Colony Stability in Least Terns

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Abstract
Colony site tenacity, turnover rates and causes of reproductive failure in least terns (S. antillarum) nesting in coastal New Jersey [USA] from 1976-1982 were examined. Least terns used 44 colony sites, although only 17-29 (mean = 22) sites were used in any 1 yr. Population levels ranged from 942-2469 birds (mean = 1817, SD = .+-. 513). The number of breeding terns increased significantly during the study period. Annual turnover rates varied from 0.16-0.30 (mean = 0.22, SD = .+-. 0.05) and were intermediate to low compared to those for other coastal-nesting terns, gulls (Larus spp.) and black skimmers (Rynchops niger). Reproductive success for the least tern colonies in New Jersey averaged 0.48 young/pair (SD = .+-. 0.22). The causes of reproductive failure were similar among colonies, except that colonies with over 80 birds suffered higher losses due to predators than colonies with fewer than 80 birds. The large, mainland colonies may be more vulnerable to predators because they are more stable (making their presence known to predators) and predators have easy access. Human disturbances accounted for over half of the reproductive failures of least tern colonies. The low turnover rate and high loss to human activity suggest that reproductive success can be improved by increased protection.

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