The California least tern is a small seabird that nests along the coast of California and northwestern Mexico. This endangered species faces threats from habitat loss, pollution, and disturbance of nesting colonies. Understanding the current distribution and abundance of the California least tern is critical for conservation efforts. This article will examine where the remaining populations of this rare bird are found today.
Breeding Range and Nesting Sites
The California least tern nests in colonies along the Pacific coast from the San Francisco Bay area south to Baja California, Mexico. Its breeding range stretches around 500 miles along the coast of California. The major nesting sites include:
San Francisco Bay
The San Francisco Bay hosts the northernmost breeding colonies of the California least tern. Important nesting sites include the former naval air station Alameda, Hayward Shoreline, Eden Landing Ecological Reserve, and the Napa River Ecological Reserve. Around 500 pairs nest in the San Francisco Bay.
Central California
Central California supports significant breeding colonies at locations such as Montezuma Wetlands near San Francisco Bay, Elkhorn Slough, and Morro Bay. Several hundred pairs nest at protected sites in this region.
Channel Islands
The Channel Islands off the coast of Santa Barbara provide crucial nesting habitat. About 2,500 adults breed on the islands, primarily on Anacapa and Santa Barbara Islands.
Southern California mainland coast
Major mainland nesting sites in southern California include Batiquitos Lagoon, San Elijo Lagoon, Newport Bay, Upper Newport Bay Nature Preserve, Bolsa Chica Ecological Reserve, Los Angeles Harbor, Venice Beach, and Camp Pendleton. Several thousand terns nest at sites spanning from San Diego to Los Angeles.
Baja California, Mexico
The southern end of the breeding range extends down the Pacific coast of Baja California. Important nesting sites include Bahía de Todos Santos and islands in the Gulf of California.Habitat protection efforts at nesting colonies in Mexico help support the Pacific population of the tern.
Winter Range
The California least tern migrates south after the breeding season. Its winter range extends along the Pacific coast of Mexico.
The major wintering sites include:
Gulf of California
The Gulf of California hosts the largest wintering population. Terns forage and roost on protected beaches and offshore islands. Important sites include Isla Tiburón, Isla San Pedro Mártir, and Isla Ángel de la Guarda.
Pacific coast of Baja California
Smaller numbers of wintering terns occur along the Pacific coast from Guerrero Negro to Cabo San Lucas at the southern tip of the Baja California peninsula.
Mainland Mexico
Scattered wintering terns reach south to Colima and Guerrero. They frequent coastal lagoons, estuaries, and bays to forage for small fish.
Migration Routes
The California least tern migrates along the Pacific Flyway, a migratory route along the western coast of North America.
During fall migration in late summer and early fall, terns travel from breeding colonies to wintering grounds. This southbound migration likely involves nonstop flights across the Gulf of California.
In spring, terns migrate back north to nesting grounds. This travel occurs in late April and May as terns move into breeding colonies. Radio telemetry data suggests northbound migration may follow the Baja California peninsula instead of crossing the Gulf.
Population and Conservation Status
Historically, the California least tern bred at sites along the entire Pacific coast of California and Mexico. Its population declined over the 20th century.
By 1970, only 600 pairs remained, primarily in a few colonies in southern California. This drastic decline was due to several threats:
Habitat loss
Coastal development destroyed nesting and foraging habitat.
Disturbance
Increased human recreation interfered with nesting. Off-road vehicles crushed eggs and chicks.
Predators
Native and non-native predators increased due to habitat changes. Predators include coyotes, raccoons, skunks, and crows.
Pollution
Chemical pollution caused eggshell thinning. Oil spills contaminated habitat and prey.
The California least tern was listed as federally endangered in 1970. Protection efforts since then have helped its population recover:
– Nesting colonies receive protection from disturbance during breeding season.
– Habitat restoration has expanded available nesting sites.
– Predator control improves nesting success.
– Public education programs raise awareness.
The population increased to around 7,000 breeding pairs by 2005. However, it remains endangered with around 25 breeding colonies across southern California and Mexico. Ongoing conservation management is critical to support recovery.
Importance of Protecting Nesting Habitat
The limited nesting habitat of the California least tern makes the conservation of breeding colonies crucial. Protecting the remaining nesting sites enables these seabirds to reproduce in successive years.
