Great Gull Island, part of a small chain off the tip of Long Island, is an ornithological study area owned by New York's Museum of Natural History. Once used for farming, then as part of a lighthouse complex, Great Gull is now decorated with the ruins of military fortifications abandoned after WW II. ""It is a child's dream of a summer place. . . a benign fortress,"" where vegetation swallows up the crumbling gun emplacements. Harwood reviews the history of the island, including some delightfully testy correspondence between lighthouse keepers and mainland officials; but mainly this is a report of his day-by-day activities as a project ""volunteer"" with the group of scientists studying the habits of the migrating tern. Harwood shares the spartan living, good fellowship, and hard work of the ornithologists: chick and egg checks, netting and banding birds, and the Sisyphian labor of ""tern farming"" (weeding plants from the beach to make more nesting room). Basic to the crisscrossing trajectories of the various working scientists is ""that order the birds impose""--terns courting, copulating, brooding eggs, squabbling over territory. . . ""those muttered dialogues and screaming flights."" A gentle, acute, and valuable tribute to a place, its creatures, and a most congenial community.