The `i`iwi may be the best known and best loved of all Hawai`i’s native forest birds. Boasting bright scarlet plumage and along, curved bill, it is easily recognized and still found on Kaua`i, Maui, and Hawai`i.
But the bird may be in trouble. A recent study of bird populations in the Nauhi area of the Hakalau National Wildlife Refuge on the Big Island found that of all bird species studied, the only bird whose reproductive rate was in significant decline was the `i`iwi.
The discovery was made by Bethany Woodworth, a biologist with the U.S. Geological Survey’s Biological Resources Division in Volcano, and a team of other scientists who studied populations of bird species found at Hakalau.
Woodworth’s team discovered most of the seven species of birds studied had adult survival rates of 70 to 90 percent, a rate comparable to those of birds in other tropical forests. Only the `i`iwi lagged behind, with an adult survival rate of 56 percent. An earlier study in the 1970s found similarly low survival rates for adult `i`iwi.
`I`iwi fell behind in the area of nesting success as well. While most species fledged young from between 40 and 80 percent of active nests, `i`iwi fledged young from just 19 percent of their nests.
In 1997 and 1998, the scientists looked at the proportion of adult birds attempting to breed in a given year. All species had a significant decline over this period, with the decline being especially noteworthy for `i`iwi and elepai`o.
Woodworth and her colleagues put all these factors together into a model that rates population growth rate. The model assigns a so-called lambda value of 1 to stable populations, where birth rates equal death rates. Values greater than 1 show a growing population, while those less than 1 indicate that the population is losing ground.
The results were good for nearly all species of bird studied. Except for the `i`iwi, all of the native forest birds had lambdas with a value greater than 1, indicating that their populations in Hakalau are likely to be stable or growing. In fact, “some populations such as omao and Hawai`i creeper,” Woodworth says, “with lambdas of about 1.3, are probably source populations for neighboring forests.”
Of those birds included in the study, only the `i`iwi was in trouble. The preliminary data show it has a lambda of about 0.71
Why?
At the Hawai`i Conservation Conference, held in Honolulu last August, Woodworth suggested that the `i`iwi’s mobility may have something to do with the low survival rate of adults. “`I`iwi are highly mobile, following the bloom of `ohi`a as it moves up and down the mountainsides,” Woodworth said. “Bird biologists have long suspected that in the course of these movements, `i`iwi might move into high disease areas and so be exposed to avian malaria.” If exposure occurs, it would almost certainly be fatal. Past studies by Carter Atkinson, a colleague of Woodworth’s at BRD, have shown that 9 out of 10 `i`iwi exposed to malaria through just a single mosquito bite have died as a result.
Then, too, weather might have been a factor. “We were only able to monitor nests during 1998 and 1999,” Woodworth noted, years characterized by El Nino and La Nina, respectively. “It is important to realize that if global warming leads to an increase in the frequency of El Nino drought years as predicted, global warming may have adverse effects on recruitment of wet forest birds, and this potential effect has not previously been appreciated.”
Even though the reproductive rates of `i`iwi are down, actual counts of the bird at Hakalau are not declining, according to regular censuses at the refuge. The mobility of bird populations – especially `i`iwi and apapane – may disguise local population processes. “Birds produced in Hakalau may grow up to breed in palila habitat at the top of Mauna Kea or along the strip road (and vice versa),” Woodworth says. “We really don’t have any knowledge of where these birds are coming from or going to.”
Regardless of the status of `i`iwi at Hakalau, researchers are troubled by the gradual disappearance of `i`iwi at mid-elevation forests on Hawai`i over the years. “Although not endangered,” Woodworth says, “they are widely considered to be vulnerable and of special concern, largely because of their susceptibility to malaria.”
Woodworth stresses that the data are preliminary and that much more work needs to be done. In addition, not enough data were available to establish population trends for the “rarest of the rare,” the akiapola`au. “I think akis are of great concern,” Woodworth says.
— Patricia Tummons
Volume 11, Number 7 January 2001
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