Barnacles dominate the winter diet of Calidris maritima (Purple Sandpiper) in Maine: A bona fide dietary shift or molecular techniques revealing an unknown diet composition?

At temperate latitudes, the overwintering period for birds is often longer than subsequent periods, and lower daily foraging success during this period may produce population-level impacts that are similar to or larger than those reported during shorter periods of the annual cycle. In this study, we use DNA metabarcoding of fecal samples to reveal previously undocumented characteristics of the winter diet of Calidris maritima (Purple Sandpiper) in Maine (USA), with potential implications for this population’s local overwintering declines during a recent 10-year period. Of the 24 taxonomic groups detected, barnacles were the dominant prey item while mussels—a common diet item in previous studies—were rare. In addition, we identified both expected prey taxa (e.g., marine snails) and several less-expected taxa (e.g., terrestrial insects). Our study is the first to employ molecular techniques to describe C. maritima diet, and as a result, may have uncovered an unknown diet composition that was difficult to detect with morphological techniques. Alternatively, our results may be at least in part due to a recent dietary shift during a period of increased environmental warming in the Gulf of Maine, which has previously been correlated with declines of marine invertebrates that are known C. maritima prey items. This period of accelerated warming coincided with a steep decline in Maine’s C. maritima overwintering population that occurred from 2004 to 2014. The relationship between a putative dietary shift in our study and the recent declines in overwintering C. maritima in Maine is intriguing, but unclear and requires further tests of winter food limitation. Regardless, utilizing a novel technique to understand C. maritima diet has shed light on this species’ food resources during the long overwintering period.