Animal Diversity Web

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)

• Biogeographic Regions
• nearctic
  • native
• neotropical
  • native

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)

• Habitat Regions
• temperate
• tropical
• terrestrial
• saltwater or marine
• freshwater

• Terrestrial Biomes
• chaparral

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

• Wetlands
• marsh

• Other Habitat Features
• urban
• suburban
• agricultural

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)

• Other Physical Features
• endothermic
• bilateral symmetry

• 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)

• Mating System
• monogamous
• cooperative breeder

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.

• Key Reproductive Features
• iteroparous
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual
• oviparous

• 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.

• Parental Investment
• male parental care
• female parental care
• pre-hatching/birth
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female

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)

• Key Behaviors
• terricolous
• flies
• diurnal
• motile
• migratory
• territorial
• social
• colonial

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)

• Communication Channels
• acoustic

• Other Communication Modes
• duets
• choruses

• Perception Channels
• visual
• tactile
• acoustic
• chemical

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)

• Primary Diet
• carnivore
  • eats terrestrial vertebrates
  • piscivore
  • insectivore
  • eats non-insect arthropods

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

• Foraging Behavior
• stores or caches food

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)

• Anti-predator Adaptations
• aposematic

• Known Predators

  • Gray fox (Urocyon cinereoargenteus), racoons (Procyon lotor), dogs (Canis lupus familiaris), and cats (Felis domesticus)

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)

• IUCN Red List

  Vulnerable Lower Risk - Conservation Dependent

• US Migratory Bird Act

  Protected

• US Federal List

  Endangered

• CITES

  No special status

• State of Michigan List

  No special status

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.

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.

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.

Tomkins, I. 1959.

Life His­tory Notes on the Least Tern.