Black Tern Habitat Model
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Draft Date:
February 20, 2001

Species:
Black tern, Chlidonias niger.  North American subspecies: C. n. surinamensis (Dunn and Agro 1995). Also known as short-tailed or semipalmated tern (Dorr 1976).

Use of Study Area Resources:
Reproduction in Maine, not recorded in New Hampshire or Massachusetts. Black terns are widespread in North America (central and southern Canada, California to New York, and extend into small areas of Maine and New Brunswick; Novak 1992). They winter from Panama to Peru. Black terns are uncommon in the Northeast; there are perhaps 100 pair in the study area (Dorr 1976). Black terns utilize marine habitats during migration and wintering.

Habitat Requirements (reproduction):
Cover. Black terns feed and nest in large shallow freshwater emergent wetlands, margins of lakes, some river edges, and semi-permanent ponds (Dunn and Agro 1995). The nest location has about 25 to 75% vegetation to open water (Dunn and Agro 1995, Novak 1992). Black terns build nests on a floating substrate of matted vegetation, often cattail or bullrush, but nesting occasionally occurs in flooded willows or heath ( Peck and James 1983 in Dunn and Agro 1995).  However, of 13 black tern nest sites in the study area (Maine BCD data, see below) only 6 were in or near areas mapped as palustrine emergent , while 7 were mapped as palustrine scrub shrub. More than vegetation type, "..interspersion of emergent vegetation and open water appears critical (Novak 1992).  Nests are typically flimsy, only 2-6 cm above the surface of the water, and shallow in depth (Dunn and Agro 1995). They are usually adjacent to or within 2 m of open water, and consequently are easily overwashed by waves, resulting in chilling or loss of eggs and young (Dunn and Agro 1995). Black terns exhibit semi-colonial nesting behavior (Dunn and Agro 1995). Woody debris such as posts, snags or floating logs are an important component of nesting habitat for perching, breeding and feeding young (Novak 1992).

Area. Wetlands or complexes > 20 ha are preferred by black terns (Novak 1992); the minimum extent of used wetlands is about 5 ha (Dunn and Agro 1995), but this may not take into account adjacent open-structured uplands. Brown and Dinsmore (1986 in Novak 1992) found terns in Iowa in marshes "in the 5-10.9 ha size class only when these smaller marshes were part of a larger wetland complex". Naugle (1997 in Shuford 1999) determined that the minimum area of semi-permanent wetland used in the Dakotas was 12.4 ha. Marshes used in Maine were > 25 ha. (Gibbs and Melvin 1990 in Shuford 1999).  We examined the size of marsh vegetation complexes at 13 nesting sites of black terns in Maine (see "BCD", below).  All were associated with lakes or ponds having at least some open water. The area of suitable vegetation associated with the nest site ranged from 14.4 to 583 ha, with a median of 89 ha.  Our measurements were based on National Wetland Inventory delineations, which may underestimate aquatic vegetation because of the limitations of the aerial photography or season during which it was taken (A. Gilbert, Univ. of Maine, pers. comm.).

Foraging. Black terns feed on a variety of aquatic insects, particularly dragonflies, damselflies, mayflies, and caddisflies, as well as small fishes and crustaceans (Novak 1992, Dunn and Agro 1995). They have been known to forage on flooded agricultural lands (Novak 1992) or to follow behind farmers plowing, taking grubs and larvae (Forbush 1939 in Dorr 1976).

Disturbance and Limiting Factors. Nests and young are readily lost to strong winds, rising water levels, wakes from boats, or even to active foraging by waterfowl around a nest (Dunn and Agro 1995). Drought conditions can expose nests to mammalian predation by raccoons, mink, and rats, while avian predators include raptors, bitterns, gulls, crows, and blackbirds. Snapping turtles may also take young (Dunn and Agro 1995). Survival of more than one chick from a clutch of two or three eggs is uncommon (Goodwin 1960, Stern 1987, both in Dunn and Agro 1995).

Model:
Our mapping relied on data from Maine's "BCD" heritage database for known colony/nesting locations. The Breeding Bird Atlas data of Maine, New Hampshire and Massachusetts (courtesy of the University of Vermont COOP Unit) disclosed possible nest localities in New Hampshire. The black tern range within the study area was delineated by selecting the USDA Forest Service Ecological subunits (Keys et al. 1995) in which the terns were known to nest, and extending this west to include the New Hampshire BBA blocks. Habitat mapping was restricted to these areas.

Certain Maine wetlands were scored highest (1.0) because black terns show strong fidelity to these areas. Other than these, habitats were mapped according to cover type (table, below), and area of wetland complex. We scored aquatic bed wetlands lowest because they offer marginal structure for nesting; scrub/shrub wetlands were scored somewhat lower than emergent marsh because of the low frequency of its use (based on the literature), and to allow them to be distinguished on maps. Wetlands of suitable composition, associated with lakes or ponds, having an area of 20 ha or more and within the used Ecological subunits, were scored as shown in the table.
NWI Designations
(wetlands only)
Cover Types Cover Suitability
(0 - 1 scale)
Upland deciduous forest
Upland coniferous forest
Upland mixed forest
Grassland
Upland scrub/shrub
Cultivated
Developed
Bare ground
PEM, L2EM Lake/pond, emergent vegetation

0.5

PFOcon Palustrine forest, conifer
PFOdec Palustrine forest, deciduous
PSSdec Palustrine scrub shrub, deciduous

0.4

PSScon Palustrine scrub shrub, conifer
PAB, L2AB Lake/pond, aquatic vegetation

0.3*

L1UB, PUB Lake/pond, unconsolidated bottom

0*

L2US Lake, unconsolidated shore

0*

L2RS Lake, rocky shore
R1UB Riverine subtidal unconsolidated
Rper Riverine perennial
E1AB Estuarine subtidal vegetated
E1UB Estuarine subtidal unconsolidated bottom
E2AB Estuarine intertidal algae
E2EM Estuarine intertidal emergent
E2RS, R1RS Estuarine, tidal river rocky shore
E2SS Estuarine intertidal shrub
E2US Estuarine intertidal unconsolidated shore
M1AB Marine subtidal vegetated
M1UB Marine subtidal unconsolidated bottom
M2AB Marine intertidal algae
M2RS Marine intertidal rocky shore
M2US Marine intertidal unconsolidated shore
other Marsh/shrub swamp sites known to be used

1.0

NOTES *Water types required to be adjacent to suitable vegetation

Sources:
Dorr, D.K. 1976. Black Tern (Chlidonias Niger) Nesting Habitat in Maine and its Relevancy to the Critical Areas Program. Maine State Planning Office Planning Rept. 18. Augusta, ME. 14 pp.

Dunn, E.H. and D.J. Agro. 1995. Black Tern (Chlidonias Niger). In A. Poole and F. Gill (eds.) The Birds of North America, 147. Acad. Nat. Sci. Phil. and Amer. Ornith. Union, Washington D.C.

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), USDA Forest Service. Atlanta, GA.

Novak, P.G. 1992. Black Tern (Chlidonias niger). Pages 149-169 in K.J. Schneider and D.M. Pence (eds.) Migratory Non-game Birds of Management Concern in the Northeast. USDI Fish and Wildlife Service, Region 5, Newton Corner, MA. 400 pp.

Shuford, W.D. 1999. Status Assessment and Conservation Plan for the Black Tern in North America. U.S. Fish and Wildlife Service. 78 pp.