The North American race of the Sandwich Tern breeds along the Atlantic and Gulf of Mexico coasts of North America, and on islands in the northern Caribbean Sea. It winters primarily in the Caribbean and along the coasts of South America, where it strays as far south as Argentina. The similar �Cayenne� Tern breeds on islands in the southern Caribbean Sea and along the Atlantic coast of South America, and has been recorded several times along the Atlantic coast of North America and once along the Pacific coast of Colombia. A few Cayenne Terns nest among larger colonies of Sandwich Tern in Puerto Rico and the Virgin Islands, and a few Sandwich-type Terns nest among larger colonies of Cayenne Tern in the Dutch islands of the southern Caribbean and in eastern Brazil. The increased variability of bill colouration in the Caribbean implies that hybridisation occurs where there ranges meet. The differences between the two taxa are discussed in this photo essay. An article describing variation and hybridisation in the Virgin Islands is being prepared by Floyd Hayes. Bill colouration.�The chief distinction between the two taxa is bill colouration. Sandwich Tern nearly always has a black bill tipped with yellow. Cayenne Tern usually has a yellowish bill that is rarely tinged with orange or red; many, however, have dark blotches on the bill, which may indicate introgression from Sandwich Tern or simply genetic variability within the taxon. Some Cayenne Terns have a mostly black bill tipped with yellow; such birds closely resemble Sandwich Tern but the yellow tip is usually more extensive. Cayenne Terns with dark bill markings occur throughout their breeding range, but appear to be most frequent in the southern Caribbean. A crucial, unresolved question is whether individuals with phenotypically 'intermediate' bill colouration (black with yellow/orange blotches or yellow/orange with black blotches) represent: (1) variant phenotypes of genotypically 'pure' Cayenne Tern; (2) hybrids between the two taxa; or (3) a mixture of both phenomena. Evidence for non-assortative mating in the Virgin Islands (click here) suggests that hybridisation may be the best explanation. Bill length and body size.�Sandwich Tern and northern populations of the Cayenne Tern are fairly similar in bill length and bill size. However, Cayenne Tern populations from southern South America have notably larger bills and body sizes. Plumage.�Cayenne Tern tends to have a darker mantle than Sandwich Tern but considerable overlap occurs. Breeding individuals of both races have a slight pinkish flush on the underparts. Caribbean and central South American populations of Cayenne Tern have a Northern Hemisphere moult cycle similar to that of Sandwich Tern. Southern South American populations of Cayenne Tern breed during the austral winter and have a Southern Hemisphere moult cycle. |