We measured habitat characteristics in colony sites and at nest sites of endangered least terns (Sterna antillarum athalassos) on an expansive alkaline flat at Salt Plains National Wildlife Refuge, Oklahoma, in 1992 and 1993. Least terns nested in 12 colony sites on the alkaline flat during 1992 and 1993. Number of nests per colony site ranged from 5–25 and density of nests ranged from 0.1–0.75 nests/ha. Density of least terns in colony sites ranged from 0–2.1 terns/ha and was not correlated with soil color (P = 0.66) or soil texture (P = 0.12). Only a small percentage of ground cover was provided by vegetation (0.0–1.2%), driftwood (0.0–0.4%) and debris (0.0–1.1%) in colony sites. Least terns selected nest sites with coarser soil (loamy sand to sandy loam) than that of random points (P ≤ 0.0003). The coarser soils were lighter in color (P = 0.0001). Least tern nests were closer to driftwood or debris than random points (P = 0.0001). Internest distances in active colony sites ranged from 21.4 to >100 m. Nearest-neighbor analyses indicated that nests were distributed randomly in 58% and uniformly in 42% of active colony sites. Nests were distributed uniformly in small active colony sites and randomly in large active colony sites. No differences in densities of nests were noted relative to size of colony sites. Increasing availability of coarse light-colored soils in areas of ≥20 ha on alkaline flats may enhance recovery of least terns by improving nesting habitat and recruitment.
