Honeycreepers
Honeycreepers
Honeycreepers are about 20 living species of birds in the family Drepanididae, which occur only on the Hawaiian and Laysan Islands and nearby islands in the equatorial Pacific Ocean. Unfortunately, a further 15–16 species of honeycreepers have become extinct in historical times as a result of ecological changes that humans have caused to the habitats of these birds. In late 2005, the last known po’ouli (Melamprosops phaeosoma), a small honeycreeper found only on Maui, died in captivity, and the species is now probably extinct. At least half of the surviving species of honeycreepers are perilously endangered, as are some of the distinctive subspecies that occur on various islands.
Most species of honeycreepers breed in native forest and shrubby habitats in the Hawaiian Islands. They are resident in those habitats and do not migrate elsewhere during their non-breeding season.
The honeycreepers are small birds, ranging in body length from 4-8 in (11-20 cm). Their bills are extremely varied, depending on the diet of the species. Some honeycreepers have small, thin beaks, ideal for gleaning arthropods from tree foliage. Other species have longer, curved beaks, adapted to feeding on nectar or on insects deep in bark crevices. The beaks of yet other species are heavier and more conical and are used to feed on plant seeds.
This extreme diversification of species with various bill shapes within such a closely related group of birds is a famous example of speciation. In the case of the honeycreepers, the speciation was driven by natural selection in favor of birds having adaptations favorable to specific ecological opportunities, which occurred in a wide variety on the Hawaiian Islands. Evolutionary biologists consider the adaptive radiation of the Hawaiian honeycreepers to be one of the clearest illustrations of evolution.
Undoubtedly, all of the many species of honeycreepers evolved from a single, probably quite small founder group that somehow arrived on the Hawaiian Islands by accident in the distant past. Because few other types of birds were present, a variety of ecological niches were unfilled or were utilized by generalist organisms. Under the pervasive influence of natural selection, the original honeycreepers slowly evolved a repertoire of differing bill shapes and other useful adaptations. Eventually, the specialized populations of birds became reproductively isolated. Ultimately, they diversified into different species that were better adapted to feeding and living in specialized ways.
The honeycreepers are also highly variable in color, which ranges from a relatively drab gray to brown, olive, yellow, red, and black. Some species are dimorphic, with the male being larger than the female.
Honeycreepers build their cup-shaped nests in trees. They typically lay two to four eggs, which are incubated by the female. Both of the parents share the duties of raising their babies.
Species of honeycreepers
The smallest of the living honeycreepers is the anianiau (Loxops parva), only 4 in (11 cm) long. The largest species is the 8 in (20 cm) long Kauai akialoa (Hemignathus procerus). This species, and the closely related akailoa (H. obscurus), have long, downward-curving bills that are about one-third of the total body length. The akiapolaau (H. wilsoni) has an especially strange bill, with the upper mandible being strongly down-curved, but the lower being straight, and only half the length of the upper mandible. This species uses the lower mandible to pry loose bark off trees, and the upper to probe and impale their food of insects.
The mamo (Drepanis pacifica) and the crested honeycreeper or akohekohe (Palmeria dolei) have relatively shorter, downward-curving beaks, useful in sipping nectar from flowers.
The liwi (Vestiaria coccinea) is a beautiful, crimson-colored bird with black wings. This species is particularly prized by aboriginal Hawaiians, who use the red feathers in the preparation of traditional garments.
The grosbeak finch (Psittirostra kona) has a massive bill, useful in cracking hard seeds to extract the edible matter inside.
Humans and honeycreepers
The Hawaiian honeycreepers have become endangered through a variety of interacting ecological stressors. Habitat losses have been important, especially those associated with the conversion of their limited areas of natural-forest habitats to agricultural and urban land-uses, which do not support these native birds. Introduced herbivores, such as goats and pigs, have caused serious damage to honeycreeper habitat, greatly changing the nature of the vegetation, even in remote places. Introduced diseases, such as avian malaria, and introduced predators such as rats, mongooses, and pigs have also caused significant damage to honeycreepers.
Today, both the United States and Hawaiian state governments have designated many of the most important remaining refuges of natural habitat as parks and ecological reserves. Some of these refuges are being managed to maintain their ecological integrity as much as possible. For example, some large areas have been fenced, and the populations of feral goats and pigs have been eliminated or reduced. Unfortunately, these sorts of ecological interventions
KEY TERMS
Adaptive radiation —An evolutionary phenomenon in which a single, relatively uniform population gives rise to numerous, reproductively isolated species. Adaptive radiation occurs in response to natural selection, in environments in which there are diverse ecological opportunities, and little competition to filling them.
Endemic —This refers to a species (or genus, family, etc.) with a restricted geographic range. For example, the honeycreepers only occur on the Hawaiian Islands and nearby islands, and are therefore endemic to that relatively small region. Some are endemic to single islands.
Founder group —A small population of original immigrants to a habitat previously not known to the species. Following the successful colonization, the population may increase, and under the influence of natural selection may diversify into various species.
Speciation —The divergence of evolutionary lineages, and creation of new species.
are required today, and will also be needed in the future if the extraordinary Hawaiian honeycreepers are to survive in their changed and changing world.
Resources
BOOKS
del Hoyo, J., A. Elliott, and J. Sargatal. Handbook of the Birds of the World. Vol. 1, Ostriches to Ducks. Barcelona: Lynx Edicions, 1992.
Ehrlich, P.R., D.S. Dobkin, and D. Wheye. Birds in Jeopardy. Stanford: Stanford University Press, 1992.
Forshaw, Joseph. Encyclopedia of Birds. 2nd ed. New York: Academic Press, 1998.
Pratt, H.D., P.L. Bruner, and D.G. Berrett. The Birds of Hawaii and the Tropical Pacific. Princeton, NJ: Princeton University Press, 1987.
PERIODICALS
Freed, L.A., S. Conant, and R.C. Fleischer. “Evolutionary Ecology and Radiation of Hawaiian Passerine Birds.” Trends in Ecology and Evolution 2 (1987): 196–203.
Simon, J.C., et al. “Reproductive Ecology and Demography of the Akohekohe.” Condor 103 (November 2001): 736–745.
Bill Freedman