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Sterna antillarumleast tern

By Leila Rojer

Geographic Range

Least terns (Sterna antillarum) have been spotted from San Fransisco Bay to San Diego along the west coast of the United States as well as around the gulf of Mexico. They are common along the eastern and western coasts of the United States during their breeding seasons. They have also been found along the southern coasts of Texas, Louisiana, Mississippi, Alabama, and along the coastlines of Florida. Least terns are also found within some central states. They are uncommonly found in the centralized states during their breeding season. During winter migration, they move from North America and northern Mexico to tropical climates, sometimes as far south as Brazil.

Least terns are found along beaches in northern South America, anywhere from Brazil to Colombia year-round. As climate change makes areas in their distribution warmer, available habitat for least terns will decrease in the central United States and increase in southwestern parts of Canada. Subcontinents where least terns are found include Central America and the Caribbean Islands. Some other countries besides the Americas where they have been commonly seen include Argentina, Colombia, Cuba, Costa Rica, Barbuda, Venezuela, among others. (Akçakaya, et al., 2003; Atwood and Kelly, 1984; "Sternula antillarum .  The IUCN Red List of Threatened Species", 2018)

Habitat

Least terns in California prefer bodies of water including gravel beaches, seacoasts, bays, estuaries, salt flats, lagoons, lakes, and large river channels. They are found on, but do not commonly dwell on, gravelly or sandy rooftops of buildings, due to their natural habitats being altered. They are mostly found along sandy coasts near shallow waters while breeding. Most inland least terns are found alongside riverbeds that contain sandbars or on lakes near salt flats. During winter they migrate to tropical coasts or even out at sea. If environmental conditions continue to fluctuate, the breeding areas for least terns may shift. They have been found to breed in Caribbean Island countries including the Bahamas, Cuba, and Puerto Rico. In South America, least terns breed in Aruba, Venezuela, and on islands. They breed in colonies on sand or mudflats near bodies of water such as coasts, rivers, islands, or gravel shores. They occasionally make their nests on stones or from discarded manmade materials found along the coastline. They are more stable when nesting in large colonies with fewer predators, less human interaction, and fewer flooding sites. Many of their natural nesting sites have been lost to human activity, domestic animal predation, and environmental changes. Terns are seeking more shelter on manmade sites such as mines or reservoirs. In places where least terns do not breed, they live offshore or near coastal drainages, where food sources are abundant. Least terns are found off the coast of Baja California and in South America from April to early May. They are very territorial - they migrate long distances and they like to live in tropical as well as temperate regions. (Akçakaya, et al., 2003; Atwood and Kelly, 1984; Massey and Atwood, 1981; Massey, et al., 1992)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • coastal

Physical Description

Least Terns are the smallest of American terns (family Laridae). During the breeding season, adults have black head caps with white foreheads. They have white eyestripes and yellow bills with black tips. Their backsides are light gray and their undersides are white. The outer edges of their narrow, pointed flight feathers have black stripes. After least terns reach sexual maturity (around the age of 2 or 3) their bills turn yellow as an indication that they are ready to breed. Non-breeding least terns have black bills and eye stripes extending to the backs of their heads, along with white caps. Chicks are very fluffy with orange legs and brown beaks. Their crowns are the same color as their backsides - light brown with darker brown smudges, though some have a darker black color. They have off-white undersides. Some chicks have light brown fur with dark brown spots all over. Juvenile least terns have barred feathers on their backs and yellow legs. They have dark-grayish bills and a black spots behind their eyes.

Adult body size for both sexes is 21 to 24 cm (8.3 to 9.1 in). They weigh 36 to 54 g (1.3 to 1.9 oz). Average wingspan length is 48 to 53 cm (18.9 to 20.9 in). Comparatively, least terns are larger than American robins (Turdus americanus) but smaller than American crows (Corvus brachyrhynchos). (Dunn and Alderfer, 2017; Libbe, 2016)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    36 to 54 g
    1.27 to 1.90 oz
  • Range length
    21 to 24 cm
    8.27 to 9.45 in
  • Average length
    22 cm
    8.66 in
  • Range wingspan
    48 to 53 cm
    18.90 to 20.87 in

Reproduction

The reproductive season of least terns lasts from April until August. Least terns are found within their nesting grounds for anywhere from 2 to 5 months. They are monogamous, and maintain the same partners all throughout the breeding year. Courtship occurs on wing, with males flying upward holding a fish in their bills with females following males. After this upward motion, they both glide down to the ground. On the ground, courtship behavior includes feeding, where males present females with food in hopes they will pair up. (Atwood and Kelly, 1984; Dunn and Alderfer, 2017; Massey, et al., 1992)

Least terns breed on the ground in sand such as beaches or banks of rivers and lakes. If not nesting on sandy beaches or riverbeds, they occasionally use gravel rooftops of buildings. Their nests are made from shallow holes in pebbles, sand, or soil. Least terns mainly nest in colonies, but occasionally in isolation. They nest on open surfaces, but mainly on the ground. Least terns make their nests by scraping shallow holes in sand, dirt, pebbles, or soil and then lining their nest with debris. Both males and females work to build nests. Although least terns favor sandy beaches, they are becoming more commonly found on gravel rooftops, as their beach habitats are being encroached upon by humans.

