NON-BREEDING TERRITORIALITY OF SEMIPALMATED SANDPIPERS

We studied non-breeding territorial behavior of Semipalmated Sandpiper s (Calidris pusilla), and documented the proportion of individuals of Least (C. minutilla) and Western (C. mauri) sandpipers exhibiting terr itoriality during fall of 1994 and 1995 at the Cabo Rojo salt flats, P uerto Rico. Territoriality was exhibited by 8-19% of the population, w ith Least Sandpipers exhibiting the highest proportion of all three sp ecies. The prevalence and size of territories of Semipalmated Sandpipe rs was influenced by the availability of high prey density sites, bird density, and aggression levels. Individuals defended territories in a reas of highest prey densities (0.16 +/- 0.009 /cm(3)). Non-territoria l birds, however, foraged in areas of significantly higher prey densit ies (0.02 +/- 0.001 /cm(3)) than overall lagunal levels (i.e., randoml y sampled plots) (0.006 +/- 0.009 /cm(3)). Territorial Semipalmated Sa ndpipers spent less time foraging than non-territorial birds (1994: 90 vs 36%; 1995: 85 vs 55%). They also spent less time in maintenance be havior (i.e., preening). Costs of territoriality may be offset by fora ging in areas of high prey densities or through increased foraging eff iciency. Prey density did not exert a direct influence on territory si ze (P = 0.67). Rather, it influenced territory size through effects on aggression. Territory size was inversely related to the mean time spe nt on aggression (P < 0.001). Territory size was also inversely relate d to bird density (P < 0.001), a factor that might contribute to highe r incidence of aggressive bouts. These findings support the contention that a facultative strategy with respect to nonbreeding territorial b ehavior seems to offer migratory species the flexibility to cope with spatio-temporal fluctuations of prey resources and shorebird density.