Platte River Management Joint Study Final Report

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37 2. INTERIOR LEAST TERN A. NestinQ Znterior least terns nest on sandy substrates on riverine sandbars or on adjacent sandpit areas (NGPC 1982-1988). On the Platte River, riverine nesting least. terns usually nest on sandbars surrounded by water (Faanes 1983, NGPC 1982-1988). Occupied sandbars are sparsely vegetated or unvegetated, usually isolated by water, and made up of dry; sandy or gravel substrate. These conditions provide wide horizontal visibility, protection from terrestrial predators, and isolation from human disturbance. Data collected on the Platte Riyer in 1987-88 suggest that least terns select nest.sites that are at the highest point on the sandbars. Least terns nesting on sandpits occupy sites possessing similar characteristics except for the lack of isolation provided by water and the isolation from human disturbance and terrestrial predators. . Physical characteristics common to some least tern nest sites on riverine sandbars in the Platte River valley in 1979 are - displayed in Table 12. The natural cycle of spring and early summer peak flows contribute to provide adequate physical and biological features including sediment deposition and some vegetation removal that support nesting least terns. Flows may also influence the subsequent selection or nest placement on the sandbars. Faanes (1983) observed nesting areas in the Big Bend of the Platte River were inundated on 21 June 1979 when flows rose from 1,810 cfs on 20 June to 3,000 cfs on 21 June. Flows were less than 1,000 cfs during nest initiation. Nesting was initiated on a sandbar near Alda, Nebraska, during 10 - 12 July 1984 (Lingle, unpubl. data). U.S. Geological Survey data indicate that previously high flows decreased from 8,000 cfs just prior to this period, and ranged from 3,500 cfs on 10 July to 2,370 cfs on 12 July (USFWS 1987a). B. ForaQinct Nesting least terns typically forage in areas less than 1 mile from the nest sites (Faanes 1983). Least terns forage almost exclusively on small fishes captured near the surface (USFWS, unpubl. data). A critical criterion is the need for an adequate abundance of small fishes which provide the only food source for this species in the Platte River valley (USFWS 1987a). Water conditions must be adequate to meet the water quality, depth, temperature, and velocity requirements of such'species as sand shiner and Plains killifish (USFWS 1987a). NGPC (1985) listed 13 fish species and their relative abundances near a least tern nest colony on the Platte River_near Overton, Nebraska (Table 13).