Trinidad and Tobago Field Naturalists' Club         Southeastern Caribbean Bird Alert         Trinidad and Tobago Rare Bird Committee
Sandwich Tern (Sterna sandvicensis acuflavida) and Cayenne Tern (S. s. eurygnatha): Page 2 of 7
Page 1: Virgin Islands (variation)
     Page 2: Virgin Islands (hybridisation)
Page 3: Aruba
Page 4: Tobago
Page 5: Brazil
Page 6: Argentina
Page 7: Argentina
Fig. 3. Adult Sandwich Tern (left) and Cayenne Tern (right) with hybrid chick at Dog Island, 22 June 2003. Photo © by Floyd Hayes. This trio spent at least 15 min walking together through the sparsely populated fringes of the colony for about 20 metres, providing conclusive evidence of being paired and the first documented instance of hybridization in the northern Caribbean between extreme phenotypes of Sandwich / Cayenne Tern. Bill coloration of chicks in the colony was variable, either yellow or grey.
Fig. 4. Incubating Cayenne Terns with pure yellow bill (above left; photo A) and 95% orange/yellow bill (right; photos A-B) about 0.5 m apart among Sandwich Terns at Dog Island, 26 June 2003. Photos © by Floyd Hayes. The Cayenne Tern at right relieved an incubating Sandwich Tern presumed to be its mate. Note that the Cayenne Tern at upper left (photo A) was relieved by a Sandwich Tern (photo B) presumed to be its mate.
     During 22 and 26 June 2003, observations confirmed that two pure yellow-billed Cayenne Terns, two predominantly yellow/orange-billed intermediate terns (with 90-95% yellow/orange bills), a half yellow/black-billed intermediate tern, and at least five intermediate terns with predominantly black bills were each mated with a Sandwich Tern. Despite the availability of mates, no matings were observed between predominantly yellow-billed terns or between intermediate terns. These observations imply non-assortative mating, supporting treatment of the two taxa as subspecies.