Least tern clutch sizes range from 1 to 3 eggs per brood and 1 brood per year (sometimes 2 in the southern United States). The egg incubation period of least terns lasts anywhere from 19 to 25 days, while their nestling period is 1 to 2 days. The average length of the eggs is 2.36 to 3.57 cm (0.9 to 1.4 in.). The average egg width is 2.07 to 2.53 cm (0.8 to 1.0 in.). Least tern eggs are pale green in color with spots of gray, brown or black. Chicks hatch with their eyes open and are able to walk, but stay in their nests. After a few days, chicks leave the nest and find their own shelter but stay near their parents for a few months.

  • Breeding interval
    Breed once a year, occasionally 2 broods in a year
  • Breeding season
    April till August
  • Range eggs per season
    1 to 4
  • Average eggs per season
    2-3
  • Range time to hatching
    19 to 25 days
  • Range fledging age
    2 to 3 weeks
  • Range time to independence
    2 to 3 months
  • Range age at sexual or reproductive maturity (female)
    2 to 3 years
  • Range age at sexual or reproductive maturity (male)
    2 to 3 years

Females incubate more often in early stages of incubation, while males incubate more often in later stages. In hotter climates, males wet their belly feathers with water to cool their eggs down. Chicks are able to fly 3 weeks after hatching, but may stay in close proximity to their parents for about 3 months. Both females and males take care of incubating and feeding their chicks.

Lifespan/Longevity

The oldest recorded least tern was reported in New Jersey to be 27 years old. The longevity of a least terns ranges anywhere between 15 and 20 years or more. However, many least terns are killed earlier due to predation. (Libbe, 2016)

  • Range lifespan
    Status: wild
    27 (high) years
  • Typical lifespan
    Status: wild
    15 to 20 years

Behavior

Least terns feed by flying or hovering over water, then plunging into the water when they see prey. If they see prey on land or the surface of the water, they dip down to capture them. While in flight, least terns sometimes catch insects on their wings or beaks.

Least terns live and nest in colonies. Parents defend their territories by making alarm calls and flying up into the air to fight against intruders. They sometimes dive at intruders to protect their nests. During mating season, least terns stay near their colony to feed, traveling around 6 km from their home to seek fish. Terns are monogamous but also very social birds. ("Journal of Field Ornithology", 1996; Libbe, 2016)

Home Range

Least terns have a variable home range depending on whether or not they are breeding. While breeding and nesting, they travel to forage within 6 km of their nesting colony.

Communication and Perception

Tern calls and songs are high-pitched, sharp sounds like “kee-zink” or “kip-kip-kip-kiddeek”. Least terns are very social birds that like to migrate and breed in colonies. Least terns are very noisy when staying in large nesting colonies, with all of them calling to each other. Least terns help each other by protecting colony nests and warding away predators together. (Tomkins, 1959)

Food Habits

Least terns are carnivorous shorebirds that feed mostly on fish and other small organisms. Their main food items include small fish, insects, small mollusks, marine worms, crustaceans, barnacles and other invertebrates. Their diets are dependent upon location and seasonal changes. (Dunn and Alderfer, 2017)

  • Animal Foods
  • fish
  • insects
  • mollusks
  • terrestrial worms
  • aquatic or marine worms
  • aquatic crustaceans

Predation

Predators of least terns include humans and mammals such as coyotes are predators. Humans destroy least tern habitats and mammals eat juveniles and eggs. Least terns are easy prey for terrestrial mammals, since they nest on the ground. Some common predators include gray foxes (Urocyon cinereoargenteus), raccoons (Procyon lotor) and domestic animals such as dogs (Canis lupus familiaris) and cats (Felis domesticus). (Tomkins, 1959)

Ecosystem Roles

Least terns are important to conserve because they help maintain healthy coastal ecosystems by nesting and controlling populations of animals upon which they prey. Least terns serve as prey for various animals and, if least tern populations decline, that puts predator populations at risk of declining as well. It is therefore crucial to protect this species so that their habitat, predators, and prey can continue to thrive.

  • Ecosystem Impact
  • creates habitat

Economic Importance for Humans: Positive

Because least terns are a species of concern according to IUCN Red list and the American Bird Conservancy, there is not only an ecosystem value to maintaining them but also an economic one. Federal and state governments, along with non-governmental affiliates in areas where least terns are present, have put restrictions on guarding the land and predators that impact them. People have also taken an active role by volunteering and donating money to conserve least tern populations by creating areas in which they can live. Actions that help least tern populations include creating areas with better water flow or suitable areas for nesting. Due to governmental and individual efforts being taken to protect least terns, they are making a comeback.

  • Positive Impacts
  • ecotourism
  • research and education

Economic Importance for Humans: Negative

There are no known negative economic impacts of least terns on humans.

