Factors Affecting Breeding Success of White Terns (Gygis alba; Aves: Laridae) in Urban Environments of Honolulu, Hawai‘i

Abstract: The White Tern (Gygis alba) is a common seabird that nests on islands in tropical and subtropical oceans. In the southeastern Hawaiian Islands, White Terns breed only in urban and suburban areas of Honolulu. The Honolulu population has grown from a single pair in 1961 to 2,300 birds in 2016. We measured breeding success of White Terns in Honolulu and examined aspects of their breeding biology and the environment that may affect breeding. We documented 3,855 breeding events in Honolulu from 2016 to 2019. Breeding success was 68.3% and varied less than 3% among years. Breeding occurred in all months, with lowest activity in the summer and a peak in the winter and spring, but the pattern varied among years. White Terns bred in 64 tree species. Breeding success varied among tree species and was related to characteristics of the trees and their attractiveness to non-native predators. The height of breeding events was 9.26 ± 0.05 m and success was related to height, with higher success in events 5–10 m above ground. The diameter of branches on which breeding occurred was 10.2 ± 0.1 cm and success was related to branch diameter, with lower success on branches >20 cm. The high breeding success of White Terns in Honolulu is related to: the small size and isolation of the O‘ahu population, which may result in reduced intra-specific competition for food; protection from predators provided by the urban environment; and trimming of trees by arborists, which unintentionally improves the value of trees as breeding sites for terns.