Yellow-billed Tern Sternula superciliaris Scientific name definitions

The Yellow-billed Tern is widespread and fairly common along major river systems and in lakes east of the Andes, ranging from Colombia and Venezuela, across much of Brazil and south into Argentina. It is found singly or in pairs along river sandbanks (up to 500m), often with Black Skimmers and Large-billed Terns. During the non-breeding season, it can also be found in coastal lagoons, river mouths, and rice fields. This small tern is mostly silvery gray above and white below, with a black crown, nape and eyeline, surrounding a contrasting white forehead. It is distinguished from the Least Tern (which is more often coastal) by its heavier bill, which lacks a black tip, a shorter and less forked tail, and a broader black wedge on the upperside of the wing, formed by the outermost primaries in flight (4-5 black feathers as opposed to 2 in Leasts). Non-breeding adults have whitish crowns and lores, with black markings around the nape and auriculars. Juveniles are brown and white scalloped above and lack a solid black cap. It has a light, rapid flight and hovers frequently before diving for fish.

23 cm; 40–57 g. Very similar to the Least Tern <em>S. antillarum</em> , but slightly larger and longer-legged, with a stouter, entirely yellow  bill  , lacking the dark tip. The legs are bright yellow in breeding birds and become pale-yellow to greenish, never black, when not breeding (1 Renaudiert, A. and Claessens, O. (2014). Field identification of Least and Yellow-billed Terns: experience from French Guiana. Neotropical Birding. 15: 22–31. Close ). Subadult has a yellowish bill with a dark horn-brown tip and dark base to the upper mandible (2 Hayes, F.E. (2001). Identification of Least Tern Sterna antillarum and Yellow-billed Tern S. superciliaris, with a sight record of Yellow-billed Tern from Tobago, West Indies. Cotinga. 15: 10–13. Close ), and greyish crown with black streaks, whereas subadult S. antillarum usually has a wholly black bill and conspicuous blackish carpal band. Juvenile has ochraceous forehead, lores and crown  , with dark streaking and dark auricular area; upperparts  greyish, with pale buffy markings, and with dark subterminal marks on each feather; dark carpal bar.

E Colombia (E of Andes) S to NE Peru, and E to the Guianas, E Brazil and CE Argentina (Buenos Aires).

Broad rivers . Breeds mainly on riverine sandbars and lake beaches, in mixed colonies with Black Skimmers Rynchops niger and Large-billed Terns Phaetusa simplex. In the Anavilhanas Archipelago, Amazonas, Brazil, all three species generally nest on larger beaches on islands that have little vegetative cover and that are exposed to open water, remote from river margins, distant from other islands and closer to large colonies of the same species (3 Zarza, R., Cintra, R. and Anciães, M. (2013). Distribution, abundance and habitat selection by breeding Yellow-billed Terns (Sternula superciliaris), Large-billed Terns (Phaetusa simplex) and Black Skimmers (Rynchops niger) in the Brazilian Amazon. Waterbirds. 36(4): 470–481. Close ). More restricted to large rivers than Phaetusa. Feeds mainly over shallow shoals on large rivers, often concentrating at tributary mouths. In Brazil, breeds mainly along inland rivers and lakes, sparingly at river mouths on coast; in Argentina along R Uruguay, R Paraná and R Pilcomayo to NE Buenos Aires; in Uruguay commonly on lakes and rivers, uncommonly along coast. Non-breeders occur mainly on coast and estuaries, also at various water bodies , from rice fields to brackish lagoons. In French Guiana non-breeders regularly fish at estuaries, brackish lagoons and freshwater ponds and lakes, as well as along coasts, unlike the Least Tern S. antillarum, which is strictly coastal in its winter quarters there (1 Renaudiert, A. and Claessens, O. (2014). Field identification of Least and Yellow-billed Terns: experience from French Guiana. Neotropical Birding. 15: 22–31. Close ). 

The principal known main movement is apparently along river valleys to coasts, where non-breeders congregate  . It is a regular non-breeding visitor to Trinidad but there is only one claimed record from Tobago, as well as two from Bonaire, Venezuela (2 Hayes, F.E. (2001). Identification of Least Tern Sterna antillarum and Yellow-billed Tern S. superciliaris, with a sight record of Yellow-billed Tern from Tobago, West Indies. Cotinga. 15: 10–13. Close ). Non-breeders are observed year-round in French Guiana, with an increase in observations in March–April and from late August to early October, corresponding with passage periods (1 Renaudiert, A. and Claessens, O. (2014). Field identification of Least and Yellow-billed Terns: experience from French Guiana. Neotropical Birding. 15: 22–31. Close ).

Records from the Pacific coast are highly exceptional; they comprise two records from Panama and, most recently, one of a bird near Lima, Peru, in January 2015 (4 Zöckler, C., 1, Chávez-Villavicencio, C., Tabilo-Valdivieso, E. and Burmeister, J. (2016). Registro de Gaviotín de Pico Amarillo (Sternula superciliaris) en el Pacífico sudamericano. Boletín UNOP. 11(1): 6–9. Close ).

Small fish  (7–45 mm), shrimps and insects; sometimes predominantly insects (Peru). Often feeds by hovering and plunge-diving from 3–8 m into shallow water, usually close to shore. Usually feeds alone. In French Guiana it is said to feed mainly by dipping, seldom diving (1 Renaudiert, A. and Claessens, O. (2014). Field identification of Least and Yellow-billed Terns: experience from French Guiana. Neotropical Birding. 15: 22–31. Close ).

Calls include a short nasal “kuk” and a slightly longer raspy “rreh”, typically repeated in loose series. During interactions, also a series of more frantic-sounding, trisyllabic notes, “chee-chee-chew”.

Virtually unstudied. November in Surinam, December in Uruguay, July in Peru; timing dependent on exposure of riverine sandbars in dry season. Colonies of up to 20 pairs, more often 3–10; nests widely spaced, often more than 50 m apart, frequently solitary. 2–3 eggs  , average 1·94. Productivity 1·04/nest; flooding is main cause of egg loss.

Not globally threatened (Least Concern). No reliable population figures available but the global population may be within the range of 25,000–100,000 individuals (5 Wetlands International (2015). Waterbird Population Estimates. Wetlands International, Wageningen, The Netherlands. Close ). Status and trends virtually unknown. Main threats are flooding and human disturbance, latter significant on some beaches.