California Least Tern Breeding Survey Analysis

California least terns listed as endangered

Due to population decline resulting from loss of habitat, 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.

Annual monitoring begins

The endangered status prompted wildlife agencies to initiate monitoring efforts to estimate the breeding population size of least terns in California. Researchers began by conducting annual monitoring of terns during the breeding season.

This information could be used to analyze their population size and productivity. The data is also necessary as part of the 5-year mean reproductive rate criteria needed for eventual delisting from endangered status.

Understanding the circumstances in which this species is able to survive and reproduce successfully is necessary in order to guide future steps for protecting the terns.

Addressing tern population needs

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. These efforts include physical substrate preparation, physical and chemical plant eradication, predator fencing, and identification of predator species.

There are likely other drivers that may be influencing least tern population trends, including changing oceanographic conditions and prey abundance and distribution.

Eventually, the success of our tern population monitoring and assessment efforts will hopefully mean delisting of the tern from its current endangered status. 

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
6. Recommendations for future monitoring and research efforts

SDSU Project Team

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