Anthropogenic climate change is affecting many bird species in a variety of ways, causing changes in their distribution, abundance, and food supply, and triggering responses such as shifting migration and nesting seasons. Most studies have focused on migratory birds that breed in temperate areas and little is known about effects of climate change on tropical birds. We used a 24 year dataset that included 1,632 nesting events of the Oahu Elepaio (Chasiempis ibidis), an endangered forest bird endemic to Hawaii, to investigate whether breeding phenology and performance were related to rainfall and whether a recent shift in the breeding season is related to changes in rainfall. Fecundity of the Oahu Elepaio was closely related to rainfall, with more offspring produced during years with higher rainfall. During the first 19 years of this study, from 1996 to 2014, all nests were initiated from November to June. From 2015 to 2019, up to 33% of annual reproduction occurred from August to October. Higher rainfall caused parallel increases in reproduction during the normal nesting season and the atypical season, with some birds nesting in both seasons. The change in breeding phenology of the Oahu Elepaio is unusual because it was not a gradual transition, but a sudden change of 6 months. The shift in breeding phenology coincided with unusually high summer and fall rainfall from tropical storms associated with a novel climate pattern in the northern Pacific Ocean from late 2014–2016 termed the warm blob. This pattern of warmer water and more frequent storms is expected to become more prevalent in the Central Pacific, so summer–fall breeding in the Oahu Elepaio is likely to continue and perhaps become more common. The Oahu Elepaio is flexible, adaptable, and may not be seriously affected by changing climatic conditions.

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