The least tern is a small seabird that is given its name due to being the smallest species in the tern family Sternidae. With an average body length of only 8-10 inches and a wingspan of 20 inches, the least tern lives up to its name as the littlest of its relatives.
What is a Least Tern?
The least tern (Sternula antillarum) is the smallest species of tern. It measures 21–24 cm (8.3–9.4 in) in length and 58–67 cm (23–26 in) across the wings. In breeding plumage, it has a black cap, white forehead, gray back, wings and tail, black primaries and outer secondaries, and a white underside. The legs are red-orange. In non-breeding plumage, the black cap is less distinct, with variable black smudging on the nape and crown. Juveniles have a dark cap, dark leading edge to the wings, and a dark collar. The North American races S. a. antillarum and S. a. athalassos differ slightly in size, proportions and coloring; S. a. browni is somewhat larger with a larger bill and longer legs.
The least tern’s call is a distinctive ker-wee. Its nest is a shallow scrape lined with shells, pebbles, or fish bones. Up to three eggs are laid which hatch in 20–25 days. Chicks fledge in 20–25 days.
Taxonomy
The least tern was described by the French polymath Georges-Louis Leclerc, Comte de Buffon in 1771 in his Histoire Naturelle des Oiseaux from a specimen collected in the French colony of Saint-Domingue (now Haiti). The bird was also illustrated in a hand-coloured plate engraved by François-Nicolas Martinet in the Planches Enluminées D’Histoire Naturelle which was produced under the supervision of Edme-Louis Daubenton to accompany Buffon’s text.
Neither the plate caption nor Buffon’s description included a scientific name but in 1783 the Dutch naturalist Pieter Boddaert coined the binomial name Sterna minuta in his catalogue of the Planches Enluminées. The type locality is Saint-Domingue. The least tern is now placed in the genus Sternula that was introduced by the French biologist Charles Lucien Bonaparte in 1838.
The genus name is from Latin sterna, “tern” and the specific antillarum, “of the Antilles”, refers to the type locality.
Description
The least tern is the smallest tern species, with a length of 21–24 cm (8.3–9.4 in), wingspan 58–67 cm (23–26 in) and a weight of 26–49 g (0.92–1.73 oz). The wings are long, narrow and pointed with a dark upper surface and pale below. The long tail is deeply forked and there is a band of grey across the outer tail feathers. The legs are short and red-orange in colour. In summer, least terns have a black cap, white forehead, grayish back and upperwings, and white underparts. The cap is solid black in females but may have a white central streak in males. In winter, the forehead becomes white and the cap partly white. Young birds have a brown cap, dark leading edge to the wings and a dark collar.calls can help distinguish this species from other dark-capped terns.
Distinguishing Features
Some key features that help identify the least tern include:
- Very small size, measuring just 21-24 cm in length
- Black cap on head (solid black in females, may have white streak in males)
- Short orange-red legs
- White forehead
- Gray back and wings
- Deeply forked tail
- Juveniles have brown cap and dark collar
- Distinctive “ker-wee” call
Distribution and Habitat
The least tern has a wide distribution across North America, Central America, the Caribbean, and northern South America. There are three recognized subspecies:
- S. a. antillarum – breeds along the Atlantic Coast of North America and winters from the southern USA south through the Caribbean to Brazil.
- S. a. athalassos – breeds inland across central North America and winters along the Pacific and Gulf Coasts from California to Chile.
- S. a. browni – breeds around the coast of Baja California and the Gulf of California.
In terms of habitat, least terns nest on sandy or gravelly beaches and feed in shallow estuaries and lagoons. They tend to avoid rocky coasts and steep beach profiles. Nesting areas are often associated with salt marshes, inlets, creeks, and flats. Outside of the breeding season, they occur along coasts from marina to offshore waters.
Behavior
The least tern is a highly social species and is nearly always seen in flocks outside of the breeding season. They feed mostly on small fish and will plunge dive from heights of 1-6 m to catch prey. Least terns also opportunistically feed on shrimp, marine worms, and insects.
Courtship displays like aerial gliding and offering small fish help form pair bonds. Nesting is colonial, with nest sites often clustered together in aggregations called colonies or rookeries. Parents share incubation and chick rearing duties. Chicks are semiprecocial and able to walk and swim soon after hatching.
The least tern is strongly migratory, traveling from breeding grounds as far north as Massachusetts and southern California to wintering areas along Central and South America. Migration peaks in late April-May and July-mid August, though individuals disperse over a broad time period.
