Two forest bird species endemic to the island of Kauai in the Hawaiian Archipelago were listed under the U.S. Endangered Species Act in 2010 due to recent population declines. This research represents the first comprehensive breeding biology study of both species, the ‘Akikiki or Kauai Creeper (Oreomystis bairdi) and ‘Akeke‘e or Akekee (Loxops caeruleirostris). The 2-year study was initiated in 2012 to determine if low nesting success may be a cause of their population declines. We monitored 20 ‘Akikiki and 8 ‘Akeke‘e nests to assess basic nesting biology parameters (e.g., brood size; nest height; length of construction, incubation, and nestling periods) and to derive estimates of nesting success and investigate causes of failure. In general, ‘Akikiki and ‘Akeke‘e breeding biology was similar to other insectivorous Hawaiian honeycreepers. Mean nest height for ‘Akikiki and ‘Akeke‘e was high (9.2 ± 2.3 m SD and 11.1 ± 2.3 m SD, respectively) compared to most Kauai forest birds. Nesting success, calculated using program MARK, was 0.77 ± 0.12 SE for ‘Akikiki and 0.71 ± 0.17 SE for ‘Akeke‘e. Three ‘Akikiki and 2 ‘Akeke‘e nests failed. One ‘Akikiki nest failed due to nest predation and the other 2 to unknown causes. One ‘Akeke‘e nest failed due to poor nest attendance and the other to hatching failure. Nest sample sizes were small and should be considered with caution; however, these results suggest that low nesting success may not be a primary cause of decline in these species. Future research on both species should assess post-fledging, juvenile, and adult survival as potential causes of their populations' declines. Determining which demographic parameters currently have the largest negative impact on these populations is imperative for guiding effective management actions to conserve these species.