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Least Tern - Sternula antillarum
Species of Concern Native Species Global Rank :
G4
State Rank :
S2B
(see State Rank Reason below)
Agency Status USFWS :
DM; MBTA
USFS :
BLM :
SENSITIVE
FWP SWAP :
SGCN1, SGIN
PIF :
1
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Copyright by Borror Laboratory of Bioacoustics, Department of Evolution, Ecology, and Organismal Biology, Ohio State University, Columbus, OH, all rights reserved.
State Rank Reason (see State Rank above)
Species is uncommon to rare along the lower Yellowstone and Missouri River drainages in eastern Montana. It was formally listed under the Endangered Species Act but had since recovered and was delisted. Populations appear stable or increasing. It faces significant ongoing threats from the changes to the hydrology of these river systems and subsequent impacts to habitat.
Least Tern (Sternula antillarum ) Conservation Status Summary
State Rank: S2B
Review Date = 10/21/2024
How we calculate Conservation Status
Rarity:
Very Uncommon Very Common
Threats:
Highly Threatened Unthreatened
Trends:
Rapidly Declining Declining Stable Increasing
Rank:
S1
S2
S3
S4
S5
Critically Imperiled Secure
See the complete
Conservation Status Report
General Description
The smallest tern in North America, the Least Tern averages 21 to 24 cm long and has a wingspan of 51 cm (Thompson et al. 1997). In breeding plumage the species is characterized by a black cap and stripe through the eye that contrast sharply with a white forehead (Thompson et al. 1997). The underparts of the bird are white, while the upperparts are gray. The outer primaries of their long, narrow wings are black. They have a short, slightly notched tail, and a slightly decurved and tapered yellow bill (unique from other tern species) with a small black tip. The sexes are virtually identical, although Whitman (1988) notes some subtle differences; the male bill is described as orange to bright yellow, while the female's is light, dull yellow, or straw-colored. The iris is dark brown (Thompson et al. 1997); the feet and legs of the male are bright orange and generally bright to pale yellow on the female (Whitman 1988).
Vocalization of the Least Tern is described as a shrill, rapid, sharp "
piDEEK-adik" or "
keDEEK" as well as a weak, nasal sounding "
whididi" and high, sharp "
kweek" or "
kwik" squeaks. The alarm call is a sharp, rising "
zreek" (Sibley 2000).
For a comprehensive review of the conservation status, habitat use, and ecology of this and other Montana bird species, please see
Marks et al. 2016, Birds of Montana. Diagnostic Characteristics
It is unlikely for the Least Tern to be confused with any other tern species in Montana. Its diminutive size, yellow bill, and white forehead are distinctive. Another tern species found in the state, the Forster's Tern, also has a black cap, but it lacks the white forehead. Also, the Forster's Tern is larger than the Least Tern, has a large orange, not yellow, bill and lacks black primaries in breeding plumage (Sibley 2000).
Species Range
Montana Range
Range Descriptions Western Hemisphere Range
Observations in Montana Natural Heritage Program Database
Number of Observations: 574
(Click on the following maps and charts to see full sized version)
Map Help and Descriptions
Relative Density
Recency
SUMMER (Feb 16 - Dec 14)
Direct Evidence of Breeding
Indirect Evidence of Breeding
No Evidence of Breeding
WINTER (Dec 15 - Feb 15)
Regularly Observed
Not Regularly Observed
(Observations spanning multiple months or years are excluded from time charts)
Migration
Little is known about the migratory patterns of the Least Tern in Montana. Most of the observations in the state have been recorded for breeding pairs, with few reported sightings of transient individuals. Spring arrival of the species occurs in mid-May, with departure in the fall generally occurring by mid-August (Montana Bird Distribution Committee 2012). The extreme early migration date in Montana is May 24, 1994 at Fort Peck (Skaar et al. 1985). Least Terns nesting in the northern U.S., including Montana, may migrate the farthest of all terns, possibly wintering in South America (Thompson et. al 1997).
Habitat
Least Terns nest on unvegetated sand-pebble beaches and islands of large reservoirs and rivers in northeastern and southeastern Montana, specifically the Yellowstone and Missouri river systems (Christopherson 1991). These wide, open river channels, and lake and pothole shorelines provide the preferred characteristics for nesting Least Terns. Sites with gravel substrate provide the most suitable sites for nesting (Montana Piping Plover Recovery Committee 1994). One of the most limiting factors to nesting site selection is vegetational encroachment; Least Terns avoid areas where relatively thick vegetation provides cover for potential predators. Fine-textured soils are easier to treat mechanically than rocky or gravelly soils when vegetation is determined as a limiting factor in an area's ability to provide suitable nesting habitat, but fine soils are not typically a preferred nesting substrate (Montana Piping Plover Recovery Committee 1994).