Conservation Status

According to a breeding bird survey done from 1966 to 2015, least tern populations in California have declined 88% in that time. Least terns are of high concern because there are only 60,000 to 100,000 breeding birds left total. Without significant conservation efforts and a reduction in threat to their habitat, least terns are at a high risk of extinction. They are common and widespread throughout the continent, but their nesting grounds are threatened due to human development and water fluctuation in rivers, which interferes with their nesting success. Nesting areas near beaches are also being threatened by beach-goers and beach communities. For the above reasons, least terns are classified as endangered on the U.S. federal list. They are protected by the U.S. Migratory Bird Act and listed as vulnerable on the IUCN Red List. The populations of least terns in internal states have been marked as endangered since 1985. Along with habitat loss, heat waves in spring endanger young chicks in their nests.

Inland least tern populations are beginning to show signs of recovery due to nesting on gravel roofs near coastlines. The recovery of inland least tern populations is due in part to protection and monitoring of nesting areas by federal and state agencies. Water flow and has been improved to provide more areas for them to nest and live on. Monitoring and controlling predation levels is another way in which their populations are being conserved. An important reason to maintain least tern species is to preserve the coastal environments they impact. Least terns can provide insight into how migratory birds respond to climate change, as they migrate throughout the year and continuously pick new homes in response to global warming. (Kirsch and Sidle, 1999; Rimmer and Deblinger, 1992; "Sternula antillarum .  The IUCN Red List of Threatened Species", 2018)

Other Comments

A group of least terns is called a “straightness”. Related birds to this species include Peruvian terns (Sterna lorata) and yellow-billed terns (Sterna superciliaris), both of which live in South America.

Contributors

Leila Rojer (author), California State University, San Marcos, Tracey Brown (editor), California State University, San Marcos, Galen Burrell (editor), Special Projects.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

agricultural

living in landscapes dominated by human agriculture.

aposematic

having coloration that serves a protective function for the animal, usually used to refer to animals with colors that warn predators of their toxicity. For example: animals with bright red or yellow coloration are often toxic or distasteful.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

choruses

to jointly display, usually with sounds, at the same time as two or more other individuals of the same or different species

coastal

the nearshore aquatic habitats near a coast, or shoreline.

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

cooperative breeder

helpers provide assistance in raising young that are not their own

diurnal
  1. active during the day, 2. lasting for one day.
duets

to jointly display, usually with sounds in a highly coordinated fashion, at the same time as one other individual of the same species, often a mate

ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

freshwater

mainly lives in water that is not salty.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

male parental care

parental care is carried out by males

marsh

marshes are wetland areas often dominated by grasses and reeds.

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

visual

uses sight to communicate

References

1996. Journal of Field Ornithology. Status of the Least Tern in the Gulf of California (Estado de Sterna antillarum en el Golfo de California), 67(1): 48-58. Accessed March 25, 2020 at www.jstor.org/stable/4514082v.

2018. "Sternula antillarum .  The IUCN Red List of Threatened Species" (On-line). BirdLife International 2018. Accessed March 11, 2020 at https://dx.doi.org/10.2305/IUCN.UK.2018-2.RLTS.T22694673A132567260.en..

Akçakaya, H., D. Breininger, C. Collins, B. Duncan. 2003. Metapopulation Dynamics of the California Least Tern.. The Journal of Wildlife Management, 67(4): 870-877.

Atwood, J., P. Kelly. 1984.

Fish Dropped on Breeding Colonies as Indicators of Least Tern Food Habits.
. The Wilson Bulletin, 96(1): 34-47. Accessed March 25, 2020 at https://sora.unm.edu/sites/default/files/journals/wilson/v096n01/p0034-p0047.pdf.

Dunn, J., J. Alderfer. 2017. Field Guide to the Birds of North America. Washington D.C.: National Geographic.

Kirsch, E., J. Sidle. 1999. Status of the Interior Population of Least Tern. The Journal of Wildlife Management, 63(2): 470-483.

Libbe, M. 2016. "Least Tern." (On-line). TheCornellLab. Accessed March 11, 2020 at https://www.allaboutbirds.org/guide/Least_Tern/overview.

Massey, B., J. Atwood. 1981. Second-Wave Nesting of the California Least Tern: Age Composition and Reproductive Success. Oxford Press, 98(3): 596-605. Accessed March 25, 2020 at https://doi.org/10.1093/auk/98.3.596.

Massey, B., D. Bradley, J. Atwood. 1992. Demography of a California Least Tern Colony including Effects of the 1982-1983 El Niño.. The Condor, 94(4): 976-983.

Renken, R., J. Smith. 1995. Interior Least Tern Site Fidelity and Dispersal.. Colonial Waterbirds, 18(2): 193-198.

Rimmer, D., R. Deblinger. 1992.

Use of Fencing to Limit Terrestrial Predator Movements into Least Tern Colonies.
. Colonial Waterbirds, 15(2): 226-229.

Tomkins, I. 1959.

Life History Notes on the Least Tern.
. The Wilson Bulletin, 71(4): 313-322. Accessed March 25, 2020 at www.jstor.org/stable/4158782.

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