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.
Resumen.--El Gaviot�n Chico Boreal Sterna antillarum nidifica en Norteam�rica y en el Caribe, y pasa el invierno en las costas de Sudam�rica. El Gaviot�n Chico Com�n Sterna superciliaris, de Sudam�rica, se encuentra principalmente en h�bitats de agua dulce, pero a veces en las costas, y hay registros de la isla de Bonaire, a 87 km del continente. El tama�o de Sterna superciliaris es un poco mayor y su pico es proporcionalmente m�s grande que el de S. antillarum, pero estas caracter�sticas son �tiles solamente cuando ambas especies est�n juntas. El color del pico es el criterio m�s seguro para distinguir las dos especies. En plumage nupcial, ambas especies tienen un pico amarillo, pero S. antillarum casi siempre tiene un �pice oscuro. En los plumages de juvenil, inmaduro y reposo sexual, S. antillarum siempre tiene un pico oscuro y S. superciliaris tiene un pico mayormente amarillo, con el �pice y base de la maxila negros; sin embargo, el color del pico de S. antillarum durante la transici�n de plumage nupcial al plumage reposo sexual es semejante al de S. superciliaris. Se registra una observaci�n de S. superciliaris en la isla de Tobago, 118 km del continente. Los registros extralimitales de S. superciliaris son m�s probables durante la �poca de inundaciones de los grandes r�os. Se anticipa encontrar S. antillarum en el interior de Sudam�rica.
Abstract [unpublished English translation].--The Least Tern Sterna antillarum breeds in North America and in the Caribbean, and winters along the coasts of South America. The Yellow-billed Tern Sterna superciliaris, of South America, is found primarily in freshwater habitats, but at times along coasts, and there are records from the island of Bonaire, 87 km from the continent. Body size of Sterna superciliaris averages larger and bill size is proportionately larger than that of S. antillarum, but these characteristics are useful only when both species are together. BIll color is the safest criterion for distinguishing the two species. In breeding plumage, both species have a yellow bill, but S. antillarum almost always has a dark tip. In juvenile, immature and breeding plumages, S. antillarum always has a dark bill and S. superciliaris has a bill that is mostly yellow, with a dark tip and base to the upper bill; however, bill color of S. antillarum during the transition from breeding to non-breeding plumage is similar to that of S. superciliaris. I report an observation of S. superciliaris on the island of Tobago, 118 km from the continent. Extralimital records of S. superciliaris are most likely during periods when the large rivers flood. S. antillarum may be expected to occur in the interior of South America.. |