Ecological Systems Associated with this Species
Details on Creation and Suggested Uses and Limitations
How Associations Were Made
We associated the use and habitat quality (common or occasional) of each of the 82 ecological systems mapped in Montana for
vertebrate animal species that regularly breed, overwinter, or migrate through the state by:
Using personal observations and reviewing literature that summarize the breeding, overwintering, or migratory habitat requirements of each species (Dobkin 1992, Hart et al. 1998, Hutto and Young 1999, Maxell 2000, Foresman 2012, Adams 2003, and Werner et al. 2004);
Evaluating structural characteristics and distribution of each ecological system relative to the species' range and habitat requirements;
Examining the observation records for each species in the state-wide point observation database associated with each ecological system;
Calculating the percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system to get a measure of "observations versus availability of habitat".
Species that breed in Montana were only evaluated for breeding habitat use, species that only overwinter in Montana were only evaluated for overwintering habitat use, and species that only migrate through Montana were only evaluated for migratory habitat use.
In general, species were listed as associated with an ecological system if structural characteristics of used habitat documented in the literature were present in the ecological system or large numbers of point observations were associated with the ecological system.
However, species were not listed as associated with an ecological system if there was no support in the literature for use of structural characteristics in an ecological system,
even if point observations were associated with that system.
Common versus occasional association with an ecological system was assigned based on the degree to which the structural characteristics of an ecological system matched the preferred structural habitat characteristics for each species as represented in scientific literature.
The percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system was also used to guide assignment of common versus occasional association.
If you have any questions or comments on species associations with ecological systems, please contact the Montana Natural Heritage Program's Senior Zoologist.
Suggested Uses and Limitations
Species associations with ecological systems should be used to generate potential lists of species that may occupy broader landscapes for the purposes of landscape-level planning.
These potential lists of species should not be used in place of documented occurrences of species (this information can be requested at:
mtnhp.org/requests ) or systematic surveys for species and evaluations of habitat at a local site level by trained biologists.
Users of this information should be aware that the land cover data used to generate species associations is based on imagery from the late 1990s and early 2000s and was only intended to be used at broader landscape scales.
Land cover mapping accuracy is particularly problematic when the systems occur as small patches or where the land cover types have been altered over the past decade.
Thus, particular caution should be used when using the associations in assessments of smaller areas (e.g., evaluations of public land survey sections).
Finally, although a species may be associated with a particular ecological system within its known geographic range, portions of that ecological system may occur outside of the species' known geographic range.
Literature Cited
Adams, R.A. 2003. Bats of the Rocky Mountain West; natural history, ecology, and conservation. Boulder, CO: University Press of Colorado. 289 p.
Dobkin, D. S. 1992. Neotropical migrant land birds in the Northern Rockies and Great Plains. USDA Forest Service, Northern Region. Publication No. R1-93-34. Missoula, MT.
Foresman, K.R. 2012. Mammals of Montana. Second edition. Mountain Press Publishing, Missoula, Montana. 429 pp.
Hart, M.M., W.A. Williams, P.C. Thornton, K.P. McLaughlin, C.M. Tobalske, B.A. Maxell, D.P. Hendricks, C.R. Peterson, and R.L. Redmond. 1998. Montana atlas of terrestrial vertebrates. Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT. 1302 p.
Hutto, R.L. and J.S. Young. 1999. Habitat relationships of landbirds in the Northern Region, USDA Forest Service, Rocky Mountain Research Station RMRS-GTR-32. 72 p.
Maxell, B.A. 2000. Management of Montana's amphibians: a review of factors that may present a risk to population viability and accounts on the identification, distribution, taxonomy, habitat use, natural history, and the status and conservation of individual species. Report to U.S. Forest Service Region 1. Missoula, MT: Wildlife Biology Program, University of Montana. 161 p.
Werner, J.K., B.A. Maxell, P. Hendricks, and D. Flath. 2004. Amphibians and reptiles of Montana. Missoula, MT: Mountain Press Publishing Company. 262 p.
Food Habits
No specific information regarding food habits in Montana is known for this species. Generally the Least Tern consumes small fishes (generally less than 9 cm long), but sometimes eats crustaceans or insects. Prey is obtained by diving from the air into shallow water usually less than 4 m deep (Moseley 1976). Interior populations depend almost entirely on cyprinids. Feeding in newly plowed fields has been observed in Texas; apparently beetle larvae were being captured (McDaniel and McDaniel 1963).
