Sandpipers, Social Systems, and Territoriality
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.