In 1938, avian malaria was discovered to have affected pigeons in the city of Honolulu (Amadon 1950). This might have seemed like a minor detail—except among breeders, pigeons do not normally elicit much concern from the average person—but it was to prove a disaster. From the pigeons, the disease spread into native birdlife of the Hawaiian archipelago and wreaked havoc. Many species living at lower elevations were wiped out, unable to withstand the disease's effects. Others were forced into remnant populations above an elevation of 1500m, where the disease's mosquito vectors were unable to survive.
Among the malaria's victims were several species of the Hawaiian honeycreepers, a group of small birds unique to the archipelago. The honeycreepers have become recognised as one of the classic examples of an island adaptive radiation, like the Madagascan vangas or the Galapagos finches. From the original colonisation of the archipelago by what was probably a fairly generalised finch-like bird, perhaps some five or six million years ago (Lerner et al. 2011), the Drepanidini have diversified into a disparate array of seed-eaters, insectivores and nectar-feeders. Some have evolved massive reinforced bills to crush the seeds of local trees such as koa or naio. Other have long slender bills that they use to reach into the depths of flowers or prise insect larvae from holes in bark. Currently, about fifty species of honeycreeper are known to have been present in the Hawaiian archipelago prior to human settlement; new ones continue to be described from fossil or subfossil remains. Sadly, due to factors such as habitat loss, competition with and predation by introduced fauna, and diseases such as the aforementioned malaria, only about twenty species remain alive today and many of those are critically endangered.
Older references will refer to the Hawaiian honeycreepers as their own family, the Drepanididae, due as much to long-standing uncertainty about their relationships to other birds as to their own distinctiveness. Many authors, such as Amadon (1950), argued for a connection between the honeycreepers and the South American flowerpiercers of the tanager family, believing that the nectar-feeders among the Drepanididae were closer in appearance to the group's original ancestor. However, recent studies, both molecular and morphological, have been unified in supporting a connection between the honeycreepers and the finches of the Fringillidae, leading to the demotion of the 'family' Drepanididae to a 'tribe' Drepanidini of the fringillids. In his original studies on the honeycreepers, Perkins recognised two subgroups: the 'melanodrepanines' were mostly nectar-feeders and were largely black and/or red in coloration, whereas the 'chlorodrepanines' were mostly seed-eaters or insectivores and usually yellow or greenish. Recent studies have supported the 'melanodrepanines' as a clade but identified the 'chlorodrepanines' as paraphyletic.
One unusual feature of many Drepanidini is that they carry a distinctive scent that has been referred to as the 'drepanidine odour' (this site describes it as a sweet, musty smell). Two primarily insectivorous genera, the po'o-uli Melamprosops phaeosoma and the ʻalauahios Paroreomyza, lack this 'drepanidine odour', and on the basis of this and a couple of other points it has been questioned whether they are properly assigned to the Drepanidini. However, the osteological analysis of Drepanidini by James (2004) confirmed their position as drepanidines, a result that has since been corroborated by molecular analyses. It seems likely that Melamprosops and Paroreomyza are basal drepanidines outside an 'odoriferous' clade (Pratt 2014). Together with the akikiki Oreomystis bairdi, these species form a basal grade of generalist feeders with fairly slender bills. It is possible that the akikiki and the Maui ʻalauahio Paroreomyza montana are the only members of this grade surviving.
