Are disproportionate losses of eastern breeding Semipalmated Sandpipers (Calidris pusilla) driving declines in the upper Bay of Fundy, Canada?

Abstract

Large numbers of Semipalmated Sandpipers (Calidris pusilla) use the Bay of Fundy, Canada, as a stopover site during their southbound migration from breeding to wintering grounds. The species exhibits a geographic cline in bill length on the breeding grounds (larger in east). Based on a previous analysis of morphometric traits that detected a small temporal decline in bill length, it has been suggested that observed declines in the Semipalmated Sandpiper population staging in the Bay of Fundy were driven by a disproportionate loss from the eastern breeding population. New data collected in the Bay of Fundy has made it possible to reassess this suggestion. We compared new bill length data from the Bay of Fundy (2009–2014) to historical data (1981–1989, 1997–2007) to examine temporal changes in bill length at this stopover site. We also examined count data at a major roost site from 1981 to 2014 to assess temporal changes in migratory passage. Variation in median passage date is explained best by a combination of year and Arctic climate data. Birds are now migrating through the region later in the season; median passage date has advanced approximately 5 d from 1980s values. Our analyses suggest that while there was a small decline in bill length between the 1980s and early 2000s, it has since reversed and bills are now comparable to historical values. Given this reversal in mean bill length, coupled with population estimates that are still low relative to historical values, there is now little evidence to support the hypothesis that the decline between the 1980s and today in birds passing through the Bay of Fundy is linked to disproportionate losses of eastern breeders. We therefore suggest that population declines may be distributed more broadly across the breeding range of birds that use the Bay of Fundy as a stopover site.