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SEellONFOUR least Tern I ,I I I '1 I J ,1 ,I IJ ,I ~ 4, 1989, hosting a maximum often juveniles and eight adults on July 18. A maximum of25 least terns (twelve juveniles, thirteen adults) was observed on July 21, 1989, on Shoemaker Island, south of AIda, Nebraska. This site hosted least terns from July 11 to August 10, 1989. Least terns traveled between 4 to 57 miles from where they nested to reach a congregation site (FWS 1997). Reproduction Success Factors Nest Fates. Predation and flooding accounted for 74 percent (37 percent each) of nest failure on river habitat. Predation was the greatest cause of nest failure on sand pits followed by human disturbance and weather. Only 3 percent of the sand pit nests were lost to flooding compared to 37 percent of the river nests. However, human disturbance and abandonment accounted for 27 percent of sand pit nests that failed and only 17 percent of the failed river nests (it should be noted that some of the losses attributed to abandonment may have been due to predators killing adults; therefore, this category is somewhat subjective). Weather took a larger toll on sand pit nests as well because the uniform substrate on the spoil piles was more susceptible to wind and water erosion than the riverine substrates. Overall, predation was the major cause of nest mortality. Dogs or coyotes (Canis latrans) were suspected in 38 cases (78 percent); skunks (Mephitis spp.) in three cases (6 percent); raccoon (Procyon lotor), great-horned owl (Bubo virginianus), and American crow (Corvus brachyrhynchos) in two cases (4 percent) each; great blue heron (Ardea herodias) and snake in one case (2 percent) each. Other potential predators included mink (Mustela vison), American kestrel (Falco sparverius), black-billed magpie (Pica pica), bullsnake (Pituophis melanoleucus sayi), and garter snake (Thamnophis spp.) (FWS 1997). Chick and Adult Mortality. The only available information includes both least tern and piping plover mortality. Circumstantial evidence suggested causes of mortality on 21 occasions involving 38 birds (33 least terns and 5 piping plovers). About 53 percent of the adult deaths were due to predation, 33 percent to weather, and 13 percent to humans. Forty-three percent of the chicks deaths were weather related, 39 percent by vehicles, and 17 percent by predators (FWS 1997). Nest Success and Flow Relationships. In 5 of the 7 years from 1985 through 1991, flooding inundated least tern eggs and/or chicks. The worst nest losses occurred in 1988 when 79 percent of the least tern nests were flooded. That summer was characterized by multiple spikes in the hydrograph. The frequency and timing of these summer spikes determined the magnitude of nest and chick loss. For example, in 1987, only nests were flooded because the timing did not allow for hatching; while in 1990, only chicks were flooded because the spike occurred well after hatching (FWS 1997). Flooding of nests is a natural phenomenon which least terns are adapted to deal with through renesting and other reproductive strategies (Sidle et aI. 1992; Kirsch and Sidle 1999). However, Lingle (1993) observed that releases from the 1-2 Return near Lexington, Nebraska coupled with local thundershowers exacerbate the flooding problem. For example, during the 44-day period from June 29 to August 11, 1990, the J-2 Return flow was zero cfs despite the fact that over 2,100 cfs was being diverted into the Tri-County Canal. The flow at Grand Island, Nebraska j 4-8 68f;OD97286lX1Ir1.aoc 6r.V1999(9.52AM)lURSG\NCFS1'2 IIBS SIelBe1 ~ CIJrIe fedetaI SlIntl:es