The next clade of drepanidines to diverge in molecular phylogenies includes the Hawaiian finches, an assemblage of often seed- or fruit-eating species with thick, strong bills (Pratt 2014). James' (2004) osteological analysis did not resolve the finches as a single clade, instead intermingling them with the aforementioned grade. Again, the finches have been hard hit by extinction, with the only survivors being the palila Loxioides bailleui, the Laysan finch Telespiza cantans and the Nihoa finch T. ultima. Amadon (1950) noted that the Kona grosbeak Chloridops kona was extremely rare even when first discovered in the late 1800s, being restricted to an area of only 'a few square miles' in the Kona district of Hawai'i. The grosbeaks of the genus Chloridops and the koa finches of the genus Rhodacanthis had particularly strongly developed bills for cracking seeds, looking almost parrot-like in the case of Chloridops (James 2004). Of uncertain relationships to the finches are two unusual extinct species, the 'o'u Psittirostra psittacea and the Lanai hookbill Dysmorodrepanis munroi. The 'o'u was a fruit-eating, large-billed bird that was once widespread on the main islands of the Hawaiian archipelago (in contrast to most other honeycreeper species, which were mostly restricted to a single island). It was last definitely recorded in 1989 and continued survival is considered unlikely. The Lanai hookbill was a particularly bizarre species in which the mandible and maxilla were curved toward each other, so that the base of the bill gaped open even when the beak was closed. The single known specimen is unusual enough that Amadon (1950) did not accept that it represented an actual species, expressing the opinion that it was probably a deformed 'o'u specimen; current authors accept it as a good species.
As noted above, the nectar-feeding 'melanodrepanines' form a well-supported clade including three surviving species: the 'i'iwi Drepanis coccinea, the crested honeycreeper or akohekohe Palmeria dolei and the 'apapane Himatione sanguinea, the last of which is one of the more abundant living honeycreepers. The melanodrepanines have slender bills, which in the species of Drepanis (the 'i'iwi and two extinct species of mamo) are long and downcurved. Also probably belonging to the melanodrepanines is the extinct ʻula-ʻai-hawane Ciridops anna, which shared their black and red plumage despite being a fruit- rather than a nectar-feeder.
The final group of drepanidines to be considered here is also the largest, and contains the most surviving species: the 'amakihis of the genus Chlorodrepanis, the 'akepas Loxops, and related taxa. These are slender-billed insectivorous forms with the more generalist species being similar in appearance to the basal genera Paroreomyza and Oreomystis. Indeed, the classification of drepanidines by Amadon (1950), which was decidedly more lumpy than the current norm, subsumed the latter two genera in an expanded Loxops. Possibly related to this group are the extinct 'akialoas of the genus (wait for it...) Akialoa, which had an extremely long down-curved bill. Two other genera of this group, Hemignathus (including the ʻakiapolaʻau Hemignathus wilsoni) and the Maui parrotbill Pseudonestor xanthophrys, are unique among passerines in having a maxilla that significantly overhangs the much shorter mandible. The Maui parrotbill, despite being primarily an insectivore, has a heavier bill somewhat reminiscent of the finch group, and James' (2004) morphological analysis (which was primarily based on skull features) associated it with Psittirostra and Dysmorodrepanis rather than with Hemignathus; the latter association, however, is supported by molecular analyses, indicating a single origin for the unequal bills.
The loss of this remarkable radiation can be regarded as nothing short of a tragedy. Only two species of Hawaiian honeycreeper are currently regarded as not threatened (as given in the IUCN listings at Wikipedia), the 'apapane and the common 'amakihi Chlorodrepanis virens. Even these species could become endangered as a warming climate allows malaria-carrying mosquitoes to encroach further on their highland refuges. And something truly wonderful could be lost from the world.
Amadon, D. 1950. The Hawaiian honeycreepers (Aves, Drepaniidae). Bulletin of the American Museum of Natural History 92 (4): 151–262.
James, H. F. 2004. The osteology and phylogeny of the Hawaiian finch radiation (Fringillidae: Drepanidini), including extinct taxa. Zoological Journal of the Linnean Society 141: 207–255.
Lerner, H. R. L., M. Meyer, H. F. James, M. Hofreiter & R. C. Fleischer. 2011. Multilocus resolution of phylogeny and timescale in the extant adaptive radiation of Hawaiian honeycreepers. Current Biology 21: 1838–1844.
Pratt, H. D. 2014. A consensus taxonomy for the Hawaiian honeycreepers. Occasional Papers of the Museum of Natural Science, Louisiana State University 85: 1–20.