Foraging conditions ‘at the end of the world’ in the context of long-distance migration and population declines in red knots

The long-distance migrant red knot (Calidris canutus ssp. rufa – Scolopacidae) alternates between the northern and southern ends of the New World, one of the longest yearly migrations of any bird and paradoxically overflying apparently suitable habitat at lower latitudes. This subspecies is sharply declining, with a major mortality event following 2000, attributed to commercial overharvesting of food resources at its Delaware Bay (USA) stop-over site. A full understanding of this peculiar migrant requires an assessment of the foraging conditions at its southern hemisphere wintering sites. Here, for a major wintering site in Argentinean Tierra del Fuego (Río Grande), we describe and compare food abundance, diet and intake rates during January–February in 1995, 2000 and 2008. The two main prey types were the burrowing clam Darina solenoides and three species of epibenthic mussels Mytilidae. In the year 2000, food availability and intake rate were higher than those recorded at other sites used by knots anywhere else in the world, contributing to the explanation of why red knots carry out this impressive migration. Intake rate in 2008 on the two main prey types was dramatically reduced as a result of birds eating smaller prey and strongly increased human disturbance; the same year we also found a high prevalence of a digenean parasite in Darina. We suggest that during the strongly enhanced winter mortality in 2000, knots did not yet face ecological problems in their southernmost wintering area, consistent with the previous evidence that problems at northern stop-overs negatively affected their numbers. However, in 2008 the ecological conditions at Río Grande were such that they would have facilitated a further decline, emphasizing the importance of a hemispheric approach to research and management.

Identification of significant shorebird areas : thresholds and criteria

Aim Conservation managers designate significant areas for shorebirds based on imperfect data. Significant wetlands for migratory shorebirds have usually been identified on the basis of whether they exceed certain thresholds, defined either by total abundance (usually 20,000 waterbirds) or percentage of a population (usually 1.0%). We evaluate the performance of existing criteria and determine if lowering thresholds would improve shorebird conservation without adding unreasonable numbers of significant sites.Location Australia.Methods We evaluated the best available data, which is used by managers to designate significant areas, to describe the effect of lowering thresholds on the number of significant sites identified and the number of shorebirds these sites support using a range of thresholds in existing criteria. We also investigated factors which may explain interspecific differences evident when lowering thresholds.Results When the threshold for total abundance was lowered from 20,000 to 2000 shorebirds, an additional 45 shorebird areas, holding 65% more shorebirds, were identified. When thresholds for the percentage of a population criterion were lowered from 1.0 to 0.1%, an additional 86 shorebird areas were identified which held 29% more shorebirds. The proportion of a species population counted within wetlands identified as significant by the application of criteria varied widely between species. The percentage of population criterion always identified a network of areas that included more individuals of each species than the total abundance criterion at all threshold levels tested. The percentage of species populations found in networks of significant areas showed greater increase as thresholds were lowered for species that were abundant, widespread and well represented at existing thresholds.Main conclusions Our results suggest lowering thresholds will substantially increase the number of shorebirds in identified significant areas. However, some species will remain under-represented, partly because of interspecific differences in distribution and inadequate sampling of some shorebird habitats.<br /