The least tern is a small seabird that nests along the coast of California. Their population has declined over the years due to habitat loss and disturbance. To monitor the status of the least tern population in California, an annual statewide breeding survey is conducted by researchers, volunteers, and agency biologists.
Introduction to the Least Tern
The least tern (Sternula antillarum browni) is the smallest of the terns found in North America. It measures about 9 inches in length and has a wingspan of 20 inches. Least terns have a black cap on their head, white forehead, gray back and wings, black tips on their primary wing feathers, and a white underside.
This species nests in colonies on coastal beaches from Southern California to Mexico. Their nests consist of small depressions or scrapes in the sand or shell debris. Least tern eggs are cream colored with black or brown spotting. The female lays 1-3 eggs per clutch from April to June. Both parents incubate the eggs for around 20 days before they hatch. Chicks fledge in about 20-25 days after hatching.
Least terns feed by diving for small fish such as anchovies, sardines, and topsmelt. They also eat invertebrates like shrimp and crabs. Their feeding habitat includes nearshore waters, estuaries, and lagoons. Least terns often forage within a mile of their nesting colonies.
Decline of the Least Tern Population
Historically, least terns nested and foraged throughout coastal California. However, the construction of harbors, dams, riprap, and other structures has led to extensive loss and degradation of nesting habitat. Development near nesting areas also increased disturbance from humans, pets, and predators.
As a result, the California least tern population declined significantly over the last century. The species was listed as federally endangered in 1970 and state endangered in 1971 due to its small population size and threats it faced.
Protective measures were implemented, such as fencing and signage around colonies, trash removal, and predator control. Some new nesting habitat was also created. Thanks to these conservation efforts, the California least tern population has rebounded from around 600 pairs in the 1970s to around 7,000 pairs today.
California Least Tern Breeding Survey
To monitor the endangered California least tern, an annual statewide survey of nesting colonies is conducted. The breeding survey is a collaborative effort between researchers, volunteers, and agency biologists.
The goals of the annual survey are to:
- Count the total numbers of breeding pairs
- Determine nesting locations and colony sizes
- Assess nesting success and productivity
- Band birds for research
- Evaluate habitat conditions
The survey period runs from April to September, encompassing the entire breeding season. Surveyors visit known nesting sites and potential habitat across California multiple times through the season. This allows them to find new colonies and track birds over time.
Survey Methods
There are several techniques used during the breeding survey:
- Colony Counts: The total number of nesting pairs at each site is carefully counted. Researchers map nest locations and habitat features.
- Nest Monitoring: A sample of nests at each colony is monitored every 3-5 days to track number of eggs laid, hatched, and fledged.
- Banding: Adult breeding birds and chicks are banded with a unique ID leg band for monitoring individual birds over time.
- Observations: Surveyors record bird behaviors, disturbances, mortality, and interactions with predators, humans, etc.
Survey data is collected on standardized data sheets then compiled into statewide and regional summaries. The survey is conducted with special permits and precautions to minimize disturbance to the sensitive nesting terns.
Key Breeding Survey Findings
Data from the annual California least tern breeding survey provides important insights into the population status and trends over time. Some key findings include:
- Since 1973, there has been a 13-fold increase in the statewide population size from around 600 pairs to over 7,000 pairs today.
- Four major nesting regions support 90% of the population: San Diego, Los Angeles/Orange County, San Francisco Bay, and Monterey Bay.
- Nesting success averages around 1 chick fledged per pair, but varies between 0.2-1.4 across sites and years.
- Population growth has slowed in recent years, suggesting the population is reaching carrying capacity.
- Breeding distribution continues to be restricted compared to historical range.
The information gathered by the annual survey guides conservation efforts for this federally endangered seabird. Monitoring is ongoing to track population trends and identify any new threats to California’s least terns.
Major Nesting Regions in California
The California least tern breeding population nests primarily in four major regions of the state:
San Diego County
San Diego County supports the largest number of breeding least terns, approximately 2,500 pairs as of 2022. Major nesting sites include beaches near Camp Pendleton, Batiquitos Lagoon, San Diego Bay, and the Tijuana River mouth.
Los Angeles and Orange County
Los Angeles and Orange counties hold nearly 2,000 breeding pairs of least terns. Important breeding sites include Venice Beach, Bolsa Chica Ecological Reserve, Los Angeles Harbor, and Seal Beach National Wildlife Refuge.
San Francisco Bay
San Francisco Bay hosts approximately 1,500 nesting pairs annually. Alameda Naval Air Station, Hayward Shoreline, and Eden Landing Ecological Reserve are significant colony locations around the Bay.
Monterey Bay Region
Monterey Bay region supports roughly 500 breeding pairs. Key nesting sites include Marina Dunes State Beach, Moss Landing Wildlife Area, and salt ponds near Elkhorn Slough.
Yearly Trend in Total Pairs of Least Terns in California from 1973 to 2022
Year | Number of Pairs |
---|---|
1973 | 609 |
1975 | 652 |
1980 | 761 |
1985 | 1,013 |
1990 | 1,724 |
1995 | 2,438 |
2000 | 4,029 |
2005 | 6,231 |
2010 | 6,869 |
2015 | 7,103 |
2020 | 7,367 |
2022 | 7,498 |
The table shows the gradual recovery of the California least tern population over time. Since 1973, the number of breeding pairs has increased over 12-fold from just over 600 pairs to around 7,500 pairs in 2022. The most rapid growth occurred between 1990 and 2005. Population growth has leveled off in the past decade as it nears estimated carrying capacity.
Threats and Conservation Challenges
While the California least tern population has rebounded significantly thanks to conservation efforts, the species still faces a number of ongoing threats and challenges:
Habitat Loss
Breeding and foraging habitat continues to be lost and degraded due to coastal development, shoreline armoring, and sea level rise. Lack of habitat availability limits population growth.
Human Disturbance
Many colonies are situated near high human activity areas. Human disturbance can lead to nest abandonment and increased chick mortality.
Predators
Terns have many natural predators, but predators drawn near colonies by human presence and food trash pose high risks. Common predators: foxes, skunks, ravens, gulls.
Climate Change
Sea level rise, increased storms, and altered fish populations due to climate change may impact nesting habitat and food availability.
Pollution
Oil spills, plastics ingestion, and bioaccumulation of toxins in prey species pose ongoing threats to both adult and juvenile terns.
Continued management and community education are needed to protect colonies, provide quality habitat, and reduce disturbances. Ongoing monitoring through the annual breeding survey helps guide conservation priorities for this still vulnerable species.
Conclusion
In summary, the California least tern breeding survey is an important statewide monitoring program that has been conducted annually since 1973. Trained biologists and volunteers count nesting pairs, monitor productivity, and band birds at colonies across coastal California.
Data from the surveys have documented the 13-fold recovery of the endangered least tern since the 1970s. The population has rebounded from around 600 pairs to over 7,000 pairs today. However, growth has leveled off, indicating the population size is nearing carrying capacity.
Four key nesting regions support 90% of the statewide population: San Diego, Los Angeles/Orange County, San Francisco Bay, and Monterey Bay. Nesting success averages 1 fledgling per pair but varies across sites and years.
Ongoing conservation efforts are still needed to protect breeding colonies from human disturbance, loss of habitat, and climate change impacts. The long-term breeding survey provides crucial data to guide management and recovery actions for California’s least tern population.