Sunday, May 19, 2013
By HERB WILSON
One of the joys of Maine is seeing or hearing a Common Raven. These massive birds belong to the perching birds, or passerines, the largest order of the birds.
All passerines have three toes pointing forward and one pointing backward, adapted to hold on to a branch or other support for perching. We normally think of perching birds as small creatures like chickadees, goldfinches and sparrows. With a wingspan of more than four feet and a weight of 3 pounds, the Common Raven is the biggest perching bird in the world.
The Common Raven is widely distributed in the Northern Hemisphere, ranging from north of the Arctic Circle to the mountains of Central America. The black plumage and large size of these birds make them conspicuous, particularly in winter. The raven figures prominently in the folklore and mythology of many northern peoples. Many early peoples associated the raven with death. This association is perpetuated in Edgar Allen Poe's well-known poem "The Raven."
Although the Common Raven is larger than a Red-tailed Hawk, ravens lack the strong bill and long, sharp talons to kill their own prey. As a result, ravens depend on carrion for most of their food. In the winter, when a dead animal is likely to be frozen, ravens have difficulty in opening the corpse to feed. They must depend on coyotes and other scavengers to tear through the skin.
The behavior of Common Ravens is winter has been the focus of a decades-long study by Dr. Bernd Heinrich of the University of Vermont. Heinrich has a camp near Mount Blue, where this project is based.
The research began one October many years ago when Heinrich observed a small group of ravens that had just located the carcass of a moose. This moose represented many meals for the ravens, enough to keep them well-fed for many months. Oddly, the ravens were quite loud, giving penetrating calls that resulted in other ravens coming to the carcass. Why would the ravens that found the moose want to share their feast?
Heinrich describes his research in the book "Ravens in Winter." The book reads like a biological detective story as Heinrich seeks to understand why ravens would show apparent unselfish behavior
The paradox was resolved only after Heinrich began to capture ravens and tag them with numbered plastic wing tags. This tagging allowed him to recognize individual ravens from day to day.
This biological mystery was solved by first noting that a mated pair of ravens -- they pair for life -- defend a territory throughout the year, not just in the summer as most of our perching birds do. This territorial pair defends their turf vigorously and will chase off any intruding raven.
From this observations of banded birds, Heinrich realized that the ravens that called to alert other ravens to the presence of an animal carcass were unmated juveniles. These birds did not have a breeding territory and wandered widely.
If a wandering juvenile located an animal carcass in the territory of a mated pair, the territorial birds would soon locate the intruder and the food and chase the intruding raven off. That observation is the key to the story. advertising the presence of the carcass to other juveniles, the number of juveniles at the food overwhelms the territorial pair. The mated pair cannot chase off all the juveniles. As soon as one is scared off, others are back feeding on the carcass.
Behavior that appears at first to be unselfish is, in fact, self-serving. If a juvenile raven found a carcass and kept quiet, it would be evicted as soon as the territorial ravens found it. recruiting other juveniles to the carcass, the juveniles can have the food for themselves because their numbers prevent the resident pair from chasing the juveniles away. It is only by sharing that the juveniles can be assured of a feast.
Sadly, the book is now out of print, but it is commonly available from used book sellers. However, a more recent book, "The Mind of the Raven," is still in print. In this book, Heinrich explores the remarkable intelligence of ravens. Ravens use tools and have remarkable abilities in problem-solving. Their vocal communication is complex.
Years ago, researchers described Common Ravens repeatedly sliding on their backs down a muddy embankment. The authors could not explain this behavior except as a form of entertainment. You have to love birds that like to play!
Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at: