Status and Monitoring Methods of a Red-tailed Tropicbird Colony on O'ahu, Hawai'i

The Red-tailed Tropicbird Phaethon rubricauda is a widespread seabird that nests in the tropical Pacific and Indian oceans. Most the global population nests in the predator-free Northwestern Hawaiian Islands. In the southeastern Hawaiian Islands, non-native predators restrict the species to nesting on coastal cliffs and small islets. I monitored 734 Red-tailed Tropicbird nests near Ha¯lona Point on O‘ahu during 2005–2020, controlled non-native predators to protect nests (beginning in 2006), and developed tools to facilitate monitoring of the species in other areas. The colony increased in size in response to predator control, from 18 eggs laid in 2006 to 94 eggs in 2020; this represented an overall growth rate of 520% and an annual growth rate of 11%. Egg laying (one egg per nest) peaked in March, hatching peaked in April, and fledging peaked in July. The mean incubation period was 44.3 days (n = 504 eggs) and the mean nestling period was 82.4 days (n = 394 chicks); together, this gives a mean breeding cycle length of 126.7 days. The apparent nest success rate was 0.631 ± 0.018 and the daily nest survival rate was 0.9962 ± 0.0002, which resulted in a mean estimated nest success rate of 0.619. The number of adult tropicbirds flying over the colony was related to the number of active nests and varied monthly. I developed three correction factors to estimate the total number of nests in a colony, using the number of nests found during a single visit in any month. These results demonstrate that seabirds can thrive on islands inhabited by people, if predators are managed.