Spacing of Wintering Shorebirds
feeding in groups along our coasts and along the margins of
inland lakes and rivers are a familiar sight to most North
American bird watchers. Such scenes are common during
migration and, in many places, throughout the winter as
well. The spacing systems found among wintering shorebirds
cover a spectrum from individual feeding territories to
large, tightly integrated foraging flocks.
At one extreme of the territoriality-flocking continuum are species such as Solitary and Spotted Sandpipers, Wilson's Plover, and Wandering Tattler which are usually seen as isolated individuals and only rarely seen in small groups. At the opposite extreme are Stilt Sandpipers, Surfbirds, Red Knots, and Long- and Short-billed Dowitchers, which are virtually always found in moderate-to-large cohesive flocks. Most shorebird species, however, fall somewhere in between, and many exhibit varying "spacing behaviors" depending upon location, time of day, and density of food resources.
Behaviors involved in defense of a feeding-site often differ from those seen in defense of a breeding territory. Conspicuous terrestrial visual displays are exhibited during feeding territoriality, for example, but aerial displays and extensive vocalizations are absent.
Like breeding territories, some feeding territories tend to have well defined boundaries, and continued occupation for weeks or even months. But here again, a continuum exists from these to territories defended for only a few hours or a few days. Some birds defend "portable" territories with boundaries that shift as food resources move (e.g., sand-dwelling invertebrates in the wave-wash zone along a beach whose abundance varies as the tide rises or falls). An extreme example is a Sanderling defending an area around a foraging Black Turnstone. As the turnstone flips through beach litter, the Sanderling forages in the newly exposed substrate (an example of "commensal feeding," in which the Sanderling benefits without harming the turnstone). When the turnstone moves along the beach, the Sanderling follows, essentially defending a moving territory centered on the turnstone.
The preponderance of evidence from migration and wintering studies indicates that nonbreeding (wintering) territoriality is primarily resource based, appearing and disappearing in response to changes in resource abundance and density. Whether or not wintering shorebirds are territorial also is sensitive to the risk territory-holders run of being eaten by falcons. Solitary small shorebirds have been shown to be more susceptible to falcon predation than those in flocks. Territoriality is most common at intermediate food densities and in places where, or at times when, the risk of predation on the territory-holder is low. When food is scarce, territoriality disappears because the amount of food within a defensible area is simply insufficient to meet the energy needs of the territory-holder. Similarly, when food is superabundant, territoriality disappears, in part because the energy cost of trying to keep out invading birds attracted to the rich food supply is too great. Likewise, the size of individual territories also shows plasticity that depends both directly and indirectly on the density of prey. Flexibility is clearly a key feature of spacing systems exhibited by nonbreeding shorebirds.
Territorial defense in wintering shorebirds shows striking parallels with defense of feeding territories by hummingbirds. Central to both groups is the nature of their food resources; only spatially clumped, energy-rich, and temporally renewable types of food can be defended -- features shared by such disparate types of food as nectar produced by flowers and invertebrates whose availability shifts with the tides.
SEE: Shorebird Feeding; Shorebird Communication; Shorebird Migration and Conservation; Sandpipers, Social Systems, and Territoriality; Territoriality; Commensal Feeding.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.