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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Monitoring populations of Western Sandpipers and Pacific Dunlins during northward migration on the Fraser River Delta, British Columbia, 1991-2013
Auteurs :	Mark C. Drever, Auteur ; Moira J. F. Lemon, Auteur ; Robert W. Butler, Auteur ; Rhonda L. Millikin, Auteur
Type de document :	Article
Dans :	Journal of Field Ornithology (85(1) MAR 2014)
Article en page(s) :	10-22
Langues:	Anglais
Résumé :	The Fraser River Delta in British Columbia, Canada, is a globally significant stopover site for shorebirds, but the population status and trends of many species that use the site remain uncertain. We describe an ongoing program to monitor population trends of the two most abundant species, Western Sandpipers (Calidris mauri) and Dunlins (Calidris alpina), during northward migration. Counts of these species were conducted at a mudflat where large flocks assembled at mid-tide from 15 April to 15 May, 1991-2013, and we estimated species-specific counts as the product of daily total flock counts and species proportions obtained during supplementary sampling. The median peak count of both species combined was 177,000 birds, and occurred between 24 April and 3 May. Ratios (proportions) of the two species followed a predictable pattern during the migration period, with a low proportion of Western Sandpipers (3%-20%) in flocks before 20 April, followed by a rapid increase to 80%-100% between 25 April and 10 May and a variable decrease to 30%-80% by 15 May. Mean counts of Western Sandpipers showed no significant trend over the study period. Mean counts of Dunlins showed a non-linear trend, decreasing until 2001 and then increasing to 2013. Bias and random error in field counts were quantified by comparing field counts to counts made from photographs taken during surveys, and analysis revealed that field counts had a downward, but predictable, bias, accounting for >90% of birds present, with a stochastic error rate of 28.0%. Uncertainty in total population estimates was high after accounting for the effect of length of stay and sampling error. Population estimates suggested that 600,000 Western Sandpipers and 200,000 Dunlins typically passed through the site during northward migration. Our estimates indicate the usefulness of daily counts at major stopover sites during northward migration as an effective tool for monitoring shorebird populations, and underscore the need for conserving such sites.

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