California least terns (CLT; Sternula antillarum browni) were listed as Endangered by the U.S. Fish and Wildlife Service in 1970 and by the California Department of Fish and Game in 1971. Critical to CLT recovery is understanding their reproductive success. CLT face multiple, interacting threats to reproductive success, including loss of nesting sites, predation, and changes in food availability. In southern California, about a third of the CLT population currently nests within the boundaries of Naval Base Coronado (NBC), adjacent to San Diego Bay, and Marine Corps Base Camp Pendleton (MCBCP) in northern San Diego County. The Department of Defense (DoD) is thus partially responsible for and invests significantly in management actions to contribute to CLT recovery. This includes protecting beach nesting habitat (for both the CLT and the threatened Western Snowy Plover), controlling potential predators, and supporting CLT research and monitoring.

Breeding populations of CLT increased statewide since monitoring and nest protection began in the late 1960s, and the population stabilized around 2003. In 2010 however, CLT breeding pairs began declining by about 21 pairs per year despite ongoing investments in nest protection and predator control, and promising population trends at some colonies. Now 12 years later, the decline is driving CLT toward functional extinction in the near future. It is still unknown as to why reproductive failure is happening and if there are management actions that could help reverse the decline.

It is known from pilot research, adults appear to be spending longer periods of time searching for food than in the past. This could be driving decreases in CLT nest productivity as parent foraging leaves eggs and chicks more vulnerable to predation, thermal stress, and/or starvation. This leads to hypothesizing that more unreliable and patchy food sources may be driving lengthier CLT foraging bouts, and lead to lower amounts and/or quality of food that CLT adults provision to chicks. The project team plans to test this hypothesis while demonstrating several new technologies to monitor foraging CLT to identify forage areas and time budgets, as well as quantifying forage quality and quantity. This project will fill known data gaps to examine drivers of CLT reproductive success at NBC and MCBCP CLT breeding colonies. The project is supported by natural resource managers at the demonstration sites and includes leveraging opportunities with other ongoing studies.

This project will build on historical investments in CLT protection and monitoring to “complete the picture” and better understand the complex drivers of CLT reproductive success by integrating and demonstrating new field monitoring technologies. The project team will use time-lapse cameras and miniaturized very high frequency transmitters to monitor CLT nest activities and foraging. The project team will quantify CLT forage fish using video from an agile remotely operated vehicle or drones, analyzed with machine learning algorithms. These technologies will provide new insight into CLT foraging behavior and forage fish communities in space and time, using faster and less labor-intensive methods to obtain these critical datasets. The project team will also apply molecular biomonitoring of fecal gut contents using 16S rRNA and 12S gene markers to analyze the microbiota communities and fish consumed by CLT from fecal samples. Finally, the project team will model CLT reproductive success based on biotic and abiotic drivers and their interactions. Models will be assessed based on expert analysis of the implied ecological mechanisms.

The DoD is responsible for managing, protecting, and monitoring a large number of threatened and endangered species on its lands and waters. These efforts can be labor intensive and costly. This project will demonstrate several technologies that can be used in DoD natural resource monitoring projects, requiring less labor and therefore lower costs. In addition, it is critical to understand the drivers of population declines, to determine whether DoD actions are responsible for these declines, or whether external factors beyond the control of the DoD are responsible. This project intends to meet this need for the CLT. The database developed under this project can also be applied to studies of threatened and endangered species at these sites such as the western Snowy Plover, Least Bell’s Vireo, and others. (Anticipated Project Completion- 2026)