Monday, September 24, 2007

The 'Io, Hawaiian hawk, holding its own

A symbol of Hawaiian royalty and one of only two surviving native Hawaiian birds of prey, the Hawaiian hawk, 'io, is on the federal Endangered Species List, but its population appears to be at least stable and may be growing a bit.

(Photo: Michael Walther, Oahu Nature Tours)

The raptor today is only found on the Big Island, although fossil 'io have been located on several of the islands from the Big Island all the way to Kaua'i.

The animal is part of a clan of hawks called buteos, making it a cousin of the red-shouldered hawk, the red-tailed hawk and the rough-legged hawk. It has the scientific name Buteo solitarius.

The birds stand up to a foot and a half tall, and can occur in either a dark-brown or pale brown color phase. Their habitat ranges from as low as a few hundred feet to a mile and a half high. For more information see

The hawk is a powerful flyer that eats insects, rodents, small birds and will even take larger birds like pheasant. It will also take other endangered species, notably the Hawaiian crow, 'alala.

One of the issues in figuring out how much protection the birds need is getting an accurate count. It was placed on the Endangered Species List at a time when nobody knew just how many there were. Over the years, the estimates have ranged within several hundred birds on either side of 2,000.

Among the issues is that the animals are fairly broadly distributed, but not real dense.

John Klavitter, now the biologist for the U.S. Fish and Wildlife Service on Midway Atoll National Wildlife Refuge, got his master's degree in part based on a thesis that discussed counting the hawks, and developing systems for figuring out whether your count accurately represents actual numbers.

In a paper written this summer, he noted that if you use recorded playbacks of 'io calls, they show up readily. The danger here is that the playback system may make it seem that there are more of them than there really are. The paper, by Klavitter and John Marzluff, “Methods to correct for density inflation biases in Hawaiian hawk surveys using attractant calls,” was published in the June 2007 issue of the Journal of Raptor Research.

His estimate in his 2000 thesis and in a 2003 paper in the Journal of Wildlife Management suggested at at the turn of the millennium, his team figured there were nearly 1,500 of the birds. (The actual estimate was 1,457, plus or minus 176.3 birds)

Based in part on that, he recommended the birds be down listed from endangered to threatened, but not delisted altogether.

The animals appear resistant to diseases like malaria and pox that severely impact other Hawaiian birds, they seem to be long-lived, and they seem to be reproducing appropriately.

There are still issues for them. They're only present on one island, meaning random events like hurricanes, fires or other catastrophes put them at risk. Furthermore, while they seem to be benefiting from some habitat change, a complete switch from native to non-native could put them at risk, researchers said. Their main nesting tree is the native 'ohi'a.

In recent work, Klavitter and co-researchers say the populations of 'io in some native wildland areas, such as around Pu'u Wa'awa'a, are as dense as those of its cousin hawks in North America, like the red-tailed hawk.

“The reason Hawaiian hawks were so dense was likely because our study plots were dominated by mature native forest with high amounts of human-created edge habitat and extensive areas with alien grass understory. Hawaiian hawks apparently have benefited from this habitat change just as several other raptors have benefited from habitat modification elsewhere,” the authors wrote.

That's not to say 'io prefer purely non-native habitat.

They still require tall, old trees—the kind they are likely to find in native 'ohi'a forest—for nesting and other purposes. Their nests of interwoven sticks can be more than 30 feet high.

© 2007 Jan W. TenBruggencate

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