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Sternula antillarum browni

(Mearns, 1916)

California Least Tern

T2T2 (G4T2T3Q) Found in 23 roadless areas NatureServe Explorer →
T2T2Global Rank
California least tern (Sternula antillarum browni). Photo by U.S. Fish & Wildlife Service, Public Domain (U.S. Government Work), via ECOS.
U.S. Fish & Wildlife Service, https://www.usa.gov/government-works
Identity
Unique IDELEMENT_GLOBAL.2.104205
Element CodeABNNM08103
Record TypeSPECIES
ClassificationSubspecies
Classification StatusStandard
Name CategoryVertebrate Animal
Endemicoccurs (regularly, as a native taxon) in multiple nations
KingdomAnimalia
PhylumCraniata
ClassAves
OrderCharadriiformes
FamilyLaridae
GenusSternula
Synonyms
Sterna albifrons browniMearns, 1916Sterna antillarum browniMearns, 1916
Concept Reference
American Ornithologists' Union (AOU). 1957. The AOU check-list of North American birds, 5th ed. Port City Press, Inc., Baltimore, MD. 691 pp.
Taxonomic Comments
Formerly (AOU 1983, 1998) included in the genus Sterna but separated on the basis of genetic data that correspond to plumage patterns (Bridge et al. 2005) (AOU 2006). The American Ornithologists’ Union (AOU) recognizes three subspecies: S. a. browni from California, the Eastern, S. a. antillarum, and the Interior, S. a. athalassos (AOU 1957).

Massey (1976) evaluated morphological, vocal, and behavioral characteristics and concluded that nominal subspecies antillarum and browni are indistinguishable. Thompson et al. (1992) examined morphological and electrophoretic variation and found little evidence of differentiation among populations of the nominal subspecies antillarum, athalassos, and browni; they recommended that the subspecific taxonomy of the S. antillarum complex be reassessed. Johnson et al. (1998) used a quantitative colorimetry analysis to study variation among antillarum, athalassos, and browni and found differences significant enough to warrant the validity of the taxa and their importance as entities for conservation.
Conservation Status
Review Date2014-12-16
Change Date2014-12-16
Edition Date2011-02-14
Range Extent20,000-2,500,000 square km (about 8000-1,000,000 square miles)
Number of Occurrences21 - 80
Rank Reasons
Restricted to California and Baja California; total population recently stable at a couple thousand pairs; threatened by human disturbance, exotic predators, environmental contamination, and other factors; nominal subspecies of the least tern are of dubious validity.
Range Extent Comments
BREEDS: Pacific coast, central California to southern Baja California, Chiapas, and Jalisco (Garcia and Ceballos 1995, Condor 97:1084-1087); since 1970, most nesting has occurred from Santa Barbara to San Diego County, California. NORTHERN WINTER: probably Baja California to southern Mexico, or, according to Ehrlich et al. (1992), possibly along the Pacific coast of Central America and north to at least Colima, Mexico.
Occurrences Comments
There were 28-29 nesting colonies in the late 1980s.
Threat Impact Comments
Major problems are human use and development of nesting habitat and predation on adults, eggs, and young by birds (e.g., kestrels, night-herons) and mammals (foxes, skunks, domestic cats and dogs). In 1988, red foxes killed 75 percent of the nesting terns in Orange County. The reduced number of suitable nesting areas has limited or eliminated the tern's anti-predator strategy of shifting among different nesting areas in different years. In southern California, levels of selenium and lead in eggs tentatively were thought to warrant more detailed study because they may be approaching levels which could impair reproductive success (Collins 1992). The El Nino of 1982-1983 had major adverse effects on population dynamics for a period of five years (Massey et al. 1992). See Carvacho et al. (1989) for information on threats to Gulf of California population.
Ecology & Habitat

Description

The smallest North American tern; breeding adult is mainly gray above, with a black cap and nape, white forehead, orange-yellow bill with or without a dark tip, grayish underparts, short deeply forked tail, and yellow-orange legs and feet; a black wedge on the outer primaries is conspicuous in flight (NGS 1983). Adult in winter plumage has a dingy cap, dark nape, a black line through the eye, a dark bill, and yellowish feet and legs (NGS 1983, Peterson 1990). Juvenile is pinkish-buff above, with brownish U-shaped marks on the back; crown is dusky; dark bar is present on the front part of the folded wing. First-summer birds resemble adults but retain the dark bar on the wing and have a dark bill and dark feet and legs, dusky primaries, a dark nape, and a black line through the eye (NGS 1983).

Diagnostic Characteristics

Similar to subspecies ANTILLARUM but underparts grayish white or distinctly tinged with pale gray (vs. pure white in ANTILLARUM), gray of upperparts slightly deeper, and black of pileum extending farther down the median portion of the nape; also, the bill frequently lacks a dark tip, and the three outer primaries are dusky (vs. the two outer primaries dusky gray or blackish slate, with their inner webs broadly edged with white, the remaining primaries being pallid neutral gray, in ANTILLARUM) (Ridgway 1919). However, Massey (1976) found BROWNI to be indistinguishable from ANTILLARUM. Thompson et al. (1992) were unable to verify previously reported distinctions among the nominal subspecies of S. ANTILLARUM and concluded that they may not be sufficiently differentiated to justify their recognition.

Habitat

Seacoasts, beaches, bays, estuaries, lagoons, lakes, and rivers (AOU 1983). Rests and loafs on sandy beaches, mudflats, and salt-pond dikes (Stiles and Skutch 1989). May roost at night on sandy beaches away from nesting areas for several weeks before nesting.

