Least Tern Habitat Model
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Draft Date:
April 2001

Species:
Least tern, Sterna antillarum; eastern subspecies, S. a. antillarum

Use of Study Area Resources:
Reproduction: Least tern (Sterna antillarum) has three subspecies which breed in the U.S. The eastern least tern, S. a. antillarum, breeds along the Atlantic and Gulf coasts from southern Maine to Texas. The interior least tern, S. a. athalassos breeds along the Mississippi River and its major tributaries, while the California least tern (S. a. brownii) breeds along the Pacific coast from the Baja Peninsula to north of San Francisco Bay. Marine coasts of Mexico, Central and South America are used for wintering habitats. (Carreker 1985).

Cover Types:
Least terns nest in loose colonies on coastal dunes and on sand or shell beaches just above the high tide line or, along major interior rivers (Hunter 1975, Blodget 1978, Carreker 1985, Thompson et al. 1997), in areas that are swept clear of vegetation. Studies in New York found that at most nest sites ground coverage by low vegetation varied from 5 to 25%, and sites with less than 20% ground coverage were preferred (Gochfeld 1983). Studies in North Carolina found a preference there for sites with less than 10% vegetative coverage (Jernigan et al. 1978). Least terns are prone to return to previous year colony sites, or move only short distances (Carreker 1985, Atwood and Massey 1988). Beaches on New York's Long Island were avoided for nesting if they were less than 10 m wide above the high tide line (Carreker 1985). The smallest beach area used is about 11,000 sq m (John Atwood, Manomet Observatory, personal communication). In the Northeast terns often nest on beaches also used by piping plovers (Jones and Camuso 1994).

Least terns forage over flats and in shallow nearshore waters on an array of invertebrates and slim-bodied fish within 15 cm of the water's surface (Carreker 1985, Thompson et al. 1997). Foraging distances for breeding colonies of least terns in California were "90-95% within 1 mile of shore in water less than 60 feet in depth"; typical foraging habitat is within 2 miles of colony sites in "relatively shallow nearshore ocean waters in the vicinity of major river mouths..." (Atwood and Minsky 1983). Jones and Camuso (1994) observed that 93 of 468 feeding episodes occurred in the marsh behind a nesting area versus 375 over the ocean in front of it. Birds feeding in the marsh tended to stay relatively near the nest area, but range more widely when feeding over the ocean.

Special Requirements:
Least terns are tolerant of urban sites; they sometimes breed on rooftops or dredge spoil sites (Thompson et al. 1997). Least terns nest colonially, and protection of their nests from predators has been relatively difficult (Jones 1993); most nest losses in Maine have been attributed to tidal flooding and fox predation (Jones and Camuso 1994). Though somewhat tolerant of human activities, they are vulnerable to disturbance and accidental impacts to nests and chicks (Carreker 1985). Abandonment or localized shifts in colony site occur in response to flooding, changes in colony size, increased vegetative cover, human activities or predation (Kotliar and Burger 1986, Atwood and Massey 1988). In feeding, marinas are avoided. Prey accessibility is limited in turbid or choppy waters, and areas heavily used by boats and jet skis (Thompson et al. 1997).

Model:
Least tern habitat was mapped using data on known nesting locations and suitability of cover for nesting and foraging. Occurrence data was obtained from the Massachusetts and New Hampshire Natural Heritage Programs, surveys from Massachusetts Division of Fisheries and Wildlife, and data from Maine least tern and piping plover surveys (Jones et al. 1999). Beach areas within 1/4 mi of the most recently observed (typically 1999) least tern nests and having suitable cover (see table, below) were scored 1.0; areas within 1/4 mi of both earlier tern nests (1985-1999 surveys) and near active piping plover nests and which had suitable cover were scored 0.5. Areas within 1/4 mi either of earlier tern nests or of active piping plover nests and having suitable cover were scored 0.4.  Other beach areas not documented as nest sites, but with adequate expanses of apparently suitable cover were scored 0.2. Mapping of potential nesting habitat was done only within the Ecological Regions (USDA Forest Service Ecological subunits, Keys et al. 1995) hosting recent nest sites.

