Shearwaters' Decline on Kaua`i May Be Result of Many Factors

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The Kaua`i population of `A`o, or Newell’s shearwaters (Puffinus auricularis newelli) is in a free-fall. Over the last seven years, numbers of the bird have declined 60 percent overall, with average annual declines of some 14 percent. The Newell’s shearwater, which is federally listed as threatened, is endemic to Hawai`i – that is, found nowhere else in the world.

What seems to have triggered the decline was Hurricane `Iniki, which ravaged the island September 11, 1992. As biologist Brian Cooper with ABS, Inc., a private biology consulting firm, reported at the Second International Conference on Albatrosses and Petrels held last month in Honolulu, of the 13 shearwater colonies whose populations were studied, 12 colonies declined from 1993 to 2000 – some by more than 90 percent.

Cooper’s group studied the bird population using radar, which tracks the shearwaters as they return each night from their ocean foraging. The plummeting population numbers he found are confirmed by reduced numbers of chicks recovered through Kaua`i’s “Save Our Shearwaters” (SOS) program, which asks citizens to rescue shearwater chicks that are disoriented by light. According to Tom Telfer of the state Department of Land and Natural Resources’ Division of Forestry and Wildlife, before `Iniki, some 1,500 to 2,000 birds were recovered each year. Since then, the number of chicks rescued annually hovers around 500 (last year, 680 chicks were rescued).

“Something is going on that is very serious, and we don’t know what it is,” says Telfer.

He, Cooper and other scientists studying the shearwaters do not have any definite answers as to the precise cause of the birds’ decline. “The most likely scenario,” Cooper says, “is that the hurricane somehow damaged habitat enough so that we had an influx of predators – cats and rats – into some of these colonies.” A related possibility is the destruction of uluhe fern by the two-spotted leafhopper. The shearwaters’ burrows are often covered by dense thickets of the fern. Anything that destroys the fern would cause some harm to the shearwaters as well.

Also, he notes, the hurricane probably disrupted the breeding cycles. This could occur in one of two ways, he said. First, like many other seabirds, shearwaters bond for life. If one partner is killed, it removes the surviving partner from the breeding cycle for at least a year. Should adult birds have been killed during the hurricane (either by suffocating in collapsed burrows or by being caught at sea in the hurricane’s winds), the ability of the surviving birds to breed the following season would have been impacted. A similar effect would occur if pairs returning to breed found that their burrow had been destroyed. “If a burrow has disappeared,” he noted, “pair bonds break down and you get the same effect of lower breeding performance.”

Still, he continued, “that really doesn’t explain the precipitous decline that has happened.”

Habitat loss is another contributing factor, Cooper said. “The hurricane probably just peeled up uluhe fern and uprooted trees, particularly on ridgetops. Some colonies could have been damaged by that.”

Then, finally, there’s the fact that young birds continue to die as a result of disorientation from artificial lights. They collide with light poles and power lines, among other things, fall to the ground, and, if not rescued, are run over by vehicles.

Cooper stresses that there is no hard data pinpointing the precise cause, or even the main cause, of the shearwaters’ falling numbers. But if he had to choose, it would probably be predation by animals whose access to the shearwaters’ colonies has been made easier since the hurricane stripped the mountains where the shearwaters nest of so much of their cover. “Predators are clearly a problem,” Cooper says. “While we don’t have solid data yet, I’d put that at the top of the list.”

To collect still more data, scientists from ABR will be returning to Kaua`i in June, revisiting colonies and gathering another year’s worth of information.

— Patricia Tummons

Volume 10, Number 12 June 2000

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