A Crow-Shaped Hole in Hawaii

By Hugh Powell
The importance of crows in the ecosystem of Hawaii

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One day around 1980, a scientist in Costa Rica found a pile of horse dung that changed the course of tropical ecology—a chance encounter that, 30 years later, is helping the case for saving the Hawaiian crow, or Alala.

Buried in that pile was the seed of a jicaro, a tree with a fruit like a small cannonball. The scientist, Daniel Janzen of the University of Pennsylvania, had never been able to figure out why the jicaro, and many other tropical trees, produce such big, hard fruits. In Costa Rica, all the fruit-eaters are small. The whole point of a fruit is to tempt an animal into moving seeds around. Why evolve such an inaccessible package for the seeds?

The answer was on the ground in front of him: a horse had easily cracked the fruit, eaten the seeds, and moved on. The original dispersers must also have been big beasts with heavy jaws: ground sloths, giant armadillos, and elephantlike animals that had roamed Costa Rica until about 10,000 years ago. Janzen’s insight was that ecosystems can be full of broken relationships. After an extinction, an animal’s absence can be seen in what remains: piles of fruit rotting under branches; trees unable to spread across the landscape. His idea has since been widely applied, including recently in Hawaii.

Weighing in at just over a pound, the Hawaiian Crow is not exactly a giant sloth—but it was one of Hawaii’s largest remaining native fruit eaters. None of these crows are moving seeds around the Big Island anymore, though; fewer than 100 remain, all in captive-breeding facilities. A group of researchers recently looked into the bird’s disappearance to see what broken relationships lay in its wake.

“There are so many [Hawaiian trees] that lack any known seed disperser, and many of them are endangered,” said Susan Culliney, lead author of the study. “Maybe the Alala are the missing seed dispersers for these large-fruited plants.”

To investigate, the researchers collected the fruits of 16 native tree species. They gave hundreds of the fruits, containing more than a million seeds, to 57 captive Hawaiian Crows. Then they watched.

The birds ate fruits from all 16 species and, in classic crow fashion, carried many of them around their enclosures to store for later. The researchers then collected the birds’ droppings and pellets, painstakingly washing through them to count how many seeds were eaten. (The answer: 7,144.) They planted seeds from six tree species and found even more evidence for a crow-plant link. For three species, seeds that the crows had eaten (stripping away the fruit and partially digesting the seed coat) showed a better chance of sprouting than untouched seeds.

The birds showed a special affinity for some plants. The hoawa tree, for example, hides its oily, black seeds inside hard pods. Early naturalists had seen Hawaiian Crows breaking these open. Captive birds—even ones that had never before seen wild hoawa—did the same. “They just immediately seemed to know what to do with them, to pound on them, hold them in their feet, and knock on them until they split open,” Culliney said.

Today, wild hoawa pods pile up where they fall, and rats eat them, Culliney said. Though some seeds may get dispersed thisway, scurrying rats are no replacement for the wings of a Hawaiian Crow. As the bird’s imprint fades, the distribution of Hawaiian trees must be changing.

At least the birds have not yet gone the way of the giant armadillo. Conservation efforts might yet succeed in darning the crow-shaped hole in Hawaii’s plant communities. Captive breeding is going well, raising the prospect of reintroducing them into the wild. “I think of how wonderful it’ll be not just to see them and hear them,” said Culliney, “but also to link up some of the past relationships of this large-bodied and large-billed bird.”

The Cornell Lab

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