Status of the Interior Population of Least Tern

Abstract

Because the interior population of least tern (Sterna antillarum) was listed as endangered in 1985. information on population status, trends, and productivity is necded to guide management of this population We compared recent estimates (1986-95) of tern numbers to objectives identified in the Recovery Plan, used linear regression to estimate trends for local areas (e.g., river segment, reservoir), and route regression to estimate trends for larger segments of the breeding range. We also compared observed estimates of fledging success to the minimum value (0.51 fledglings/pair) thought necessary for population maintenance to determine whether observed productivity could support recent population trends. Although the interior population exceeded the recovery goal of 7,000 birds in 1995, this was due to large increases in tern numbers along a 901-km stretch of the Lower Mississippi River, and numbers for most breeding areas have not reached recovery levels. Trend (λ) was significant for 7 (5 positive, 2 negative) of 31 local areas for which trend could be calculated. At larger scales, λ was not discernibly different from 1 for the Platte and Missonri river drainages, but λ was > 1 for the Lower Mississippi River drainage. Overall trend for the interior population was 1.090 (95% CI = 1.056-1.111), and 1.024 (95% CI = 0.998-1.045) when data from the Lower Mississippi River were excluded. Fledging success ranged from 0.00 to 2.33 fledglings/pair, and was <0.51 in 9 arcas. Based on available fledging success estimates, there is no evidence that productivity within the interior range caused recent increases in tern numbers. Improved rangewide monitoring of numbers and productivity, and information on movements and postfledging survival. are needed to assess recovery criteria and management options for this population of least terns.