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SEcnONFOUR leastTem I ,I I t , 1 I J ~ \] J I '~ winter along portions of the Central America coast and along the northern coast of South America from Venezuela to northeastern Brazil (FWS 1990). 4.4 POPULATION STATUS 4.4.1 Status Partial surveys of the interior population of least tern began in the late 1970s and became more comprehensive in the mid-1980s (Kirsch and Sidle 1999). Annual estimated numbers of least terns throughout their intenor range for 1988 to 1997 are provided in Table 4-1. During this period estimated numbers have ranged from almost 5,400 to 9,000 birds. Although there are annual fluctuations wit!i.i~ specific survey locations, much of the variation is due to the number of areas that are surveyed within a specific year. During this period, the majority (54 to 77 percent) of the least terns was located along the lower Mississippi River. The least terns estimated to be located along the central Platte River represented between 1.3 and 3.0 percent of the annual estimates during the 1988 through 1997 period. A population analysis of the least tern throughout its range for the 1984 to 1995 period provides information on the status of the population (Kirsch and Sidle 1999). Breeding population estimates for least terns for 35 local areas censused during the 1984 to 1995 period were compiled by Kirsch and Sidle (1999). Least tern numbers at local breeding areas fluctuated substantially, perhaps reflecting changes in local and regional habitat availability or differences in emigration, immigration, and local recruitment. Population trends were estimated for 31 of the local areas that had been surveyed 3 or more years during the 12-year period. Their estimates indicated that five of the areas had significant positive trends, two of the areas had significant negative trends, and no trends were detected at 24 of the local areas. The central Platte River was one of the local areas for which no trend was detected. The trend for the entire population was positive and this trend was heavily influenced by the positive trend of the lower Mississippi River drainage. If the lower Mississippi River drainage data are not included, the overall trend is still positive, but not significantly positive (Kirsch and Sidle i 999). Population modeling by Kirsch (1996) indicated that 0.51 fledglings per pair is required for population maintenance. Information compiled by Kirsch and Sidle (1999) included fledging success estimates for I or more years were available for 22 local areas. Fledging success was extremely variable and least terns failed to produce fledglings in 1 or more years in several of the local areas. Fledging success was less than the 0.51 fledgling per pair needed for population maintenance at eight local areas. Three of the local areas were slightly higher, and the remaining 11 local areas were substantially higher than the 0.51 with the highest rate for a local area being 1.60 fledglings per pair (Kirsch and Sidle 1999). Based on their analysis, they concluded that the data from much of the interior least tern range suggest that productivity in many areas was not sufficient for local population maintenance. Moreover, there are insufficient fledging success data over the interior breeding range of the least tern to assess whether fledging success is sufficient for population maintenance. Nesting habitats are ephemeral in quality and abundance; 4-4 68F0097286CXl1r1.doc 6I21'999(9:52AM)llJRSGWCFSJ2 ."... ~ant""* FeIIrRI SIrrIl:es