PIERSMA T & SPAANS B (2004) The power of comparison : ecological studies on waders worldwide. LIMOSA 77 (2): 43-54.

This is a brief review of the international research on waders, or shorebirds, entertained by teams from the University of Groningen and the Royal Netherlands Institute for Sea Research in The Netherlands. Extending the ecological studies to mudflat systems and shorebirds other than the Wadden Sea is logical in view of the very migrant nature of the shorebirds themselves, and because so much can be concluded from comparisons between species and sites. We discuss why many birds winter in the tropics, despite the finding that tropical intertidal areas are not necessary rich in food. However, the tropics may have other attractions (like low maintenance costs and, perhaps, more highly predictable food conditions than in the north), and the eventual relative pay-offs of different wintering destinations should be reflected in annual survival rates, and the apportionment of mortality over the seasons. A lot of our work has focused on the worldwide migration system of Red Knots Calidris canutus. These sandpipers breed only on high arctic tundra but move south from their disjunct, circumpolar breeding areas to nonbreeding sites on the coasts of all continents (apart from Antarctica), between latitudes 58° N and 53° S. Due to their specialised sensory capabilities, Red Knots generally eat hardshelled prey found on intertidal, mostly soft, substrates. As a consequence, ecologically suitable coastal sites are few and far between, so they must routinely undertake flights of many thousands of kilometres. In contrast to prediction (based on the low costs of living and thus the freedom to allocate nutrients to fuel storage), Red Knots at tropical intertidal sites have lower fuelling rates than birds at more southern or northern latitudes. The highest fuelling rates occur at the most northerly sites, before the flight to the tundra for breeding. These northern fuelling areas offer smaller benthic biodiversity but greater harvestable biomass than circa-tropical sites, at least in spring. We have now started comparative demographic work on Red Knots wintering in North-western Europe and in West-Africa. The first results show that Knots in West-Africa are very sitefaithful to a particular small area whereas Knots in the Wadden Sea operate on a much larger scale, indicating a higher predictability of food availability in the tropics. In the near future we will be able to compare annual survival between these two populations of Knots and indeed between survival of Bar-Tailed Godwit Limosa lapponica as well. Red Knot Calidris canutus

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