The Black Tern differs from other North American terns in several respects. It is small, has a black head and underparts in breeding plumage, eats insects as well as fish, and nests in freshwater habitats. This bird is described well by Bent ( Bent 1921 : 297) as “a restless waif of the air, flitting about hither and thither with a wayward, desultory flight, light and buoyant as a butterfly. Its darting zigzag flight as it mounts into the air to chase a fluttering moth is suggestive of a flycatcher or a nighthawk; as it skims swiftly over the surface of the water it reminds me of a swallow; and its true relationship to the terns is shown as it hovers along over the billowing tops of a great sea of tall waving grass, dipping down occasionally to snatch an insect from the slender, swaying tops.”
The Black Tern nests semicolonially amidst emergent vegetation in biologically rich fresh-water wetlands. Nests are flimsy, often floating, and are easily destroyed by wind or changing water levels. Reproductive success varies greatly. Adaptations to marsh nesting include frequent renesting, low site tenacity, and eggshell morphology suited to damp conditions.
This highly social species often forages in flocks. It migrates mostly through the inland United States, then shifts primarily to marine habitats in winter. There it favors productive marine waters, especially off the Pacific Coast of Panama, and often concentrates where predatory fish have driven small prey to the surface.
Populations of this tern in North America and Europe have declined markedly, at least since the 1960s. In North America, Breeding Bird Survey (BBS) data show an average annual decline of 3.1% over the period 1966 - 1996, with the steepest declines evident prior to 1980. Declines were largest in the prairie provinces of Canada. Loss of wetlands on breeding grounds and migration routes is likely a major cause of these declines, but food supplies may have been reduced through agricultural control of insects and overfishing in the marine winter range.
Recent studies have focused on identifying broad scale habitat suitability through the use of remote sensing, modeling, and landscape level land cover ( Naugle et al. 1999b , Naugle et al. 1999a , Naugle et al. 2000b , Naugle et al. 2000a , Naugle 2004 ), as well as analyses of nest site selection within wetlands ( Hickey 1997 , Hickey and Malecki 1997 , Mazzocchi et al. 1997 ). Analyses investigating habitat and conspecific variables that influence nest success ( Maxson et al. 2007 , Heath 2004 ) have elucidated factors influencing predation. In Maine, factors influencing breeding productivity -- including water level increases, food resources, and predation -- have also been investigated ( Gilbert 2001a , Heath 2004 , Gilbert and Servello 2005a , Gilbert and Servello 2005b , Heath and Servello 2008 ). Research on the effects of nesting platforms and investigator disturbance on breeding success ( Shealer and Haverland 2000 , Shealer et al. 2006 ) have provided insight into management and research alternatives.
Given the recent declining trends in populations of this species in North America, identifying critical wetland complexes for conservation and filling gaps in our knowledge of breeding ecology and wintering ecology remain important priorities across the its range.
This account deals almost entirely with the North America subspecies of Black Tern (C. n. surinamensis). For details on this species in Europe (C. n. niger), see Cramp ( Cramp 1985c ).