Ecology
Least Terns are a recent breeder in the state. The first confirmed nesting occured in 1987 (Montana Piping Plover Recovery Committee 1989, 1994). In Montana, nesting colonies are particularly vulnerable to inundation in the Missouri River system.
Reproductive Characteristics
Least Terns are known to nest along the shorelines of Fort Peck Reservoir, the Missouri River below Fort Peck Dam, and along the Yellowstone River, downriver from Miles City (Hanebury, personal communication). Least Terns have been observed nesting in association with Common Terns at Fort Peck Reservoir and are often closely associated with Piping Plovers in the limited areas of the state in which they breed (Montana Piping Plover Recovery Committee 1994).
Management
In October 2019, the USFWS published a notice in the
Federal Register proposing to delist the interior population of Least Tern based on a
Five Year Review of the species status and a current estimated population of 18,000 terns and 480 nesting sites across this population.
Appropriate water management, that which includes natural seasonal flows, is identified as the major consideration for Least Tern conservation in Montana, for the greatest threat to breeding pairs, in some years, is the loss of existing nesting sites from inundation by high water at unusual times of the breeding season (Montana Piping Plover Recovery Committee 1994). Rising water levels late in the nesting season can also decrease overall island size, and may result in assisting local avian predators to locate nests (containing eggs or nestlings) more easily (Erickson and Prellwitz 1999). These conditions reinforce the need to manage reservoirs and dammed rivers in a manner that mimics more natural seasonal fluctuations for the protection of Least Tern populations. Other management activities beneficial to the species include: instituting grazing management practices more appropriate to the conservation of the Least Tern; controlling access to key nesting locations; moving nests upslope from areas where flooding of nests is imminent; relocating eggs to nests of other Least Terns for foster incubation; signing of beaches to indicate nesting by Least Terns (though in areas where there is hostility toward the species, or toward listed species in general, this is not recommended); beach enhancement (grading or burning to remove unwanted encroaching vegetation); raising island elevation to make room to move nests in years with rising water during the nesting season (Montana Piping Plover Recovery Committee 1994); and timing spring flow releases from Fort Peck Dam to more closely mimic the natural seasonal flows of the river (MTFWP 2003). Other management activities to enhance habitat or affect better protection for this species includes reducing human, dog, and vehicular disturbance during nesting (MTFWP 2003).
Additional information on the biology and management of Least Tern populations can be found on the U.S. Fish and Wildlife Service's
Environmental Conservation Online System Species Profile Stewardship Responsibility
Based on the Montana Natural Heritage Program's latest
predicted habitat suitability model
Total species' range in Montana 22,771 km2
(6% of Montana)
Area predicted to have 3,481 km2
(<1% of Montana)
Stewardship responsibility for the predicted area of suitable habitat can be broken down as follows
Total Suitable
Optimal Suitability
Moderate Suitability
Low Suitability
Federal
22%
<1%
3%
19%
State
2%
<1%
1%
1%
Local
<1%
<1%
<1%
<1%
Conservation Lands/Easements
<1%
<1%
Private/Tribal/Unknown
76%
3%
21%
52%
See the Habitat Suitability for Biodiversity task in Map Viewer
for a more detailed look at stewardship responsibilities within a variety of local jurisdictions.
References
Literature Cited AboveLegend: Charadrius melodus ) and least tern (Sterna antillarum ) in Montana, summer 1990. Unpublished report for the Montana Piping Plover Recovery Committee. 60 p.Charadrius melodus ) surveys; Nelson Reservoir, Bowdoin National Wildlife Refuge, Hewitt Lake National Wildlife Refuge. Unpublished report. PRT-704930. 10 p.Sterna antillarum ) on Long Island, N.Y. The Seatuck Research Program in cooperation with the New York State Department of Environmental Conservation. 70 pp.Charadrius melodus ) and Least Tern (Sterna antillarum ) in Montana - summer 1988. 39 pp.Charadrius melodus ) and least tern (Sterna antillarum ) in Montana. Unpublished report. 116 pp. plus appendices. Additional ReferencesLegend: Do you know of a citation we're missing? Effects of recreation on Rocky Mountain wildlife: a review for Montana . [Montana]: Montana Chapter of the Wildlife Society.Charadrius melodus ) and least tern (Sterna antillarum ) in Montana. 117 pp. plus appendices.Sterna antillarum ). Species Account Number 290. The Birds of North America Online (A. Poole, Ed.). Ithaca, NY: Cornell Laboratory of Ornithology; Retrieved 3/25/2008 from The Birds of North America Online database Web Search Engines for Articles on "Least Tern"
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