Reproduction and Breeding
The breeding season for least terns runs from May to August, depending on latitude. Courtship involves aerial gliding displays and offering fish to prospective mates. Nest sites are simple scrapes or depressions in the sand or gravel and may have some small pebbles or shell fragments lining the nest bowl.
Clutch size averages 2-3 eggs (range 1 to 4) which are incubated for 20-25 days by both parents. The eggs are buff to tan with dark brown spotting. Parents share brooding duties and chicks hatch asynchronously over several days.
Chicks are precocial and leave the nest within 1-3 days to seek food. They are able to fly at 20-25 days old. Annual productivity is highly variable based on weather, food availability, and human disturbance. Average clutch survival rates are often only 10-40%.
Key Breeding Facts:
- Nesting is colonial, with <10 to >1000 pairs per site
- Lay 1-4 eggs (usually 2-3)
- Incubation is 20-25 days
- Chicks fledge at 20-25 days old
- Double brooding is common
- Breed from May-August depending on latitude
Diet and Feeding
Least terns areSpecialized and efficient plunge divers. They feed mainly on small, schooling fish like anchovies, silversides, and killifish. Other prey items include shrimp, marine worms, and insects. Terns hunt by flying over water and plunge diving to depths of 1-6 m when prey is spotted.
Feeding habits shift during breeding when fish make up over 90% of chick diet. Parents capture fish singly and carry them back to chicks in the bill. Most foraging occurs within sight of the colony to allow quick return to nests.
Least terns occasionally kleptoparasitize food from other seabirds by harassing them in flight. This aerial piracy helps them supplement their fish diet with other prey like crabs and squid stolen from gulls.
Key Facts on Feeding:
- Plunge dive from heights of 1-6 m to catch fish
- Prey predominantly on small schooling fish
- Also take shrimp, marine worms, insects
- Nestlings fed mostly fish carried in parent’s bill
- Forage near colony during breeding season
- Pirate food from other seabirds
Population and Conservation
The least tern has experienced population declines across much of its range due to habitat loss and disturbance. However, coordinated conservation efforts since the 1970s have helped stabilize numbers in many areas. Current global population estimates are:
- S. a. antillarum (Atlantic coast) – 45,000 to 70,000 breeding pairs
- S. a. athalassos (Interior) – 25,000 to 50,000 breeding pairs
- S. a. browni (Pacific coast) – 1,000 to 2,500 breeding pairs
Major threats include human disturbance, development, invasive predators, and climate change impacts. Least terns nest in exposed sandy beaches that experience high rates of human recreation and vehicular use. They are easily flushed from nests, exposing eggs and chicks to overheating and predators.
Habitat loss from shoreline development, water diversion, and sea level rise also impact terns. Predators including gulls, crows, foxes, coyotes, and feral cats consume many eggs and young. Conservation efforts involve public education, nest protection, predator control, and habitat management.
Subspecies | Population Estimate | Conservation Status |
---|---|---|
S. a. antillarum | 45,000 – 70,000 pairs | Least Concern |
S. a. athalassos | 25,000 – 50,000 pairs | Least Concern |
S. a. browni | 1,000 – 2,500 pairs | Endangered |
Significance to Humans
For most people, least terns are valued for their ecological roles and inherent worth as wild species. Watching their aerial agility and fishing skills is an entertaining wildlife viewing experience. Least terns may also benefit humans by controlling insect pests and eating invasive fish.
However, their need for undisturbed nesting habitat can conflict with human recreational interests. Temporary beach closures, leash laws, and usage limits aim to allow both tern breeding and human activities. With thoughtful planning, people and least terns may successfully coexist and appreciate the beaches and shores they share.
Fun Facts
- Least tern eggs blend in remarkably with beach sand and gravel, camouflaging them from predators.
- If a predator gets too close to a colony, least terns will mob it with loud alarm calls and aerial dives.
- The San Diego Zoo successfully bred least terns in 1979, the first successful captive breeding of the species.
- A group of nesting least terns is called a “colony”, “rookery”, or “creche”.
- Least tern chicks can swim within a day after hatching if needed to escape floods or predators.
Conclusion
In summary, the least tern earns its name as the smallest species in its family, with a body length of just 21-24 cm. Despite its diminutive size, it makes epic migrations and defends nesting colonies tenaciously. Loss of nesting habitat has caused population declines, and thoughtful conservation efforts are needed to protect beach nesting areas. With appropriate habitat, the aerial agility and lively flocks of least terns will continue gracing shorelines and reminding us that small can be mighty.