The
most widespread sandpiper in North America is the Spotted,
and it has one of the most unusual breeding systems found in
birds -- polyandry (one female mating with more than one
male). Unlike most sandpipers, the Spotted has invaded
temperate areas to breed. Apparently it has found Polyandry
to be the most successful reproductive strategy for taking
advantage of the relatively long breeding season (compared
with the season in the arctic and subarctic breeding areas
used by most members of this family). The Spotted Sandpiper
can be characterized as a "pioneering species" that quickly
and frequently colonizes new sites, emigrates in response to
reproductive failure, breeds at an early age, lives a
relatively short time (breeding females live an average of
only 3.7 years), lays many eggs per female per year, and has
relatively low nest success. Initiation of breeding in
the Spotted Sandpiper is influenced by the abundance of
insects, and cessation of breeding normally occurs abruptly
in early July, in anticipation of the decline of insect
populations later in the month (hatchlings from later
clutches would starve). Females arrive first on the breeding
grounds and must compete for males as they appear. Their
vigorous displays occasionally lead to physical combat.
Characteristically, females defend territories encompassing
the individual territories of their male consorts.
Ornithologist Lewis Oring and his colleagues have uncovered
fascinating details of Spotted Sandpiper breeding biology
through their long-term study of an island population in
Minnesota. The maximum clutch size in
these sandpipers is four eggs, each of which weighs about 20
percent of the adult female's body weight. Apparently, it is
physiologically impossible for a female to increase the
clutch size beyond four, but during the six-to-seven-week
breeding season she can lay up to five complete clutches of
four eggs each. Each clutch requires about three weeks of
incubation, so a female would be hard-pressed to hatch and
raise even two broods. Multiple mates enable a female to
increase her reproductive output by freeing her from the
responsibility for incubation and care of the
young. Nonetheless, because of a
chronic shortage of available males, females produce an
average of only eight eggs per breeding season, although
physiologically capable of producing twenty. That explains
why females compete so vigorously for mates; indeed, it is
fighting among females that limits the number of breeding
females on the island. Early in the season there are fewer
males than females present. As more males arrive over the
following few weeks, the ratio of available males to females
becomes more even but only temporarily. When males begin to
incubate clutches, they are effectively removed from the
pool of prospective mates and, at the same time, the females
completing clutches are available again for remating. During
the second half of the breeding season, it is not unusual to
find six or seven females for every potentially available
male. This shortage of males,
rather than any scarcity of food resources, limits the
reproductive effort of female Spotted Sandpipers. Not
surprisingly, experienced females acquire more mates, lay
more eggs, produce more chicks, and successfully fledge more
young than do inexperienced females. Spotted Sandpipers thus
forgo the advantages of having two parents care for eggs and
chicks. At least in the study population, however, that does
not seem to affect the fledging rate greatly, and the
Spotted Sandpipers' reproductive strategy seems to be highly
successful. SEE: Polyandry;
Monogamy;
Polygyny;
Promiscuity;
Sandpipers,
Social Systems and
Territoriality;
Spacing
of Wintering Shorebirds. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.