John V. Dennis, 86, a biologist, ornithologist and botanist who wrote handbooks for homeowners on how to attract birds to their gardens and guidebooks for beachcombers on the seedlings and plants that float in with the tides, died of cancer Dec. 1 at home in Princess Anne, Md.

Mr. Dennis was a freelance ecologist and conservationist, an enthusiastic outdoorsman and a passionate bird-watcher. In 1948, he journeyed to the remote mountains of Oriente Province in Eastern Cuba in search of the rare ivorybill woodpecker, which had been thought to be near extinction. He found a nest and three adults of an ivorybill subspecies.

Eighteen years later in the Big Thicket region of Eastern Texas, he had what he described as "my only good look at a North American ivorybill," but this sighting was never recognized or verified by ornithological organizations.

On the windswept moors of Nantucket, he netted and banded migratory birds. He explored the flora and fauna of the Congary Swamp in South Carolina. While serving in the Army Signal Corps during World War II, he went birding in the rugged mountains and valleys of Western China.

His best-known book was "A Complete Guide to Bird Feeding," published in 1975. It includes information on how to select and locate bird feeders, how to protect bird feeding sites from predators, how to use the planting and topography of a garden to attract birds and how to select, prepare and set out bird food. It was said to have helped make bird feeding popular among suburban Americans.

Once begun, a bird feeding program should not be halted in a time of scarcity, such as winter, Mr. Dennis warned in his book. "Many birds become so dependent upon our bounty that they would not be able to manage on their own if we suddenly discontinued our feeding at a critical time of the year," he wrote.

Mr. Dennis was born in Princess Anne and grew up in Washington. His father died when he was a teenager, and for a time he helped his mother operate a boardinghouse in Washington. Later he attended George Washington University. During World War II, he was a radar technician with the Flying Tigers aircraft unit in Yunnan, China. After the war, he graduated from the University of Wisconsin, where he studied political science. It was at Wisconsin that he began to develop an interest in ornithology. A friend used to wake him early in the morning to go bird-watching, and Mr. Dennis found that he liked it.

He received a master's degree in botany at the University of Florida and started work on a doctorate in ornithology at the University of Illinois. For most of the next 25 years, he lived part of the year in a stone farmhouse on the banks of the Potomac River near Leesburg and on Nantucket, where he taught school for a brief period.

He did contract work for the Smithsonian Institution and the Nature Conservancy. In the 1960s, he invented a woodpecker repellent to protect wooden telephone poles. He had a contract with the American Petroleum Institute to study the effects and origins of oil deposits on Florida beaches. He traveled extensively in the United States on a variety of nature- and conservation-related projects.

With C.R. Gunn he wrote the "World Guide to Tropical Drift Seeds and Fruits," in 1976, which, in effect, is a catalogue of the material sometimes known as "sea beans" that washes up on beaches around the world, sometimes after drifting on ocean currents for up to 15,000 miles. From this material, scientists can calculate the direction and distance of ocean currents, and they can often glean cultural and anthropological information.

Another of Mr. Dennis's books, "The Great Cypress Swamp," was published in 1988.

About 1975, Mr. Dennis returned to Princess Anne, and he had lived there since.

Survivors include his wife, Mary Alice of Princess Anne; three children, Louisa Dennis Wilking of Concord, Mass., Laura Dennis of Salisbury, Md., and John V. Dennis Jr. of Ithaca, N.Y; a brother, Alfred P. Dennis of Leesburg; and six grandchildren.