Studies
of breeding sandpipers in different arctic and subarctic
habitats have shown how differences in mating systems and
territoriality may be related to ecological factors such as
the distribution and abundance of food resources and the
need to avoid predators. For instance, in northern Alaska
where food is relatively scarce and unpredictable,
monogamous Dunlins establish territories sufficiently large
to provide the pair with enough food to carry it through the
leanest of years. This keeps densities of breeding
populations low; when male Dunlins holding territories were
removed experimentally, other males promptly replaced them.
The replacements apparently had either been nonterritorial
or occupied inferior locations. In contrast, in the southern
part of Alaska where the longer growing season provides a
more predictable and abundant food supply, the territories
are smaller and the population density higher. In both
situations, however, the nests are well spaced because of
male aggressiveness. Experimental evidence shows that wide
spacing of nests in gull colonies reduces predation, and
that may apply to sandpipers also. Since foxes, weasels,
gulls, and jaegers prey heavily on sandpiper nests, one
result of such spacing may be higher nesting success.
Dunlins are monogamous, and the pair bond allows one adult
to incubate while the other forages. In addition to limiting
egg predations, the presence of both parents on the
territory reduces egg chilling, which otherwise would delay
the chicks' hatching and could reduce their chances of
survival. Some monogamous species,
such as the Western Sandpiper, do not defend feeding
territories. The Western's nesting habitat is patchily
distributed but located close to abundant food. Western
Sandpipers nest in "islands" of shrubby vegetation in the
tundra that provide some protection from predators, but feed
outside of their territories. Monogamous, territorial social
systems like those of the Dunlin and Western Sandpiper are
also found in the Red Knot, Surfbird, and the Purple,
Semipalmated, Least, Baird's, Rock, and Stilt
Sandpipers. Other species of sandpipers
exhibit a variety of nonmonogamous mating systems.
Sanderlings, Temminck's Stints, and Little Stints (neither
of the latter two breed in our area) are serially
polyandrous -- a female lays two or three clutches, which
are normally fathered by more than one male, and the males
care for them, or she lays two clutches and the male broods
one clutch and the female a second one. White-rumped,
Curlew, and Sharp-tailed Sandpipers are polygynous. Those
males displaying on the best territories may have more than
one mate, but do not assist in incubation. Pectoral
Sandpipers, Buff-breasted Sandpipers, and Ruffs (which
rarely breed in our area) are promiscuous. Males display
vigorously, the Buff-breasted and Ruffs on leks, but play no
part in incubation, and females may associate with several
males in close succession. This second group of
sandpipers shows much more variation in territorial
strategies and reproductive effort than do the monogamous
species. They are more "opportunistic," adapting their
strategies to local conditions, especially the temporary
availability of abundant food. For instance, instead of
being conservative like Dunlins and setting up a territory
that will contain enough food in any circumstances, Pectoral
Sandpipers acquire fat reserves during their northern
migration that permit them to ride out periods of food
shortage. Like the White-rumped Sandpiper, they may breed at
very high densities, with males holding small territories,
since a continuous food supply is assured. Rather than
guarding their nest to avoid predation, Pectorals keep the
nest hidden, and incubation by only one adult reduces
telltale traffic to and from the nest. Single adult
incubation may also help to conserve food supplies for that
adult and the young, since a second adult will not be
depleting resources near the nest. Opportunistic arctic
breeders accept the increased hazards of nesting in highly
productive locations such as lowland marshes. They get a
rich harvest of insects in exchange, but risk increased
predation associated with more closely spaced nests and
flooding by summer rains (water runs off the tundra very
rapidly because it cannot soak into the ground, which is
frozen just below the surface). The various mating systems
offer different advantages, and those advantages accrue
differentially to the two sexes. The females of polyandrous
species gain more than males. They are freed from the
necessity of incubating the first clutch, so their
reproductive output is increased. Polygynous males with good
territories may greatly increase their reproductive output,
gaining more benefit than the two or more females sharing
the territory. This may generate an evolutionary pressure
toward a lek system that frees the female, once mated, from
any constraints inherent in the male's territorial needs.
The best territory for a displaying male may not be the best
one for a female to use for nesting and rearing her young.
Females in promiscuous species are free to optimize their
choice of nest sites. Several of the sandpiper
species usually considered monogamous are occasionally
polygamous (polygynous or polyandrous) and show other
tendencies toward more opportunistic strategies, such as
communal feeding away from the territory while maintaining a
monogamous pair bond. Also, they will renest if the first
clutch is lost -- suggesting a possible evolutionary
stepping-stone to the strategy of the male rearing the first
clutch and the female the second, which in turn could open
the door to polyandry. We have simplified the
account here, and more research is needed to confirm various
aspects of sandpiper social systems, but as you can see,
different members of this structurally rather uniform group
of birds have developed very different ways of solving the
problems of successful reproduction in northern
environments. SEE: Monogamy;
Polygyny;
Polyandry;
Polyandry
in the Spotted Sandpiper;
Territoriality;
Shorebird
Communication. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.