Nests usually on open, flat beaches along lagoon or estuary margins; sometimes on mud or sand flats a distance from the ocean or on artificial islands created from dredge spoils (Ehrlich et al. 1992). See Spendelow and Patton (1988) for many further details on nesting habitat. Usually nests in same area in successive years; tends to return to natal site to nest (Atwood and Massey 1988).

Ecology

Larger colonies may incur greater losses to predation than do smaller colonies because the former are more stable and hence known to predators (e.g., Burger 1984).

Nonbreeding: usually occurs singly or in small loose groups; in larger flocks when migrating.

At Venice Beach, annual survival rate for most adult age classes was about 80-90% (Massey et al. 1992).

Reproduction

Lays clutch of usually 2-3 eggs, mostly May-June (July-August nests probably are renests). Incubation usually 20-25 days, mostly by female. Young tended by both parents, brooded for several days, fly at about 3-4 weeks, dependent for a few weeks more. First breeds at 2-3 years. At Venice Beach, the expected breeding life of an adult (once it has bred) was 9.63 years, with a lifetime productivity of 1.49 (= number of progeny per adult expected to survive to breeding age) (Massey et al. 1992). Colonies may be divided into subcolonies (Massey 1974).
Terrestrial Habitats
Sand/dune
Palustrine Habitats
Riparian
Other Nations (1)
United StatesN2B
ProvinceRankNative
ArizonaS2MYes
CaliforniaS2Yes
Threat Assessments
ThreatScopeSeverityTiming
1 - Residential & commercial developmentUnknownExtreme - moderateHigh (continuing)
6 - Human intrusions & disturbancePervasive - restrictedSerious - moderateHigh (continuing)
6.1 - Recreational activitiesPervasive - restrictedSerious - moderateHigh (continuing)
7 - Natural system modificationsPervasive - largeExtreme - seriousHigh (continuing)
7.3 - Other ecosystem modificationsPervasive - largeExtreme - seriousHigh (continuing)
8 - Invasive & other problematic species, genes & diseasesPervasive - largeExtreme - seriousHigh (continuing)
8.1 - Invasive non-native/alien species/diseasesPervasive - largeExtreme - seriousHigh (continuing)
8.2 - Problematic native species/diseasesPervasive - largeExtreme - moderateHigh (continuing)
9 - PollutionPervasive - largeUnknownHigh (continuing)
11 - Climate change & severe weatherPervasive - largeSerious - moderateHigh - low
11.1 - Habitat shifting & alterationPervasive - restrictedSerious - moderateHigh - low

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Roadless Areas (23)
Arizona (19)
AreaForestAcres
Black CrossTonto National Forest5,966
BoulderTonto National Forest40,359
Butterfly Roadless AreaCoronado National Forest42,296
Catalina St. Pk. Roadless AreaCoronado National Forest951
Cdo WsaCoronado National Forest1,955
GoldfieldTonto National Forest15,257
Happy ValleyCoronado National Forest7,972
Horse MesaTonto National Forest9,146
Lime CreekTonto National Forest42,568
Lower RinconCoronado National Forest3,278
Lower Romero WSRCoronado National Forest10
MazatzalTonto National Forest16,942
Middle Romero WSRCoronado National Forest60
Oracle RoadlessCoronado National Forest22,365
Santa RitaCoronado National Forest6,078
TumacacoriCoronado National Forest44,594
Upper Rincon RoadlessCoronado National Forest2,991
Upper Romero WsrCoronado National Forest150
WhetstoneCoronado National Forest20,728
California (4)
AreaForestAcres
Chalk PeakLos Padres National Forest7,472
JuncalLos Padres National Forest12,289
MatilijaLos Padres National Forest5,218
White LedgeLos Padres National Forest18,632
References (40)
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  2. American Ornithologists' Union (AOU). 1983. Check-list of North American Birds, 6th edition. Allen Press, Inc., Lawrence, Kansas. 877 pp.
  3. American Ornithologists' Union (AOU). 2006. Forty-seventh supplement to the American Ornithologists' Union Check-list of North American Birds. The Auk 123(3):1926-936.
  4. Atwood, J. L., and B. W. Massey. 1988. Site fidelity of least terns in California. Condor 90:389-394.
  5. Bent, A.C. 1921. Life histories of North American gulls and terns. U.S. Natl. Mus. Bull. 113. Washington, D.C.
  6. Biosystems Analysis, Inc. 1989. Endangered Species Alert Program Manual: Species Accounts and Procedures. Southern California Edison Environmental Affairs Division.
  7. Bridge, E. S., A. W. Jones, and A. J. Baker. 2005. A phylogenetic framework for the terns (Sternini) inferred from mtDNA sequences: implications for taxonomy and plumage evolution. Molecular Phylogenetics and Evolution 35:459-469.
  8. Buckley, P. A., and F. G. Buckley. 1984. Seabirds of the north and middle Atlantic coast of the United States: their status and conservation. Pages 101-133 in Croxall et al., eds. Status and conservation of the world's seabirds. ICBP Tech. Pub. No. 2.
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  17. Draheim, H. M., M. P. Miller, P. Baird, and S. M. Haig. 2010. Subspecific status and population genetic structure of least terns (<i>Sternula antillarum</i>) inferred by mitochondrial DNA control-region sequences and microsatellite DNA. The Auk 127(4):807-819.
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  38. van Halewyn, R., and R. L. Norton. 1984. The status and conservation of seabirds in the Caribbean. Pages 169-222 in Croxall et al., eds. Status and conservation of the world's seabirds. ICBP Tech. Pub. No. 2
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  40. Whittier, J. B., D. M. Leslie Jr, and R. A. Van Den Bussche. 2006. Genetic variation among subspecies of Least Tern (Sterna antillarum): implications for conservation. Waterbirds 29(2):176-184.