Of Hawaii's birds, the honeycreepers (Drepanidinae) are most famous, having put on what is arguably the world's most dazzling display of adaptive radiation--an explosion of species from a single unspecialized ancestor to at least 54 species that filled available niches in the islands' habitats. In fact, speciation in the Hawaiian honeycreepers dwarfs the famed radiation of Darwin's 14 Galapagos finches.
Robert Fleischer, Cheryl Tarr, and Carl McIntosh at the National Zoo's Molecular Genetics Laboratory estimate that the honeycreepers' ancestor arrived three to four million years ago; others put the arrival farther back, at closer to seven million years ago. This ancestor--one colonizing species of finch, possibly a Eurasian rosefinch (Carpodacus sp.) or, less likely, the North American house finch (Carpodacus mexicanus)--started what proved to be an evolutionary snowball. "There must have been a lot of open niches, and the birds hit the islands and speciated very rapidly," says Fleischer, who studies the genetics of fossil and living Hawaiian birds. Rapidly, in terms of geologic time, is thought to be within the first 200,000 to 300,000 years after the first finch touch-down.
Nectar-feeding honeycreepers evolved dramatically curved bills designed for probing and extracting the nectar from the flowers of Hawaii's endemic lobelias and other plants. Insectivorous honeycreepers developed thin, warbler-like bills for picking insects from the foliage. Seed-eaters developed stouter, stronger bills for cracking tough husks. Some species probed or cracked bark with strong hooked bills seeking wood-boring insects, thereby filling a niche woodpeckers do elsewhere. Honeycreepers shared the islands with an array of other unique bird species. In 1991, Storrs L. Olson and Helen F. James of the Smithsonian's National Museum of Natural History described for the first time 32 extinct species they identified from bones found in lava tubes, sinkholes, dunes, and excavated Polynesian refuse piles (middens) on the main Hawaiian Islands over the past 19 years. Three others had been previously described. When their analyses are through, at least 20 more species will likely be added.
These recent findings conjure up a vision of an almost mythical world where birds, not mammals, dominated. Large flightless waterfowl called moa nalos were the islands' large herbivores. A harrier, a hawk, an eagle, and four owls topped the food chain as predators. No mammals patrolled the ground (Hawaii's only native land mammal is a bat), and, with the need to fly gone, many of the castaway bird species, such as endemic ducks, ibis, and rails, lost their powers of flight.
The rare `akia pola`au occurs in only a few areas of upper elevation on the Big Island. Its bill is the most unusual in the honeycreeper family. The lower bill is short, straight, and stout. With mouth agape, it is used to chisel holes, woodpecker style. The upper bill is long, curved, and slender. It is used to probe, pierce, and pull insects and caterpillars from the hole. The male is brilliant yellow with a black mask; the female is dull green with a less distinctive mask and slightly shorter bill. The `akepa is an insect-eating bird with a short, straight bill. The male is blaze orange; the female is gray-green with tinges of yellow or orange on the breast. It is the only Hawaiian honeycreeper that always nests in natural tree cavities. The largest endangered forest bird in Hawaii is the 'Io (Hawaiian Hawk). It is frequently seen soaring high above the tree canopy in search of birds, large insects, mice, and rats. Rarely seen in the 1960s and 1970s, hawks are now frequently observed from the coast to the tree line on mountain slopes.