The Yellow-billed Tern nests primarily along large rivers in South America and frequently wanders to the coast, especially during the non-breeding season. It closely resembles the Least Tern, which breeds along coasts and larger rivers of South America and on many Caribbean islands, including those off northern South America, and winters along the coasts of South America. The differences between these two species were discussed by Hayes (2001) and are summarized in this photo essay. Immature plumage.--Immatures of both species are readily distinguished from adults by the presence of a dark carpal bar on the lesser wing coverts, which is markedly darker in the Least Tern and typically faint in the Yellow-billed Tern. The crown is white, often with dark flecking, and a thick, dark stripe extends from the eyes across the nape (Fig. 1). Basic (non-breeding) plumage.--Individuals in this plumage resemble immatures but lack the dark carpal bar. The outer primaries become worn and more extensively dark than in alternate (breeding) plumage. There appear to be be no consistent differences in plumage between the two species at this stage. Alternate (breeding) plumage.--Individuals in this plumage possess a dark crown and nape with a V-shaped, white forehead patch extending just behind the eye in the form of a narrow supercilium (Fig. 1). In freshly moulted individuals the supercilium is slightly narrower in the Yellow-billed Tern, though given the variability in the stage of moult this feature is seldom useful for identification. Although not mentioned in the article by Hayes (2001), the black "arrow" between the bill and eye appears to connect lower on the bill and extend lower on the face in Least Tern. In Least Tern, the outermost two (rarely three) primaries are dark compared with three (rarely two to four) in Yellow-billed Tern (contra several references which state that the outermost four primaries in Yellow-billed Tern). Thus, the Yellow-billed has a more extensive black wedge on the wingtip. Body size and shape.--The Yellow-billed Tern is slightly larger than Least Tern, with a 9% difference in mean wing length and only slight overlap. In freshly moulted individuals the tail is proportionately longer and more forked in Least Tern, but because of variation in moult and wear this feature is seldom useful for identification. Bill size.--Yellow-billed Tern has a proportionately heavier bill that averages 8% longer than that of Least Tern (Fig. 1). This feature is readily apparent in most individuals, but slight overlap occurs between long-billed male Least Terns and short-billed female Yellow-billed Terns. Bill colouration.--In alternate plumage, Yellow-billed Tern has an entirely yellow bill; the Least Tern usually has a dark tip to the bill (Fig. 1), but in some individuals it is entirely yellow. The differences between the two species are most pronounced in basic plumage, when Least Tern has a dark bill and Yellow-billed Tern has a mostly yellow bill with a dusky tip and a dusky patch around the nostrils (contra several references which state the bill is entirely yellow. These differences are illustrated below. Leg colouration.--Although not mentioned in the article by Hayes (2001), the legs of Yellow-billed Tern may be more consistently yellowish in all plumages than in Least Tern, in which they are usually more orangeish or darker.
LITERATURE CITED
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.
PHOTO INDEX Perched (page 1) Flight (page 2) |