|SOUTHEASTERN CARIBBEAN BIRDS
|Trinidad and Tobago Field Naturalists' Club Southeastern Caribbean Bird Alert Trinidad and Tobago Rare Bird Committee
|Least Tern (Sterna antillarum) and Yellow-billed Tern (S. superciliaris): Page 1 of 2|
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.
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.
Perched (page 1)
Flight (page 2)
|Fig. 1. Bill coloration and head patterns in Least Tern and Yellow-billed Tern. Note the larger bill size of Yellow-billed Tern and marked differences in bill coloration between the species in non-breeding and immature plumages. Illustration from Hayes (2001), © by the Neotropical Bird Club.|
|Fig. 3. Immature Yellow-billed Tern at Trincity, Trinidad, 4 May 2002. The dark carpal bar on the wing is indicative of immaturity. Note the large bill with a dark tip and a dark patch around the nostrils; an immature Least Tern would have an entirely dark bill. The white crown contrasts strongly with the thick black stripe extending from the eyes across the crown. Photo © by Floyd Hayes.|
|Fig. 4. Yellow-billed Terns at Trincity, Trinidad, 4 May 2002. The upper two birds appear to be in 1st alternate plumage with near-adult head pattern and entirely yellow bills, but retaining traces of a carpal bar. Note the dark tip and nostril area of the bill in the lower immature. Photo © by Floyd Hayes.|
|Fig. 5. Yellow-billed Terns at Trincity, Trinidad, 4 May 2002. These birds appear to be in 1st alternate plumage with adult head pattern and bill color but retaining traces of a carpal bar. Note the unusually downcurved bill of the bird on the right. Photo © by Floyd Hayes.|
|Fig. 6. Yellow-billed Tern at Trincity, Trinidad, 4 May 2002. This bird appears to be in 1st alternate plumage with adult head pattern and bill color but with traces of a carpal bar. Photo © by Floyd Hayes.|
|Fig. 8. Yellow-billed Tern at Trincity, Trinidad, 22 September 2001. This bird appears to be in an adult (carpal bar lacking) moulting into basic plumage. The bill has already acquired a dark tip and nostril area. Photo © by Floyd Hayes.|
|Fig. 2. Juvenile Least Tern along central Texas coast, late June 1999. Note the heavy dark markings on the back and wings that are typical of juvenile terns. Note also the short, black bill and orangeish legs. The distinctions between Least Tern and Yellow-billed Tern in this plumage have not been investigated. Photo © by Martin Reid.|
|Fig. 7. Alternate adult Least Tern along central Texas coast, late June 1999. In this photo the bill does not appear proportionately smaller than that of Yellow-billed Tern. The subterminal black spot on the bill is unusual. Note the more extensive black "arrow" between the bill and the eye extending lower on the face and connecting lower on the bill than in Yellow-billed TernNote the Photo © by Martin Reid.|