HINESBURG -- Four young ravens, glossy black and big as chickens, listened curiously as University of Vermont zoologist Bernd Heinrich called them. Old enough to fly but not old enough to fend for themselves, the birds looked warily at Heinrich through the open door of their aviary. He waved a bite-sized chunk of veal at them.

"OK, guys, come out. You guys hungry?" he asked, sounding like a parent coaxing anxious 2-year-olds.

Finally, one bird, called Orange for the orange band around its leg, hopped up on Heinrich's hand. He fed the bird from an old Cool Whip container. The other birds opted for freedom rather than food and flew off. Heinrich wasn't worried that his research subjects had, shall we say, flown the coop.

"They'll be back in an hour, when they are hungry again," he said with a smile.

Ravens have dominated Heinrich's life for 15 years now. With the publication this summer of his 10th book, called "Mind of the Raven," Vermont's most well-known and most influential biologist once again poses tough-to-answer questions about the natural world. Why do ravens take baths? Not, Heinrich has found, just because they're hot or dirty. Do birds such as ravens lord their power over other birds just because they can? Yes. Ravens can be as unreasonable as an overworked boss on a Friday afternoon.

"He is one of the few who combines a career as a first-rate scientist -- he is a superb physiologist and ecologist, as well as a researcher in field behavior -- with an extraordinary ability, as evidenced by his books, to conduct long, meticulous patient field studies," said Edward O. Wilson, a Harvard professor, naturalist and two-time Pulitzer Prize-winning author.

At 59, with a well-muscled body that's as fit as a 30-year-old's, Heinrich climbs trees, hunkers down in bird blinds, hauls 1,000-pound carcasses to the back woods. He roams the woods in Maine and behind his Hinesburg home for his life's work.

Learning to eat

Unlike many animals -- and much like humans -- ravens live in a variety of habitats, from high arctic tundra to mountaintops to the sea. Some birds, such as flycatchers or robins, survive by specializing and eating specific kinds of food. But a raven will eat a variety of foods, from maggots to grubs to woodchucks to eggs to frogs.

Heinrich knows.

He's constantly scrounging for food to feed the growing birds, which can increase their body weight by 50 percent or more per day. The sticker on the bumper of his battered red pickup gives evidence of his passion for scrounging: "This car stops for roadkills."

Six 5-week-old nestlings can eat a hindquarter of a Holstein calf in one day. Another day's meal might consist of two gray squirrels, five frogs, six eggs and six mice. For those who want to know more, there's a four-day menu in the book.

Studying the familiar

What's most engaging about Heinrich's quest is that he explores the world that's found in many Vermont backyards and wild places. He studies ravens that "quork" and circle over Camels Hump, but that's not all. He's monitoring phoebes that nest over the light at his back door. He's investigating wood frogs -- the frogs that chorus from vernal pools along Vermont dirt roads.

"The researcher doing the work -- that is where the fun is," Heinrich said. "When I look at the book, this encapsulates a lot of the experience of doing the work."

Testing and trying

Living in so many different environments, ravens need to be able to distinguish what's edible from what's not. Some of that's learned from watching parents forage, Heinrich knows. Some of it, though, is learned by simply testing and trying. Like babies who constantly put things in their mouths, young ravens are always probing and poking at anything they find as a way to explore their world.

"Some behavior is hard-wired," Heinrich said, like sitting on eggs to get them to hatch. "Where intelligence is involved, there is unpredictability."

However, Heinrich said, if things are always the same, it doesn't make sense to waste time or energy thinking about them. Here's an example: If a bird lays eggs in its nest, does the bird need to distinguish its eggs from other eggs, or even other objects? Or is it so unlikely that any other object would find its way into a raven's remote nest that it would be pointless for the bird to question what's in the nest?

"Ravens have to respond appropriately to things that have been unchanged for millions of years," Heinrich said.

When two of his older birds mated this year and built a nest, Heinrich was set. He painted rocks to look like eggs and put them in the nest before the female bird, Red, laid any eggs. She rolled them out. But once she had laid her own eggs, anything Heinrich put in her nest was left alone.

"She would accept chicken eggs, potatoes, flashlight batteries, rocks painted like eggs, anything," he said of all the items he put in her nest. That made sense to Heinrich, based on what he knew. Then he saw something he'd never seen before.

