Accelerating decline of an important wintering population of the critically endangered Spoon-billed Sandpiper Calidris pygmaea at Sonadia Island, Bangladesh

Abstract

The critically endangered Spoon-billed Sandpiper Calidris pygmaea breeds in Russia and winters between Bangladesh and China. World population estimates during 2014–2019 suggested a decline at a mean rate of 8% per year. Several surveys of local populations also indicated declines. We report a rapid decline of another local population of the species based on a 9-year series of annual surveys during the boreal winter at Sonadia Island, Chattogram Division, Bangladesh. We made bounded-count estimates of local populations of the Spoon-billed Sandpiper and 25 other shorebird species based on monthly counts in each of nine winters (2012/2013 to 2020/2021). For three of these winters (2017/2018 to 2019/2020), we also made Lincoln–Petersen estimates of the local population of Spoon-billed Sandpipers using resightings and scan surveys of individually marked birds. Population and trend estimates for the two methods were similar during the 3-year period when results from both were available. Bounded-count estimates of Spoon-billed Sandpipers declined markedly over the 9-year period. Analysis of combined data from both methods indicated that an exponential decline at a mean rate of 9.5% per year during the period 2012/2013 to 2017/2018 was followed by a much more rapid decline at 49.1% per year during 2018/2019 to 2020/2021. Bounded-count estimates of the combined population of 25 other shorebird species in each winter showed no decline during the 9-year period, which suggests that the decline in the local population of the Spoon-billed Sandpiper was not part of a general decline in populations of wintering shorebirds due to local factors. Estimates of the mean trend of other Spoon-billed Sandpiper local populations and of the world population all indicate declines at broadly similar rates. We recommend immediate reassessment of threats faced by the Spoon-billed Sandpiper and improved conservation interventions.