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interior least tern is a small shorebird with a black-capped crown, white
forehead, gray wings and back, and yellow or orange legs and bill. The
male’s legs and bill are brighter than the female’s, but the sexes are
similar. At just 8 to 9 ˝ inches long, the least tern is the smallest
species in the family Laridae. Least terns feed almost exclusively on
small fish which they catch by skimming over the water and plunging in.
Least terns are listed as endangered on both the South Dakota state and
the federal endangered species list.
 In
South Dakota, least terns nest primarily on sandy unvegetated beaches and
sandbar islands along the Missouri River. They tend to nest in large
communal colonies, often with the state and federally threatened piping
plover. When an intruder enters the nesting area, least terns
respond by flying and diving at the interloper while vocalizing loudly.
The nest is little more than a shallow scrape in the sand in which the
female lays one to three eggs. The young are able to walk almost
immediately after hatching, but they generally remain in the nest for
about two days. Parents care for the young for several weeks, protecting
them from extreme weather and feeding them. The young tend to fly after
about three weeks. If a first nest fails, the parents will readily re-nest,
especially early in the nesting season.
Least terns spend about four months in South Dakota annually, arriving
in late April and staying until late August to early September. They
probably over-winter in the Gulf of Mexico and the Caribbean Islands,
although wintering habits have not been well documented. Conditions on the
wintering grounds, in particular habitat loss and pesticide use, are also
a concern for the species’ survival and recovery.
The species' decline began in the late 1800’s, when thousands of least
terns were killed for the millinery (hat) trade. The population began to
rebound in the early 1900s, but beginning in the 1930’s, dam construction
and subsequent channelization and water regulation of the Missouri River
reduced available nesting habitat and frequently inundated nests.
Increased human activity on the river causes colony failure by directly
impacting nests or increasing predation by domestic animals or natural
predators.
Historically, high spring flows in the Missouri River purged
existing islands of vegetation and sediment carried by the water created
new islands. Today however, large flood events are rare and the dams trap
sediment so the water no longer carries sufficient material to create new
islands. Islands erode without being replenished or become covered with
vegetation that provides habitat for potential predators. High water in
1996-1997 led to an increase in appropriate nesting habitat in South
Dakota and the least tern population
and fledge
ratio (the number of young per adult pair that survives to
approximately 21 days when they can fly) have shown a corresponding
increase in subsequent years. As the islands created in that flood event
become degraded however, least terns are expected to decline.
References:
Bent. 1929. Sterna antillarum (Lesson). Least tern habitats.
Bulletin 113, United States National Museum: 270-279.
National Academy of Sciences. 2002. The Missouri River Ecosystem:
Exploring the Prospects for Recovery. National Academy Press. Washington
D.C.
Thompson, B.C. J.A. Jackson, J. Burger, L. Hill, E.M. Kirsch, and J.L.
Atwood. Least Tern, Sterna antillarum, Excerpts. The Birds of North
America, No. 290, 1997. Website accessed January 21, 2004.
http://birds.cornell.edu/birdsofna/Excerpts.html.
USFWS Website. Accessed January 14, 2004. http://www.fws.gov/.
USFWS. 1990. Recovery Plan for the Interior Population of the Least
Tern, Sterna antillarum;. U.S. Fish and Wildlife Service. Twin
Cities, MN.
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