New estimate of the trend in world population size of the Spoon-billed Sandpiper suggests continuing decline

Establishing the size and trend of the world population of the Critically Endangered Spoon-billed Sandpiper Calidris pygmaea is vital for assessing the priority and effectiveness of global conservation measures, but these parameters are not yet known precisely enough for effective monitoring. We used a global mark-resighting analysis combined with 17 scan surveys at five sites during the non-breeding season and a Lincoln-Petersen method to make estimates of the species’ world population. This study extends the series of survey results using this approach beyond that reported previously from ten surveys to 17 surveys and increases the duration of the monitoring period from six to nine years. The estimated mean world population size at the end of the breeding season, averaged over the whole survey period, was 443 mature individuals and the trend of the population estimates over time suggested a decline at an average rate of 5% per year. The precision of this estimate of population trend was low, but although the suggested decline was not statistically significant at the two-tailed 5% level, the evidence that the population is declining is now substantial and we wish to update our trend estimate now to draw attention to the species’ continuing precarious conservation status. The precision of the trend estimate has improved substantially with the expansion of the dataset, in line with predictions from simulations based upon previously published results, so it should be possible to reach more reliable conclusions about population trend soon, but only if it is possible to continue to apply individual marks.