The Akikiki (Oreomystis bairdi) and Akekee (Loxops caeruleirostris) are two honeycreepers endemic to Kauai, Hawaii, that were listed as federally endangered in 2010. Both species are rare, little-studied, and occur in a remote, roadless area. We analyzed high-resolution airborne lidar data to identify forest structure and topography metrics associated with Akikiki and Akekee nest locations (88 for Akikiki and 22 for Akekee) and occurrences (3706 for Akikiki and 1581 for Akekee) from 2012 to 2017 on the Alakai Plateau to predict their distribution in unsurveyed areas. Akikiki and Akekee nested in areas with similar forest structure at 10 m resolution, but different maximum tree heights. Akikiki and Akekee foraged in areas with significantly different forest structure (maximum tree height, mean canopy height, relative heights) and topography (slope) based on occurrences. Elevation was consistently one of the most important metrics for predicting both species nest locations and occurrences across scales (10, 100, 250 m) and it appears that both species are at the upper limits of their elevational range. We estimate that the area of suitable nesting habitat for Akikiki is 17.59 km 2 while the area of suitable nesting habitat for Akekee is 11.10 km 2 at 10 m resolution. The Akikiki has a potential range of 38 km 2 while the Akekee has a range of 58 km 2 at 100 m resolution. We produce predictive nest and occurrence maps at 10 m and 100 m resolutions to spatially target conservation actions. Results suggest that if avian malaria cannot be controlled and both species populations do not stabilize over the coming years, translocation may be needed to insure their viability.

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The effects of population decline on culturally transmitted behaviours in animals have rarely been described, but may have major implications to population viability. Learned vocal signals in birds are of critical importance to behaviours associated with reproduction, intrasexual interactions and group cohesion, and the complexity of vocal signals such as song can serve as an honest signal of an individual's quality as well as the viability of a population. In this study, we examined how rapid population declines recently experienced by Hawaiian honeycreepers on the island of Kaua‘i (USA) may have influenced the diversity, complexity and similarity of learned honeycreeper songs. We analysed the acoustic characteristics of songs recorded during three time periods over a 40-year time frame for three species of declining Kaua‘i honeycreepers. We detected a loss of song complexity and diversity over the 40-year time period that paralleled dramatic population declines. Concurrent with the loss of complexity, we also found that the acoustic characteristics of the three honeycreepers' songs became more similar to one another. To our knowledge, this is the first documentation of convergence of acoustic characteristics among rapidly declining species. The reduction in song complexity and diversity and convergence of songs not only signals a loss of culturally transmitted behaviours in these endemic Hawaiian honeycreepers, but also potential challenges to the recovery of these rapidly declining species. Moreover, the present study highlights that there is a ‘hidden’ cost to declining populations beyond just the loss of individuals that is not often considered, the loss of culturally transmitted social behaviours.

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