Use of dredged material substrates by nesting terns and black skimmers

While islands formed as the result of dredging operations are becoming incr easingly 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, sil t, and clay), we studied black skimmers (Rhyncops niger), least terns (Ster na antillarum), and gull-billed terns (S. nilotica) to determine if their r eproductive success differed between the 2 substrates. We also tested the h ypotheses that the substrate composition might affect vegetation growth, su bstrate surface temperatures, and nest visibility. The average proportion o f eggs that hatched was higher for nests on shell than for nests on fine-te xtured dredged materials (P = 0.025); however, the proportion of nests in w hich at least 1 egg hatched was not different between substrates (P = 0.214 ). Vegetation densities and cover were higher on fine-textured dredged mate rials than on shell after 2 growing seasons (P less than or equal to 0.010) , but not after only 1 season (P greater than or equal to 0.112). The amoun t of time substrate temperatures were potentially stressful (greater than o r equal to 42 degrees C) did not differ between substrates (P = 0.987). Hum an observers took longer to locate black skimmer and gull-billed tern nests on shell than on fine-textured dredged materials (P less than or equal to 0.008). Gull-billed terns (P less than or equal to 0.001), but not black sk immers (P = 0.343), nested on supplemental deposits of shell in larger numb ers than on nearby areas of fine-textured dredged material. Our results sug gest 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 is lands for nesting terns and black skimmers.