Population Research Priorities for Black Terns Developed from Modeling Analyses

Abstract

-Black Tern (Chlidonias niger) populations are threatened in several regions of North America and are increasingly the subject of monitoring and research efforts. To focus future studies on critical information needs, I used a population model to determine the sensitivity of population growth rate (r) of Black Terns to reproductive and survival estimates commonly measured in field studies, and developed research priorities based on these results and a review of the literature. Sensitivity was estimated by two methods: 1) percent change in rresulting from a 10% increase in a parameter's value and 2) percent change in r resulting from an increase in the value of a parameter by an increment equal to 10% of the range of field estimates for that parameter. For 10% changes in baseline rates, rwas more sensitive to adult (168% increase in r) and sub-adult survival (62%) rate than to reproductive estimates (4-31%) as expected; however, when sensitivity was examined relative to variation in estimates for wild populations, adult survival, chick survival and nest success had similar effects (33-38%). Clutch size, hatchability, renesting rate, proportion of 2-year-old terns breeding, and longevity had relatively less influence on r (4-10%). Adult and chick survival rates should be high priorities for future study because adult rates are important for evaluating breeding success data and chick survival rates are needed to estimate productivity. Sub-adult survival is important, but will be difficult to study in small populations. Clutch size and hatchability information should have lower priorities. Although sensitivity to renesting rate was low, the lack of information justifies a greater emphasis. For nest success, the research emphasis needs to change from monitoring rates to quantifying limiting factors. Received 19 April 2000, ac-