Several strategies effectively protect tern nesting habitat:
Area closures
Closing access to nesting beaches during breeding season prevents disturbance and trampling. This reduces stress, nest abandonment, and loss of eggs and chicks.
Predator control
Lethal removal and non-lethal deterrents of key predators like coyotes maintains nest success. Predator fences may also exclude land predators.
Habitat improvement
Clearing vegetation, adding substrate, and creating islands improves nesting habitat quality.
Public outreach
Explaining closures prevents human interference. Signage raises awareness of terns among beachgoers.
Continued monitoring ensures habitat protections remain effective over time at actively managed colonies. Ongoing stewardship and funding secures habitat for successful breeding.
Foraging Habitat Needs
In addition to safe nesting habitat, California least terns require access to sufficient marine foraging habitat. Terns are pursuit divers that capture small fish near the water’s surface.
Key foraging habitat needs include:
Shallow water
Terns forage in shallow nearshore areas, bays, estuaries, and lagoons where small schooling fish concentrate.
Productive conditions
Upwellings and estuary plumes that deliver nutrients and attract bait fish provide good tern feeding habitat.
Low disturbance
Minimal boating and human activity allows terns to effectively hunt prey.
reduced pollution
Lower contamination levels provide healthier prey populations.
Protecting water quality and productivity close to breeding colonies ensures terns can provision chicks effectively. Linking colony and foraging site conservation provides comprehensive habitat management.
Colony Monitoring
Regular monitoring tracks the status of California least tern breeding colonies. Annual surveys record key population parameters:
Nest counts
Researchers census nests early in incubation to estimate breeding pairs. Nest numbers reflect colony size.
Fledgling success
Surviving chicks are banded and counted before fledging. This productivity metric indicates reproduction success.
Return rates
Re-sightings of banded terns across years estimates adult survivorship and site fidelity.
Comparing data over time identifies population trends. Updated information guides adaptive habitat management and conservation actions.
Monitoring is conducted cooperatively across the range by federal, state, and local agencies along with non-profit groups. Banding terns enables collaborative tracking across sites.
Threats and Conservation Challenges
Despite population increases in recent decades, California least terns still face threats that require ongoing management:
Nesting habitat loss
Coastal squeeze between rising seas and shoreline development reduces nesting sites. More habitat restoration and protection is needed to compensate for future losses.
Extreme weather
Exceptional storms, high tides, and rainfall damage nests and drown chicks. Predicted climate shifts may increase these events.
Human disturbance
Nesting terns remain vulnerable to disruption from beach recreation, fishing, drones, pets, vehicles, and other human activities near colonies. Enhanced law enforcement can deter illegal activities.
Predation impacts
While managed, nest predators still reduce productivity. Predator populations recover quickly, so control requires annual effort.
Pollution
Runoff, oil spills, and remaining legacy toxins like DDT cause ongoing concern. Improving coastal water quality benefits terns.
Sustained monitoring and protection in both nesting and foraging habitat is essential to enable the California least tern population to stabilize and potentially continue increasing.
Citizen Involvement in Conservation
Public participation supports California least tern conservation in several key ways:
Beach closures
Respecting closed areas prevents disturbance to sensitive nesting habitat during breeding season. Staying out of closed sections of beaches gives terns space.
Litter removal
Picking up trash, especially plastics and fishing line, improves nesting areas and marine foraging grounds. This helps reduce chick mortality.
Site stewardship
Volunteering with habitat restoration and management provides hands-on help. Opportunities exist through local organizations.
Monitoring
Reporting tern sightings aids tracking of migration and wintering patterns. This contributes to range-wide understanding.
Outreach
Education programs raise awareness so more people understand tern conservation challenges. This builds public support.
Through small personal actions and larger community efforts, individuals can make a difference for the rare California least tern. Public involvement remains integral to the recovery of this species.
Conclusion
The endangered California least tern has rebounded from the brink of extinction but still requires active management. Breeding primarily along the coast of California and Baja California, nesting habitat protection and close monitoring enables populations to recover. Ongoing challenges like sea level rise necessitate expanded habitat conservation. Public participation in restoration, policy, and education complements scientific management. Targeted conservation strategies focused on sensitive nesting colonies and adjacent foraging areas offer the best hope for the future of this rare seabird. Recovery is possible through diligent effort integrating habitat protection, directed research, and public engagement.