Energetic Condition of Semipalmated and Least Sandpipers during Northbound Migration Staging Periods in Delaware Bay

Semipalmated and Least Sandpipers (Calidrispusilla, C. minutilla) were captured in Delaware Bay, USA, from 1995-1997 and from 2000-2008 to investigate changes in energetic condition during northbound migration staging. For Semipalmated Sandpiper, a model containing the effects of Julian clay, period (Early: 1995-1997, Middle: 2000-2004, Late: 2004-2008) and habitat. type (marsh, beach), as well as all interaction terms for these variables (the "full" model) had the strongest support (i.e. lowest AIC(c) score) compared to six reduced models. Results suggested that mean size-adjusted mass declined significantly across periods (Early > Middle > Late). Also, rate of increase in size-adjusted mass within staging periods was significantly greater during the Early compared to Middle and Late periods. For Least Sandpiper, a reduced model that included Julian clay and Period had the strongest support. Size-adjusted mass was significantly lower in the Late compared to Early and Middle periods, which were not different from each other. While in Delaware Bay, Semipalmatecl Sandpipers feed primarily on horseshoe crab eggs during episodes of rapid mass accumulation, but harvest pressure from 1995-2005 dramatically reduced egg availability. Least Sandpipers appear less dependent on this resource, which may explain differences between the two species regarding changes in energetic condition during staging periods in Delaware Bay. Long-term population trend indices suggest that Semipalmated Sandpiper has declined significantly since the 1980s, especially populations migrating along the Atlantic Coast en route to eastern Canadian breeding areas, which could be related, in part, to changes in food availability, in Delaware Bay. Received 26 July 2011, accepted 2 December 2011.