Use of Dredged Material Substrates by Nesting Terns and Black Skimmers

Abstract

While islands formed as the result of dredging operations are becoming increasingly important as nesting habitat for colonial seabirds, the effect of the composition of these islands on their usefulness as colony sites is not adequately understood. Because some seabirds apparently prefer to nest on shell rather than on fine-textured dredged material (a mixture of sand, silt, and clay), we studied black skimmers (Rhyncops niger), least terns (Sterna antillarum), and gull-billed terns (S. nilotica) to determine if their reproductive success differed between the 2 substrates We also tested the hypotheses that the substrate composition might affect vegetation growth, substrate surface temperatures, and nest visibility. The average proportion of eggs that hatched was higher for nests on shell than for nests on fine-textured dredged materials (P = 0.025); however, the proportion of nests in which at least 1 egg hatched was not different between substrates (P = 0.214). Vegetation densities and cover were higher on fine-textured dredged materials than on shell after 2 growing seasons (P ≤ 0.010), but not after only 1 season (P ≥ 0.112). The amount of time substrate temperatures were potentially stressful (≥42°C did not differ between substrates (P = 0.987). Human observers took longer to locate black skimmer and gull-billed tern nests on shell than on fine-textured dredged materials (P ≤ 0.008). Gull-billed terns (P ≤ 0.001), but not black skimmers (P = 0.343), nested on supplemental deposits of shell in larger numbers than on nearby areas of fine-textured dredged material. Our results suggest shell substrates are important for vegetation control, egg camouflage, and reproductive success. Where possible substrate composition should be manipulated for increased levels of shell when managing dredged material islands for nesting terns and black skimmers.