What is the habitat of the Forster’s tern?

The Forster’s tern (Sterna forsteri) is a small seabird that breeds primarily in coastal areas of North America. Its preferred habitat during the breeding season consists of marshes, beaches, barrier islands, and other coastal areas. Forster’s terns nest on the ground in colonies, often alongside other tern species. Their nests are simple scrapes in the sand or gravel that may be lined with bits of debris and vegetation.

Breeding Habitat

Forster’s terns breed in coastal areas from southeastern Alaska across southern Canada and the northern United States down to Baja California and the Gulf Coast of Mexico. The core of their breeding range centers around the Great Lakes and westward through the prairie provinces of Canada to British Columbia.

During the breeding season, Forster’s terns are most abundant in the northern prairie regions of the United States and Canada. Their preferred nesting habitat consists of large marshes and wetlands, along the marshy borders of lakes and ponds, and on islands or peninsulas in lakes and estuaries. Vegetation in nesting areas is typically sparse or absent.

Beaches and sandbars are also used for nesting in some areas, especially along the East and Gulf coasts. Nests may be placed among scattered vegetation, debris, or other cover on open sandy or gravelly areas. Forster’s terns appear to prefer flat or gently sloping sandy beaches over wide, flat expanses of mudflat as found in tidal marshes.

Throughout their breeding range, Forster’s terns nest close to shallow-water feeding areas such as estuaries, saltmarshes, ponds, flooded fields, and shorelines. Proximity to food resources is an important factor in their nest site selection.

Colony Sites

Specific types of sites used by nesting colonies include:

• Sandy beaches and sandbars in coastal areas
• Gravel pads and island in rivers and lakes
• Small islands in estuaries and salt marshes
• Peninsulas or exposed reefs in lakes
• Open and sparsely vegetated areas near water
• Bare or thinly vegetated islands in prairie marshes

Forster’s terns nest in dense colonies that may contain hundreds or thousands of pairs. Colony size ranges from as few as 25 pairs up to over 10,000 pairs. Large colonies are often located in areas relatively inaccessible to mammalian predators.

This species exhibits high colony site fidelity, reusing nesting sites in successive years as long as suitable habitat conditions persist. However, habitat alteration or increased disturbance levels may cause abandonment of traditional colony locations.

Non-Breeding Habitat

During the non-breeding season, Forster’s terns inhabit coastal areas, lakes, rivers, and wetlands across much of Mexico, the southern United States, and Central America. Their winter range extends as far south as Panama.

On the Atlantic Coast, these birds winter from Massachusetts southward with concentrations in the southeastern states. On the Pacific Coast, they winter from southern British Columbia south to Baja California and the Revillagigedo Islands off western Mexico.

Preferred foraging habitats in coastal areas include saltmarshes, tidal creeks and flats, estuaries, bays, and coastal lagoons. Inland habitats include lakes, ponds, rivers, flooded fields, and wetlands. Forster’s terns often roost on sandy beaches, sandbars, jetties, piers, and buoys when not feeding.

While wintering along the Gulf Coast, Forster’s terns can be found taking refuge on oil platforms and other manmade structures. They have also been observed feeding in the wake of shrimp boats.

Migration Stopover Sites

During spring and fall migration, Forster’s terns stop to rest and feed at wetlands, lakes, and rivers across much of the central United States. Important stopover sites include:

• Northern prairie marshes and lakes
• Great Salt Lake in Utah
• Large wetland areas of the Mississippi Delta
• Coastal marshes and estuaries along the Gulf and Atlantic coasts

Migrating and wintering Forster’s terns often mix loosely with other tern species including the Common Tern and Black Tern while feeding and roosting.

Habitat Preferences

The availability of suitable nesting habitat limits Forster’s tern distribution and abundance across much of its range. This species prefers to nest in areas with the following characteristics:

• Low, sparse vegetation with sandy or gravel substrates
• Isolation from mammalian predators
• Proximity to shallow water feeding areas
• Low levels of disturbance during nesting season

Suitable nesting habitat may be naturally occurring, such as sparsely vegetated islands in lakes and estuaries. Or it may be created through human alteration, including dredge spoil islands, evaporation ponds, gravel pits, and rooftop nesting sites.

Forster’s terns often rely on the early successional habitat found in newly created or restored wetlands. However, vegetation succession can quickly render an area unsuitable for nesting within a few years if the habitat is not maintained in an early successional state.