The baby birds hatched, and then, one by one, they disappeared. There were no predators -- Red was nesting in a chicken-wire fenced aviary. Finally Heinrich figured it out: Red was eating the babies. Heinrich doesn't know why -- he thinks it's because Red was too immature to understand how to rear her own young. She's well-fed in the aviary, and might have matured sexually earlier than she would in the wild. Most ravens nest when they are 3, and Red was only 2, with her mouth lining still pink, colored the way only immature birds are colored. He continues to wonder. Animal cannibalism, particularly parental infanticide, doesn't make evolutionary sense, unless it's an unrelated animal doing the killing.

"I still don't have an explanation," he said.

Having fun

Heinrich attributes part of his lifelong curiosity to his childhood during World War II, living off the land for five years with his familyin an abandoned nature club cabin outside of Hamburg, Germany. Heinrich picked berries, poached trout, looked for beechnuts and acorns, birds' eggs and mice. The family ate everything, and wasted nothing. They caught mice to skin them and sell the skins to natural history museums around the globe. They even collected the fleas off the mice for a British researcher. Then they ate the mice. Heinrich savors fried mice to this day.

Eventually, he found his way to the United States to study zoology. He became fascinated by temperature regulation in sphinx moths, bumblebees and other insects. Part of his doctoral dissertation at the University of California, Los Angeles, involved performing heart surgery on sphinx moths to see how circulation helps insects keep cool.

Heinrich's inventiveness and curiosity drive his research still. It's what keeps him studying ravens, or thinking about why wood frogs breed in tiny ephemeral spring pools (he's put six artificial pools in his back woods), or what exactly happens after a female moth emerges from her cocoon (the males find them within hours he's got a female Polyphemus moth tied with sewing thread to a red oak outside his house, where she mated and laid eggs).

"Detailed observations are important," he said. "You are talking about very subtle differences here. That's what interests me. Looking at the details, you see how complex ... (organisms) are."

For fans like Harvard's Wilson, Heinrich's insatiable curiosity is just what the field of biology needs.

"Bernd is generally regarded, and I heartily agree, to be one of the best naturalists in this country," Wilson said.

Still questioning

The date Heinrich first decided to study ravens is fixed in his mind, like the date of a wedding, or the birth of a child: Oct. 29, 1984. A crowd of young birds was loudly calling in other birds to a moose carcass. Why, he wondered, would they want to share?

"I had no idea I would be working with ravens this long," he said. "I saw them calling in other birds. It was behavior that reminded me of insects and I got going."

Heinrich eventually discovers the reason for their behavior, detailed in his 1989 book, "Ravens in Winter." The birds that call are juveniles. They gather like roving teen-agers who converge at the mall, thus outnumbering any nearby nesting pairs that might want to claim the kill as their own. Along the way, Heinrich has found more and more questions to ask. He's asking questions still. While he spends much of his research time in the woods and watching birds on the wing, he's also arranged his Hinesburg home as an informal laboratory.

He's built two chicken-wire aviaries into the wooded hillside behind his cedar-sided house and not far from the large vegetable gardens and chicken coop. He can watch the birds from the big picture window in the small, wood-paneled study where he does his work. There are paintings and drawings on the walls -- Heinrich illustrates his writings -- and books aplenty. And yes, there's even a copy of Edgar Allen Poe's "The Raven." But there is no computer. Heinrich writes all his books in longhand.

"I thought I would be done with ravens when I wrote 'Ravens in Winter,''' he said from the study as he watched the ravens playing with bath water he put out for them. "But with ravens, there is always something new, all the time," he said. "I am still trying to make sense of ravens."

BOX: A Bernd Heinrich reader

University of Vermont zoologist Bernd Heinrich has written or edited almost a dozen books in the past 20 years. Some are scholarly and some are for the general reader. One is a children's book. Here's a list of his books and their publication date:

"Bumblebee Economics," Harvard University Press, 1979
"Insect Thermoregulation," Wiley and Sons, 1981
"In a Patch of Fireweed," Harvard University Press, 1984
"One Man's Owl," Princeton University Press, 1987
"Ravens in Winter," Summit Books, 1989
"An Owl in the House: A Naturalist's Diary," Little Brown, 1990
"Hot-blooded Insects: Strategies and Mechanisms of
"Thermoregulation," Harvard University Press, 1993
"A Year in the Maine Woods," Addison-Wesley, 1994
"Trees in My Forest," Cliff Street Books/Harper Collins, 1997
"The Mind of the Raven," Cliff Street Books/Harper Collins, 1999