Foraging habitat was mapped as suitable estuarine or marine cover types within 1 mile of recent least tern nest sites, that was mid-intertidal to 60 feet deep. Such areas were much larger than the nesting areas and therefore scored 0.3, both because of their relative abundance, lack of specificity in mapping, and to distinguish feeding and nesting areas in the single (combined) output grid.
NWI Designations
(wetlands only)
Cover Type Cover Suitability
(0 - 1 scale)
Upland deciduous forest
Upland coniferous forest
Upland mixed forest
Grassland 1.0*
Upland scrub/shrub
Cultivated
Developed
Bare ground 1.0*
PEM, L2EM Lake/pond, emergent vegetation
PFOcon Palustrine forest, conifer
PFOdec Palustrine forest, deciduous
PSSdec Palustrine scrub shrub, deciduous
PSScon Palustrine scrub shrub, conifer
PAB, L2AB Lake/pond, aquatic vegetation
L1UB, PUB Lake/pond, unconsolidated bottom
L2US Lake, unconsolidated shore
L2RS Lake, rocky shore
R1UB Riverine subtidal unconsolidated 0.3**
Rper Riverine perennial
E1AB Estuarine subtidal vegetated 0.3**
E1UB Estuarine subtidal unconsolidated bottom 0.3**
E2AB Estuarine intertidal algae 0.3**
E2EM Estuarine intertidal emergent
E2RS, R1RS Estuarine, tidal river rocky shore 0.3**
E2SS Estuarine intertidal shrub
E2US, R1US Estuarine, riverine intertidal unconsolidated shore 1.0*, 0.3**
M1AB Marine subtidal vegetated 0.3**
M1UB Marine subtidal unconsolidated bottom 0.3**
M2AB Marine intertidal algae 0.3**
M2RS Marine intertidal rocky shore 0.3**
M2US Marine intertidal unconsolidated shore 1.0*, 0.3**
NOTES *Nesting: dunes, vegetated beach ridge, edge of upper tidal flats
**Foraging: if within 1 mi of known nesting area; mid-intertidal to 60' deep

Sources:
Atwood, J.L. and D.E. Minsky. 1983. Least tern foraging ecology at three major California breeding colonies. Western Birds 14:57-72.

Atwood, J.L. and B.W. Massey. 1988. Site fidelity of least terns in California. The Condor 90:389-394.

Blodget, B.G. 1978. The effect of off-road vehicles on least terns and other shorebirds. M.S. Thesis, University of Massachusetts, Amherst, MA. 79 pp.

Carreker, R.G. 1985. Habitat suitability index models: least tern. FWS/OBS- Biological Report 82(10.103). 29 p.

Gochfeld, M. 1983. Colony site selection by least terns: physical attributes of sites. Colonial Waterbirds 6:205-213.

Hunter, M.L., Jr. 1975. Least tern breeding range extension in Maine. Auk 92(1):143-144.

Jernigan, L.R. , J. Parnell, and T. Quay. 1978. Nesting habitats and breeding population of the least tern (Sterna albifrons antillarum) in North Carolina. National Oceanic Atmospheric Administration, Sea Grant Publication UNC-SG-78-07. 39 pp.

Jones, J. 1993. Recommendations for managing least terns and piping plovers in Maine. Maine Audubon Society, Falmouth, ME. 14 p.

Jones, J.J., G. Giumarro, and K. Williamson. 1999. 1998 Piping Plover and Least Tern Project Report. Maine Audubon Society, Falmouth, ME. 19 p.

Jones, J.J. and J. Camuso. 1994. 1994 Piping Plover and Least Tern Project Report. Maine Audubon Society, Falmouth, ME. 40 p.

Keys, J.E., Jr., J.C. Carpenter, S. Hooks, F. Koenig, W.H. McNab, W. Russell, and W. Smith. 1995. Ecological units of the eastern United States - first approximation (map and booklet of map unit tables), Atlanta, GA: U.S.

Kotliar, N.B. and J. Burger. 1986. Colony site selection and abandonment by least terns Sterna antillarum in New Jersey, USA. Biological Conservation 37:1-21.

Thompson, B.C., J.A. Jackson, J. Burger, L.A. Hill, E.M. Kirsch, and J.L. Atwood. 1997. Least tern. The Birds of North America, 290.