The foraging ecology of Royal and Sandwich Terns in North Carolina, USA

We studied the foraging ecology of Royal Terns (Sterna maxima) and Sandwich Terns (S. sandvicensis) in North Carolina during the breeding seasons in 1999 and 2000. Foraging habitats were surveyed using 95 fixed-point observations (30-45 min each) and over 100 h of inshore and offshore transects. Stable isotopes (C-13:C-12, N-15:N-14, and O-18:O-16) in eggshells were analyzed for evidence of differences in foraging habitats. Water masses were delineated using satellite imagery of sea surface temperatures. We determined diet for each species with observations of chick provisioning and fecal analyses. Although Royal and Sandwich Terns nested together in the same colonies, their prey and foraging habitats differed. Sandwich Terns fed their chicks 48% anchovies (Anchoa sp.), 39% herring (Clupeidae)/jacks, (Carangidae)/mackerels (Scombridae), and 9% drums (Sciaenidae)/porgies (Sparidae) mullets (Mugil sp.), and foraged primarily in the marine coastal environment. Conversely, Royal Terns provided their chicks with 9% anchovies, 30% herring/jacks/mackerels, and 41% drums/porgies/mullets, and fed more frequently in the estuary. The species' foraging ranges overlapped at inlets and the river mouth. Stable isotope ratios also suggested that foraging differences occurred between colonies. The results of this investigation indicate that Royal and Sandwich Terns in North Carolina partition food resources by habitat and prey choice.