California Least Tern Breeding Survey Analysis

The California least tern was listed as endangered by the U.S. Secretary of the Interior in 1970 and the California Fish and Game Commission in 1971 due to a population decline resulting from loss of habitat. The endangered status prompted wildlife agencies to initiate monitoring efforts to estimate the breeding population size of least terns in California. Consideration for delisting is based on the annual breeding population size among a set number of coastal management areas and a 5-year mean reproductive rate. Thus, annual monitoring of terns during the breeding season and subsequent analysis of tern population size and productivity is necessary to assess whether the California least tern meets these delisting criteria.
There are numerous potential barriers to least tern population growth and recovery, including conditions on breeding and wintering grounds. Considerable effort has been put into habitat restoration and protection for least terns on the breeding grounds, including physical substrate preparation, physical and chemical plant eradication, predator fencing, and identification of predator species. Current tern monitoring considers these efforts and a summary of these are included in previous CDFW reports. There are likely other drivers that may be influencing least tern population trends, including changing oceanographic conditions and prey abundance and distribution.

Project Outcomes:

1. Estimation of the number of breeding pairs of nesting terns;
2. Assessment of productivity of nesting terns;
3. Quantification of predation impacts on tern productivity and survivorship;
4. A summary of the above data, in context with data from previous years of tern monitoring;
5. Creation of an R script that succinctly analyzes the data, to be used in future years of tern monitoring analysis; and
6. Recommendations for future monitoring and research efforts.

SDSU Project Team

Rebecca Lewison, Principal Investigator and Student Advisor
Corey Clatterbuck, Project Analyst