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. |
