoa suggest that these species feed less efficiently (intake/attempt) when in larger groups. Dunlin Calidris alpina are not patchily distributed within flocks, nor are Avocets' feeding successes clustered temporally, suggesting that food is not very patchy. When Willets Catoptrophorus semipalmatus fly to a new position in the flock, they l and further from individuals than already-feeding birds are to each other. Individuals do not conspicuously avoid feeding in areas already used. There is little evidence that birds follow others to better feeding areas from high-tide roosts. Predation appears to be the major influence on shorebird flocking. Measures of individual alertness show that birds in larger groups and in interior or high-density regions of groups are less alert, both in feeding flocks and roosts. Calculations show that even so, larger flocks are probably better detectors of predators. Tightness of cohesive flocks appears to be correlated with vulnerability to predation. Avocets form tighter flocks when they are feeding with their eyes under water. All species form cohesive flocks when feeding in the salt marsh, where the vegetation may interfere with vision. Single Willets are more alert in the salt marsh than on the mud flats, and Willet flocks become tighter as vegetation height increases. Finally, a general correlation between feeding posture and flocking exists. Those species which feed with their heads down (e.g., small s and pipers) form cohesive flocks, while head-up feeders (e.g., plovers) do not.Ph.D.EcologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/157886/1/8025651.pd