Current distribution, abundance, and breeding biology of White Terns (Gygis alba) on Oahu, Hawaii

Abstract

White terns (Gygis alba) are common on many islands in the tropical and subtropical Pacific, Atlantic, and Indian oceans, but in the southeastern Hawaiian Islands they are found only on Oahu, where they are listed as threatened by the State of Hawaii. In 2016, we censused the White Tern population on Oahu, mapped their distribution, and investigated their breeding biology. The total White Tern population on Oahu was 2,308 birds, including 1,408 breeders and ∼900 nonbreeders. The number of White Terns on Oahu has increased 282% since the previous census in 2002, for an average annual population growth rate of 7.7%. We observed White Terns only in urban and suburban areas of Honolulu and not in other parts of the island. Breeding occurred year-round, with 2 peaks in egg-laying in March and October. Hatching success was 69%, fledging success was 83%, and overall breeding success was 57%. Pairs made an average of 1.75 (SE 0.09) breeding attempts per year and produced an average of 1.08 (0.09) offspring per year (range 0–3). Terns used >58 tree species for breeding and roosting, most of which were large (>45 cm diameter). Average height in trees of breeding attempts was 9.1 (0.2) m. White Terns are thriving in urban Honolulu and are not seriously affected by numerous potential sources of human disturbance or predation from nonnative mammals, but some efforts will help to ensure their status remains secure. Received 7 July 2017. Accepted 6 January 2018.