White Terns (Gygis alba) commonly breed on remote atolls and islands in tropical and subtropical regions, including the Northwestern Hawaiian Islands (NWHI). In 1961 a pair of these seabirds was reported to be breeding at Koko Head on Oahu (Ord 1961), the only main Hawaiian Island where they have been found. Since then the terns have expanded their nesting area some 16 km to the civic center of Honolulu. Harrison et al. (1984) estimated 50–100 pairs in the main Hawaiian Islands.
Dorward (1963) found that the White Tern was one of the few species on Ascension Island to have a distinct annual breeding season, laying about January. Individual pairs laid on the same ledge in successive seasons with an interval of close to a year. On Christmas Island in the Central Pacific Ocean Ashmole (1968) found that incubation and chick-rearing took about 5 months and molt about 5.5 to 7 months. Howell (1978) found the incubation period to be 35 days. Pettit et al. (1981) found the White Tern to be one of only two terns with prolonged incubation and to have the longest incubation in relation to its egg mass of any tern. A single egg is laid but no nest is built.
Here I present the results of my study of White Tern breeding biology on Oahu. I describe differences occurring in individual breeding pairs; these previously undescribed differences concern the lengths of intervals between breeding which, in turn, affect the number of offspring produced in one year.