The least tern (Sternula antillarum) is the smallest member of the tern family Sternidae found in North America. This migratory seabird breeds in coastal areas across North America and winters along the coasts of Central and South America.
The least tern nests in a simple scrape on open, sandy beaches or flats along coastlines, rivers, and lakeshores. Its specific nesting habitat requirements make it vulnerable to disturbance and habitat loss. Understanding the details of the least tern’s nesting ecology is key to conservation efforts for this species.
What time of year does the least tern nest?
The least tern returns to its breeding grounds and establishes nesting colonies in May and June. The timing varies slightly across their range, arriving earlier in southern areas and later in northern areas.
Egg laying typically begins in late May and June. The breeding season lasts around 3 months until the young fledge in August and September. Adults and juvenile birds then migrate back to their wintering grounds in September and October.
Where does the least tern nest in North America?
The least tern has a wide breeding distribution across North America:
- Atlantic coast from Maine to Florida
- Gulf coast from Florida to Texas
- Caribbean islands
- Pacific coast from California to Washington
- Interior areas near large rivers and lakes such as the Mississippi River, Rio Grande, Great Lakes and Great Salt Lake
Some key nesting areas include Cape Cod, Massachusetts, Long Island, New York, coastal Virginia, Texas Gulf Coast, San Francisco Bay, California and the Lower Colorado River.
What type of habitat does the least tern require for nesting?
The least tern nests in open habitats with little vegetation, typically on sandy beaches, sandbars, barrier islands, dredge spoil islands, dry salt flats, and alkali lakeshores.
Key requirements for nesting habitat include:
- Open, sparsely vegetated sites near water – ocean, rivers, lakes
- Sandy or gravel substrates – beaches, flats, dredge islands
- Access to high protein fish prey within flying distance
- Lack of predators and human disturbance
Vegetation above a few inches tall obscures visibility and provides cover for predators, so sparsely vegetated sites are strongly preferred. Sand and gravel substrates provide camouflage for eggs and chicks. Proximity to water provides feeding areas to capture small fish.
How does the least tern choose a nest site?
Least terns are colonial nesters, meaning they nest in groups called colonies or breeding aggregations. Colonies can range from a few pairs up to hundreds of nesting pairs.
When selecting a nesting site, least terns look for the following habitat cues:
- Open visibility with low vegetation
- Substrate matches egg color for camouflage – pale eggs on light sand
- Some shelter is provided by objects like driftwood, debris piles, dunes, or clumps of vegetation
- Other least terns are present
- No predators or humans disturbing the area
If a site has appropriate habitat cues, the least tern scrapes out a shallow depression in the sand or gravel to form the nest. The nest is typically 2 to 6 inches across and 1 inch deep. Egg laying begins once the nest site is established.
How does the least tern nest colony use the habitat?
Within a nesting colony, least tern pairs space their nests approximately 1 to 4 feet apart. This spacing helps minimize conflict over territories while still providing some protection by aggregating in a group.
Key areas used by the colony include:
- Nesting area – Open interior areas for nest scrapes
- Loafing areas – Open areas near nests for resting and sheltering chicks
- Chick rearing areas – Feeding and sheltering areas often near driftwood, debris or dunes
- Flight paths – Access corridors to feeding areas over water
Maintaining open sight lines throughout the colony is critical. Any encroaching vegetation is usually trimmed back by the birds through trampling. Human activity and predators can disrupt nesting if access corridors are altered.
How does proximity to water influence nest site selection?
Access to feeding areas with abundant small fish is key requirement for nesting. Least terns frequently fly 1 to 8 miles from the colony to fish, but prefer to nest within a mile or two of high quality fishing areas.
Preferred nesting sites provide quick access to marine waters, estuaries, shoals, tidewaters, river channels, and lakes with adequate fish prey. If a nesting area becomes isolated from feeding areas due to changing water levels, the terns will abandon the colony.
How do substrate and topography influence nest site selection?
Least terns evolved to nest on light colored, sandy or gravelly beaches and flats. This habitat provides camouflage for their pale colored eggs and chicks. The color match helps hide the eggs from potential predators.
The terns prefer flat, open areas of loose substrate with good drainage to nest. Flat sites provide better visibility to watch for predators. Loose sand or pebbles allows the birds to scrape a shallow nest depression. Good drainage prevents flooding of nests during high tides or rainfall.
In natural settings, nesting habitat is maintained by shoreline dynamics and flooding that turns over and flattens the substrate. On river systems, seasonal flooding and drought shape sandbars and beaches used for nesting. Coastal dunes and washover areas also provide suitable nesting habitat.
How does competition influence nest colony selection?
Prime nesting habitat is limited, so least terns often compete with other colonial waterbirds for preferred nesting sites. Competition is especially high with related terns and gulls that share similar habitat needs.
Species that often overlap with nesting least terns include:
- Black skimmers
- Gull-billed terns
- Sandwich terns
- Royal terns
- Common terns
- Forster’s terns
- Black skimmers
- Laughing gulls
- Ring-billed gulls
- Herring gulls
Least terns may nest in mixed-species colonies or occupy segregated areas apart from competitor species. Aggressive interactions help maintain boundaries between species. Predation and competition for food can become issues in mixed colonies.
How does predation influence colony site selection?
Least terns nest in open habitats with good visibility to help detect approaching predators. Primary nest predators include gulls, crows, raptors, foxes, coyotes, raccoons, and snakes.
During site selection, least terns avoid areas with signs of predators like tracks, scat, nests, and perch sites. Established colonies may relocate if disturbance or predation becomes excessive. Adults mob intruders with aerial attacks to drive predators from the area.
Areas isolated from mainland predators, like islands and shell rakes, are preferred nesting sites. In natural habitats, spring floods and high tides provide seasonal protection from mammals by isolating colonies.
How do disturbance and human activity influence nest sites?
Least terns are highly susceptible to disturbance from human activity in nesting areas. They avoid nesting near areas with steady activity like boat ramps, marinas, and beaches used for recreation.
Nesting colonies are often displaced from traditional sites by new construction, vehicle traffic, dredging, and intensive recreation. Terns are frequently attracted to nest on rooftops when ground sites are limited.
Various strategies are used to protect tern colonies from human disturbance, including fencing, signage, access restrictions, and time-of-use limits. Patrols and monitoring help enforce protections and assess disturbance levels.
Conclusion
In summary, the ideal least tern nesting habitat consists of open, sparsely vegetated sites near high quality fishing areas. Specific requirements include flat sandy or gravel substrates with little vegetation, isolation from land-based predators, and protection from human disturbance.
Understanding the habitat needs and nesting ecology of the least tern is essential for effective conservation of this threatened species. Targeted habitat management and protection of sensitive nesting colonies can help ensure the least tern continues thrive across its breeding range.
