Snowy-crowned Tern Sterna trudeaui Scientific name definitions

Snowy-crowned Tern is a striking species of South American near-coast marshes and inshore marine waters.  Typically, it is found from Rio de Janeiro south to central Argentina along the Atlantic Coast, and in central Chile along the Pacific Coast as well. Incredibly, and perhaps incorrectly, the type specimen of Snowy-crowned Tern was reported by Audubon as being from Cape May, New Jersey, USA (though there are no widely accepted North American records). Though it behaves generally like other Sterna terns, this species is unusual in structure, having a rather large, blocky head and a thick neck.  The Snowy-crowned Tern is pale gray overall with a small black eye mask instead of the typical Sterna cap, an orange bill with a black band in the breeding season (black with pale tip in non-breeding), long, pearly white wings, and orange legs.

35 cm; 146–160 g; wingspan 76–78 cm. Head  and upper neck white, with black spot in front of eye and dark band behind; upperparts and upperwings  pale grey, with whitish rump and uppertail-coverts; underparts  greyish white; iris dark brown; bill  orange, with broad subterminal black band and yellow tip; feet  reddish orange. Like non-breeding S. forsteri, but latter has all black bill and dusky nape. Non-breeding adult similar, but with stripe on side of head greyer and bill black with yellowish tip. Juvenile  has back and scapulars patterned with dark and white markings, dark carpal bar, tail feathers with dark subterminal mark, bill black, and legs dark with yellowish webs. Immature creamy white.

Coast and interior of South America, breeding from SE Brazil and Uruguay to EC Argentina (S Buenos Aires, rarely to Santa Cruz), and in Chile (Aconcagua to Llanquihue).

Fresh and saline wetlands, both inland and coastal . Breeds on vegetated lagoons of pampas and Patagonia, mainly in marshes, but also on dykes and islands in saline lagoons. Feeds at wetland edges, and also over fields.

Outside breeding season, ranges north on the Pacific coast, rarely as far as southern S Peru, where reported north to Ica (2 Schulenberg, T. S., D. F. Stotz, D. F. Lane, J. P. O’Neill, and T. A. Parker (2010). Birds of Peru. Revised edition. Princeton University Press, Princeton, New Jersey, USA. Close ), and on the Atlantic coast to Rio de Janeiro region. Some linger as far south as Chubut into July. Accidental in the Straits of Magellan. Considered abundant on the coast of Uruguay and at mouth of the Río de la Plata in Mar; numerous on the Buenos Aires pampas in September–February, but less common there during the austral winter. It is most abundant on the coast of Rio Grande do Sul, Brazil, in autumn and winter (3 de Souza Petersen, E. and Petry, M.V. (2011). Riqueza e abundância sazonal de Laridae e Sternidae no litoral médio do Rio Grande do Sul, Brasil. Revista Brasileira de Ornitologia. 19(4): 493–497. Close ).

Small fish and insects. In Chile, feeds mainly on pejerrey (Austromenidia laticlavia). Of 84 fish prey observed during one study of foraging at the Mar Chiquta lagoon, Argentina, 24% were identified as silversides (Odontesthes sp.), 5% as flatfishes (Paralichthys sp.), while the rest remained undetermined. Most of the fish prey (55.7%) were smaller than 35 mm, 25.7% were 35–50 mm in length, and the remaining 18.6% exceeded 50 mm (4 García, G.O. and Mariano-Jelicich, R. (2005). Foraging behavior of the Snowy-crowned Tern (Sterna trudeaui) at Mar Chiquita, Buenos Aires Province, Argentina. Ornitología Neotropical. 16: 563–566. Close ). Forages over shallow clear water on edges of lagoons, rivers and estuaries, but also over ploughed fields. Plunge-dives for fish.

The call is a series of rapid notes "je-je-je-je", or a short, grating "jeeer" (2 Schulenberg, T. S., D. F. Stotz, D. F. Lane, J. P. O’Neill, and T. A. Parker (2010). Birds of Peru. Revised edition. Princeton University Press, Princeton, New Jersey, USA. Close ).

Virtually no data published. Recorded Oct in N, Nov in Buenos Aires, Dec in Chubut. One colony in a lagoon held 30–40 nests. Often nests with Brown-hooded Gulls (Larus maculipennis), in densely vegetated marshes. Builds floating platform of plant stems, free or anchored to emergent vegetation, in water 10–150 cm deep. 3 eggs  (2–4); adults vigorously attack and even strike intruders. Productivity not studied, but one report of 55 adults accompanied by 37 juveniles  in Dec.

Not globally threatened (Least Concern). The global population is not known with any precision but is tentatively put within the range of up to 10,000 birds in the southwest (Pacific) region and 25,000–100,000 birds in the southeast (Atlantic) region (5 Wetlands International (2017). Waterbird Population Estimates. Close ); its status is unclear but the southwestern population at least is regarded as stable. It nests north to SE Brazil on the Atlantic, breeding in Rio Grande do Sul and is common in coastal lagoons. Does not nest on small lakes, so maintenance of largest freshwater wetlands essential. Loss of wetland habitat and lack of protection of existing wetlands are potential major problems. In Chile, uncommon, nesting from Aconcagua to Llanquihue.