Hayes, F. E. 2004. Variability and interbreeding of Sandwich Terns and Cayenne Terns in the Virgin
Islands, with comments on their systematic relationship. North American Birds 57:566-572.
Abstract.--A mixed colony of 410 breeding pairs of Sandwich Tern (Sterna sandvicensis acuflavida) and Cayenne Tern (S. s. eurygnatha) was studied at Dog Island off St. Thomas, U. S. Virgin Islands, during May-July 2003. An estimated 94% of breeders were typical Sandwich Terns (bill black with yellow tip), two (0.24%) were typical Cayenne Terns (bill pure yellow), and the remaining 6% were highly variable intermediates (bill black with yellow / orange blotches or yellow / orange with black blotches). The proportion of Cayenne Terns and intermediate individuals in the Virgin Islands appears to be increasing since the 1980s. Two pure yellow-billed Cayenne Terns, two predominantly (90-95%) yellow / orange-billed intermediate terns, a half-yellow, half- black-billed intermediate tern, and at least five predominantly black-billed intermediate terns were each mated with a Sandwich Tern, suggesting non-assortative mating, but sample sizes of yellow-billed birds are small and the sex of yellow-billed individuals unknown. The intermediacy and variability of bill coloration in breeding populations throughout the Caribbean imply extensive interbreeding between the two taxa. Nevertheless, the persistence of presumably �pure� phenotypes where the two taxa overlap within the Caribbean suggests a tendency toward assortative mating. In the absence of direct evidence for assortative mating, however, maintaining the two taxa as conspecific seems appropriate. |
