Optimal colony size for Least Terns: An inter-colony study of opposing selective pressures by predators

I compared breeding success and causes of nesting failure for Least Terns Sterna antillarum nesting in 11 colonies ranging from 2 to 606 nests in Connecticut, USA. Although the percentage of nests lost due to predation decreased with increasing colony size, the relationship between colony size and the impact of predators varied with the type of predator. A significant positive correlation was observed between colony size and nest loss due to Black-crowned Night-Herons (Nycticorax nycticorax). In contrast, a significant negative correlation was observed between colony size and nest loss due to small mammals, Herring Gulls (Larus argentatus), and American Crows (Corvus brachyrhnchos). Thus, colonies of approximately 150 nests appear to be large enough for low impacts by small mammals, gulls, and crows, but small enough to be relatively unattractive to Black-crowned Night-Herons. Nest losses and chick mortality were significantly higher during 1988, mainly the result of increased levels of Black-crowned Night-Heron predation on large colonies. Overall, both hatching and hedging success of Least Terns did not increase with increasing colony size, although all colonies with fewer than 10 nests failed completely. Hatching and hedging success did not correlate with increasing nesting synchrony or increasing nest density during either year. However, nesting synchrony showed a significant negative correlation with both hatching success during both years and hedging success during 1988. Thus, less synchronous colonies showed a tendency to be more successful.