Declines in migrant shorebird populations from a winter-quarter perspective

Abstract

Many long-distance migrating shorebird (i.e., sandpipers, plovers, flamingos, oystercatchers) populations are declining. Although regular shorebird monitoring programs exist worldwide, most estimates of shorebird population trends and sizes are poor or nonexistent. We built a state-space model to estimate shorebird population trends. Compared with more commonly used methods of trend estimation, state-space models are more mechanistic, allow for the separation of observation and state process, and can easily accommodate multivariate time series and nonlinear trends. We fitted the model to count data collected from 1990 to 2013 on 18 common shorebirds at the 2 largest coastal wetlands in southern Africa, Sandwich Harbour (a relatively pristine bay) and Walvis Bay (an international harbor), Namibia. Four of the 12 long-distance migrant species declined since 1990: Ruddy Turnstone (Arenaria interpres), Little Stint (Calidris minuta), Common Ringed Plover (Charadrius hiaticula), and Red Knot (Calidris canutus). Populations of resident species and short-distance migrants increased or were stable. Similar patterns at a key South African wetland suggest that shorebird populations migrating to southern Africa are declining in line with the global decline, but local conditions in southern Africa's largest wetlands are not contributing to these declines. State-space models provide estimates of population levels and trends and could be used widely to improve the current state of water bird estimates.

Climatic variation in Africa and Europe has combined effects on timing of spring migration in a long-distance migrant Willow WarblerPhylloscopus trochilus

“…The AA and the climate indices were time series, so they potentially contained autocorrelations between successive values (Cowpertwait & Metcalfe, 2009;Crawley, 2013;Simmons et al, 2015;Barshep et al, 2017). The missing values of the AA (in 1993AA (in , 2011 were imputed for the autocorrelation tests, with the values predicted from their regression trend against the years (Crawley, 2013).…”

Metal Levels in Blood of Three Species of Shorebirds during Stopover on Delaware Bay Reflect Levels in Their Food, Horseshoe Crab Eggs

“…Many shorebirds migrate long distances from their Arctic breeding grounds to their wintering grounds in South America [ 9 , 10 , 11 ], stopping to refuel along the Atlantic coast [ 12 ]. Over the last 30 years, many species of shorebirds have declined sharply [ 13 , 14 , 15 ]. In the 1990s, about 20% of the world’s shorebirds were listed as species of special concern [ 16 ], and now Andres et al [ 17 ] estimate that 61% of American shorebird populations have declined in the last 30 years.…”

State-space models’ dirty little secrets: even simple linear Gaussian models can have estimation problems

“…Fitting methods for SSMs, such as the Kalman filter, are now used to facilitate both state and parameter estimation 15 . In many cases, SSMs are used to estimate variance parameters because they are designed to differentiate measurement error from process stochasticity 16 17 . While estimating parameters is often a means to estimate the unobserved states (e.g.…”