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<br />nesting starts when the river flows are high and small amounts of sand are <br />exposed. The size of nesting areas depends on water levels and the extent <br />of associated sandbars. An examination of the interior least tern's <br />nesting ecology on the Missouri River (Schwalbach et al. 1988) illustrates <br />. the changes caused by varying river flows. Along one stretch of the <br />Missouri River in South Dakota the average size of nesting sandbars was 12 <br />and 31 ha in 1986 and 1987, respectively; nest elevation and nest to water <br />.distance differed by a factor of three in both years. <br />The Lower Mississippi River is very wide and carries a tremendous <br />volume of water and sand. Sandbars form annually, are washed away, and <br />shift position. Many sandbars are .over 3.2 km long and 1.2 km wide. Nest <br />sites are often several hundred meters from the water (Rumancik 1987, <br />1988). Thus, nesting areas usually are several hundred hectares in size. <br />Mississippi River levels at the onset of nesting also influences the <br />number of nests at a colony. Smith and Renken (1990) observed Mississippi <br />River colonies that averaged 100 nests/colony when habitat was restricted <br />by high water early in the nesting period, but which averaged only 19.3 <br />nests/colony during a year of more moderate river levels. <br /> <br />Artificial Nestin2 Habitat: Least terns nest on artificial habitats <br />such as sand and gravel pits and dredge islands (Dryer and Dryer 1985, <br />Haddon and Knight 1983, Kirsch 1987-89, Larkins 1984, Morris 1980). In <br />North America the coastal and California least terns commonly nest on a <br />variety of artificial nesting habitats, even roof-tops (Altman and Gano <br />1984, Atwood et a1. 1979; Fisk 1975, 1978; Jernigan 1977, Massey and <br />Atwood 1980, 1983; Swickard 1974). <br /> <br />The interior least tern nests on dike fields along the Mississippi <br />River (Smith and Stucky 1988; Smith and Renken 1990), at sand and gravel <br />pits (Kirsch 1987-89), ash disposal areas of power plants (Dinsmore and <br />Dinsmore 1988, Johnson 1987, Wilson 1984), along the shores of reservoirs <br />(Boyd 1987, Chase and Loeffler 1978, Neck and Riskind 1981, Schwalbach <br />1988) and at other manmade sites (Shomo 1988). The percentage of interior <br />least terns nesting on pits adjacent to the lower reach (Columbus to <br />Plattsmouth) of the Platte River varies depending on the flow and amount <br />of exposed sandbar habitat (Kirsch 1987-89). Suitable nesting habitat in <br />the upper Platte River channel has been severely reduced (Sidle et al. <br />1989) and in many stretches of the river, sand and gravel pits annually <br />provide the only nesting habitat (Lingle 1989). It is unknown to what <br />extent sand and gravel pits, dike fields, reservoir shorelines and other <br />artificial habitats have replaced natural habitat. In the lower <br />Mississippi River alone, 7,518 ha of bar and island habitat were lost in <br />diked reaches between 1962 and 1976 (Nunnally and Beverly 1986, Smith and <br />Stucky 1988). <br /> <br />21 <br />