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One of the last surviving of the extraordinary “finch-billed” Hawaiian honeycreepers (Drepanidinae), the Palila epitomizes specialization for seed-eating as it relies almost entirely on immature (soft) seeds, flowers, and other resources produced or supported by mämane (Sophora chrysophylla: Fabaceae), an endemic dry-forest tree. Mämane seeds are highly nutritious, but a strong, sharp bill is necessary to remove and rip open the tough, fibrous pods that protect them. In addition, the seeds contain high levels of potentially toxic secondary compounds, which may also explain why they are eaten by so few other native or alien species. The Palila’s close association with mämane influences not only its foraging behavior but also its distribution, abundance, dispersal, and breeding ecology. Like most drepanidines, the Palila produces small clutches, usually only 2 eggs. However, Palila embryos and chicks develop slowly, so eggs and nestlings are threatened by storms and predators for a longer time than many other small passerine species.
The Palila generally prefers large mämane trees but is highly selective with regard to individual trees in which it forages, nests, and roosts. Populations thrive in large tracts of forest arrayed along a gradient of elevation. Mämane flower and seed availability varies annually among trees, but the timing of production is strongly influenced by elevation. Thus, food resources are available year-round in forest tracts that extend broadly down and around mountain slopes, and small, loose flocks of these social birds track the availability of mämane pods up and down steep, volcanic slopes. However, in contrast to nectar-seeking birds that seasonally invade mämane forests, Palila are relatively sedentary and tend not to travel more than a few kilometers from their natal nests. Where the forest has been truncated or fragmented, particularly by cattle ranching, populations slowly die out. Populations disappear more rapidly when the forest is browsed by ungulates, which, by their preference for seedlings and tender branch tips, have degraded and reduced huge areas of dry forest since their introduction about 200 years ago. Dependence on dry mämane forest increases the vulnerability of Palila to fire threats, which in turn are exacerbated by the invasion of alien grasses and other weeds into Palila habitat following disturbance by ungulates and other factors.
As with other Hawaiian forest birds, the Palila’s range has contracted dramatically in modern times, but few species are limited to an area as small as that of the Palila. Fossils reveal that this species once occurred on the islands of Kaua‘i and O‘ahu, but these populations apparently disappeared long before the nineteenth century and live specimens are known only from Hawai‘i I. The Palila’s present confinement to Mauna Kea Volcano—and then almost entirely to the western slope—represents only a tiny fraction of its range when Polynesians first encountered the species some 1,500 years ago. Again, like nearly all other native forest birds, the Palila recently became stranded in the uppermost portion of its historic range. This is due partly to habitat loss and partly to the distribution of introduced, disease-bearing mosquitoes, which have not yet colonized forests much above 1,220 meters elevation. Not only has the Palila lost access to much of its former habitat, it no longer interacts with the full range of native birds, arthropod prey, and other species with which it evolved. Instead, this species’ survival and future are increasingly threatened by introduced mammalian predators, alien insects that reduce insect prey, weeds, and other invasive species.
Despite formidable threats to Palila conservation, there are opportunities for restoration. The ecology of the Palila is more thoroughly known than for any other Hawaiian forest bird, in part because the species and its habitat are relatively accessible and amenable to field study. Early research emphasized breeding biology (van Riper 1978, 1980a) and population distribution and abundance patterns (van Riper et al. 1978; Scott et al. 1984, 1986), and although additional studies of nesting behavior (Pletschet and Kelly 1990) and demography (Fleischer et al. 1994, Lindsey et al. 1995, Jacobi et al. 1996) have followed, emphasis has more recently shifted to habitat and limiting factors (Fancy et al. 1993; Lindsey et al. 1997; Pratt et al. 1997; Hess et al. 1999, 2001; Banko et al. 2002a) and habitat restoration (Fancy et al. 1997). Unlike the case with most Hawaiian birds, research and conservation efforts were initiated before the Palila declined to very low numbers or its habitat was lost. However, recovery of the Palila is not assured, despite a population numbering in the low thousands and a substantial degree of habitat protection.
Banko, Paul C., Luanne Johnson, Gerald D. Lindsey, Steven G. Fancy, Thane K. Pratt, James D. Jacobi and Winston E. Banko. 2002. Palila (Loxioides bailleui), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/679