Temporal patterns in reproductive success in the endangered roseate tern (Sterna dougallii) nesting on long island, new york, and bird island, massachusetts

Roseate Terns (Sterna dougallii) nest in few colonies in the northeastern United States, and the population is listed as endangered. We compare reproductive success from 1987 through 1990 at Cedar Beach, Long Island, and in 1980 and 1987 through 1990 at Bird Island, Massachusetts, to examine yearly and seasonal differences. Productivity was highest for terns breeding in the first six days of the egg-laying period and decreased thereafter. Clutch size, hatching success, and productivity declined significantly during the season, with some variations in pattern among years and between colony sites. Pairs initiating nests after 22 June fledged almost no young. Patterns of reproductive success were more irregular at the smaller colony (Cedar Beach) than the larger colony. Reproductive success was related to age of adults; young birds (two to three years old) had lower clutch sizes, had lower reproductive success, and laid later than older birds. The effective reproductive population of the colony included primarily birds that bred in the early and peak periods; thus, monitoring reproductive success only from early or peak nests overestimates overall reproductive success. Selection against even earlier breeding in this species may be due to lower food resources early in the season and higher predation rates on early nests.