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.