Response of palila and other subalpine Hawaiian forest bird species to prolonged drought and habitat degradation by feral ungulates

Extinction has claimed half of all historically-known Hawaiian passerines, and today many extant species are increasingly threatened due to the combined effects of invasive species and climate change. Habitat disturbance has affected populations of feeding specialists most profoundly, and our results indicate that specialists continue to be most vulnerable, although even some abundant, introduced, generalist species also may be affected. Surveys of passerines during 1998-2011 in subalpine woodland habitat on Mauna Kea Volcano, Island of Hawai′i, revealed that the abundance of the critically endangered palila (Loxioides bailleui), a seed specialist, declined by 79% after 2003. The ′akiapōlā′au (Hemignathus munroi), an endangered specialist insectivore, was not detected in the survey area after 1998. The Hawai′i ′amakihi (Hemignathus virens virens), a generalist feeder and the most abundant species on Mauna Kea, was the only native species to maintain a stable population. The Japanese white-eye (Zosterops japonicus), a well-entrenched generalist and one of the three most common introduced species, declined. Drought prevailed in 74% of months during 2000-2011, and dry conditions contributed to the recent decline of the palila by reducing the annual māmane (Sophora chrysophylla) seed pod crop, which influences palila breeding and survival. Sustained browsing by introduced ungulates also lowered habitat carrying capacity, and their elimination should reduce the effects of drought and promote forest restoration. Our results illustrate how the feeding ecology of a species can influence its response to interacting environmental perturbations, and they underscore the value of long-term monitoring to detect population trends of sensitive species.