Using stable isotope ratios to unravel shorebird migration and population mixing: a case study with Red Knot Calidris canutus

Identifying demographic mechanisms is fundamental to understanding the causes of population change in waterbirds. This may be relatively easy for static breeding and wintering populations, but populations of mixed breeding or wintering origin often occur in stopover sites in spring and autumn, and thus estimates of survival and recruitment from these areas are inevitably representative of all the birds marked, rather than individual populations. We used stable isotope analysis of flight feathers to identify the different wintering populations of Red Knot Calidris canutus rufa that passed through Delaware Bay, north-eastern USA, in the springs of 2004 and 2005. Here, they feed and fatten on an abundance of Horseshoe Crab Limulus polyphemus eggs before flying to their Arctic breeding areas. δ13N values separated birds from wintering areas in southern South America (“southern” birds) and Brazil/south-eastern USA (“northern” birds). Northern birds were further separated using δ13C values. Approximately 55% of the birds caught within Delaware Bay were from the southern population, 22.5% from Brazil, and 12.5% from the south-eastern USA, while 10% were of unknown (although most likely “northern”) origin. At a site on the Atlantic coast of Delaware Bay, where only Mussel Mytilus spp. spat were available, the proportion of short-distance migrants from the south-eastern USA was much higher, and is most likely related to their shorter-hop migration strategy that allows them to take advantage of this hard-shelled prey resource