On Monday, Allan Keith was out birding at the Sheriff’s Meadow Sanctuary in Edgartown looking for ducks on the pond when he saw a huge bird overhead. It turned out to be a golden eagle, a rare visitor to the Island.

How rare?

“Probably about 10 sightings in the last 200 years,” Mr. Keith said.

In the birding world, forming a positive identification is a precise business, especially when out birding alone and no photograph accompanies the sighting. But Mr. Keith has 70 years of birding experience, having started the lifelong passion at age 13.

“I was walking home from school one day and saw this funny looking bird and went home and looked it up,” he said of his birding origins.

On Monday he was able to quickly narrow the identification possibilities due to the enormous size of the bird.

“A golden eagle’s wing span is five and half to six feet and there are only two other birds of that size around here: turkey vultures and bald eagles,” he said. “Turkey vultures are routine here, and their wings are in a dihedral or V-shape. A bald eagle holds its wings very level like an old barn door going by in the sky, absolutely flat. But a golden eagle has a slight dihedral, you have to wait for it to soar a bit to see it. Also, a turkey vulture has a very small head and beak, whereas a bald eagle has a huge head and bill that sticks out in front. A golden eagle doesn’t have the big bill but it has a fair-sized head.

“This was a classic golden eagle and it was an adult, with no white on its plumage.”

Golden eagles are more prominent in mountainous areas, both in the West and on the East Coast. Usually when migrating south for the winter, the eagles stick to the hilly areas of New Hampshire, Pennsylvania and New Jersey, riding the ridge lines where the warm air flow collides with the hills and is forced upward, Mr. Keith said.

“They ride this cushion of rising air, following the direction of the hills and mountains without having to flap much,” he said. “They don’t tend to go out on the coastal plain.”

When Mr. Keith saw the golden eagle it was soaring above him, “on its way somewhere.”

Mr. Keith has been coming to the Vineyard since 1943 and has lived full time in Chilmark since 2000 on the Keith Farm. Usually at this time of year he birds farther up-Island in Aquinnah, when he has the time.

“I have a 30-acre farm to take care of,” he said. “That’s a lot of time on the tractor. But when I’m out I’m always looking for birds.”

To see a golden eagle up close head to the Chilmark Library, home to a stuffed golden eagle shot in 1906 by Robert Flanders at the Brickyard in Chilmark.