Niche Relationships Among Six Species of Shorebirds on Their Wintering and Breeding Ranges

The dynamics of the organization of a community of six species of migrant predatory shorebirds (Least Sandpiper, Semipalmated Sandpiper, Dunlin, Short—billed Dowitcher, Lesser Yellowlegs, and Semipalmated Plover) was appraised by studying foraging behavior and habitat utilization under winter conditions in southern Florida and under summer conditions in the eastern Canadian Arctic. Eight foraging methods, defined primarily on the basis of how the bill is used and the pattern of locomotion, constitute the behavioral repertoire of the species. Each foraging method is correlated with a particular rate of locomotion (distance/time) and rate of feeding (pecking or probing/time). Feeding and locomotion rates seem independent of air temperature, number of conspecifics, and total number of shorebirds foraging nearby. Instead, the seasonal changes in these rates are probably related to food density. On the basis of these findings and of differences in rates of feeding and locomotion between species the most reasonable hypothesis is that food density is higher in summer on the breeding grounds and foraging is more selective. Each foraging method is combination with a particular microhabitat defines a statistically different resource. During the winter, on intertidal habitats of southern Florida, shorebirds on the whole exhibit a low behavioral and microhabitat diversity and low resource overlap between species. The small niche breadth in winter is probably a response to food limitation, and each species exists in its exclusive niche where it is optimally adapted and therefore has high foraging efficiency. In summer tundra and taiga habitats of the Arctic, shorebirds generally have a higher behavioral and microhabitat diversity (broader niche) and higher overlap between species. Exceptions to these general patterns exist among the study species. Seasonal differences in prey density, prey behavior, time available for foraging, feeding and locomotion rates, and the pattern of resource partitioning imply that shorebird populations are regulated through competitive processes occurring on their wintering habitats. Conclusions concerning coexistence mechanisms in migratory bird species and residents in seasonal environments may be erroneous if populations are studied only in the breeding season.