During the non-breeding season, Forster’s terns use a wider variety of coastal and inland wetland habitats.Their habitat use during migration and winter is influenced more by food availability than nesting requirements.

Range-Wide Habitat Trends

Forster’s tern populations expanded during the late 19th and early 20th centuries as human activities inadvertently created nesting habitat across their breeding range. Activities such as dredging, dam construction, and wetland drainage combined with widespread hunting of predators lead to an increase in nesting colonies.

However, in the late 20th century, Forster’s tern numbers declined due to habitat loss caused by factors including:

• Filling of marshes and wetlands for development
• Flood control and bank stabilization projects along inland rivers
• Increased vegetation overtaking nesting areas
• Expanded predator populations
• Recreational disturbance of nesting colonies

Ongoing wetland losses combined with disturbance continue to put pressure on Forster’s tern habitat across much of their range. Targeted habitat management and protection efforts are needed to maintain nesting populations in many regions.

Regional Habitat Trends

Some notable regional habitat trends include:

• Declines in the prairie regions of the U.S. and Canada due to wetland drainage and increased predation.
• Reduced nesting habitat availability along the Atlantic and Gulf Coasts caused by development and stabilization of barrier islands and beaches.
• Disturbance of nesting colonies in the Great Lakes by expanding gull populations and human recreation.
• Habitat losses in coastal California due to wetland filling and expanding human populations.

Targeted habitat conservation and management programs have helped stabilize or increase Forster’s tern populations in some regions. But habitat availability remains a primary limiting factor across significant portions of its breeding range.

Habitat Management Methods

A variety of approaches can be used to create, maintain, and protect nesting habitat for Forster’s terns:

• Artificial Island Construction: Building dredge spoil islands and gravel pads provides safe nesting sites isolated from predators.
• Vegetation Control: Removing or thinning vegetation through grazing, mowing, herbicide use, or burning helps prevent overgrowth of nesting areas.
• Water Level Management: Manipulating water levels can create suitable mudflat or beach habitat during nesting season.
• Predator Management: Lethal and non-lethal techniques help reduce nest predation by limiting gulls, crows, coyotes, and other predators.
• Signage & Fencing: Informational signs, access restrictions, and symbolic fencing help protect colonies from human disturbance.

Habitat creation or enhancement projects are most successful when tailored to the specific conditions at a given location. Ongoing habitat management and monitoring are also needed to track impacts over time.

Migratory Habitat Needs

In addition to habitat protection at nesting colonies, conservation of migratory stopover habitat is crucial for Forster’s terns. Destruction of wetland habitat along migration routes may limit food supplies needed to replenish energy reserves during migration.

Forster’s terns exhibit low site fidelity to stopover areas used during migration. They appear to be opportunistic in selecting stopover sites, using a variety of habitat types that offer adequate food resources.

This flexibility makes identification of critical migratory habitats more difficult compared to breeding habitats. However, conservation efforts focused on protecting wetlands along major river systems and other known migratory corridors will benefit Forster’s terns and numerous other waterbird species.

Climate Change Impacts

Climate change poses a significant threat to coastal nesting habitat for Forster’s terns in the coming decades. Projected impacts include:

• Accelerated sea level rise inundating low-elevation sites
• Increased storm activity and severity
• Shifting ocean currents impacting food supplies
• Altered precipitation patterns changing inland water conditions

Nesting habitat could be lost or degraded due to increased flooding, erosion, sedimentation, and saltwater intrusion at coastal sites. Vegetation zones may shift as precipitation and temperature changes alter wetland plant communities.

If global warming leads to expanded predator populations and ranges, increased conflict with nesting terns could result. Rising temperatures may also allow inland expansion of nesting but with the tradeoff of increased competition with other tern species for limited habitat.

Conserving adequate habitat across a wide geographic range will be important in providing Forster’s terns with options to adapt to climate change impacts.

Conclusion

In summary, Forster’s terns nest in a variety of coastal and inland wetland habitats across North America. They require areas with sparse vegetation and isolation from predators for successful breeding. Conservation efforts that protect, enhance, and actively manage nesting habitat are crucial for maintaining populations. Ongoing loss of coastal wetlands and prairie marshes continues to put pressure on breeding colonies across significant portions of their range.