Sometime in the early 1950s, Chilmark resident Allan Keith remembers spotting a funny looking bird while out for a walk. Intrigued, he looked the bird up in a book as soon as he got home. A short time later, he spotted a different bird and again pulled out a book to try to identify it. Six decades later, Mr. Keith is recognized as one of the leading birders and naturalists on the Vineyard.

“There’s not a week goes by that I don’t go out and at least spend an hour or two, all year long,” he said. “I’m a little more fanatical about it than most people.”

On Sunday, Jan. 3, several dozen Islanders who share Mr. Keith’s fanaticism will come out to take part in what has become a yearly ritual on the Vineyard and across the country: the annual Christmas Bird Count. And this Sunday, the Island celebrates the 50th anniversary of its version of the count. For Mr. Keith, it will be one more in a long line of bird counts that he has participated in since he coordinated the first official count in 1960.

“It’s like a lot of birdwatching. It’s fun, it’s kind of a game,” he said of the annual birding event. “It’s also a bit of a challenge to go out on a specific day and see how many different birds you can find. In a modest way here, it’s a social occasion. People like to go out. There are a lot of people then for whom this is the one big sort of birdwatching event of the winter.”

The longstanding tradition dates back to Christmas Day in 1900, when ornithologist Frank M. Chapman coordinated a bird count in 25 locations throughout the Northeast. The count spread quickly in subsequent years, but there is some debate about when it actually started on the Island. An article published in a 1951 edition of the Gazette referred to an extraordinary 1932 count in which a crew had included in their count the last living heath hen in the world. Robert A. Culbert, who has been the count compiler since 1993, believes the Island counts date back to the first decade of the 20th century, when a small group held an informal count in Edgartown. But the counts were held only sporadically until 1960, when it became an annual event. Mr. Keith said that was also the first time the event was organized and executed by Islanders; earlier counts were performed by people from the Massachusetts Audubon Society who came from the mainland to explore the bird-rich Island.

The count is still held under the auspices of the National Audubon Society, which drew up national guidelines a little over half a century ago to regulate the fast-growing project. Now expanded throughout the states, Canada, Latin America and the Caribbean, the count is limited to a single day within a two-week period at the end of December. Field areas are limited to a circular area of land no more than 15 miles in diameter.

For the event, the Island is divided into 13 different territories, with a captain and a team of birders assigned to each. People who prefer to count birds from the comfort of their homes are also invited to join what Mr. Culbert called feeder watchers. This year, the groups will assemble at Felix Neck Wildlife Sanctuary at 5:30 p.m. on Sunday to tally their findings.

“Over time and across the huge areas covered by the counts, it’s been found that the results really are meaningful in terms of what the counts tell you about long term trends,” said the Nature Conservancy’s Island program director Matt Pelikan, who has been participating in the annual bird counts for nearly 35 years, and has done about a dozen since he moved to the Island. The count is not a controlled experiment, so results aren’t perfect, but Mr. Pelikan said that the information gathered does help to create a picture of how bird populations and distribution are changing.

Mr. Keith said this is part of the appeal. “I think people have a sense that they are contributing to a quasi-scientific activity. All of the information on the numbers and distribution of birds is very useful, and can be analyzed . . . to study the movement of bird populations,” he said. “For example, some winters we get flights of birds from the north if the food supply in the north is poor, [but] if the food supply is ample in the north, those birds just simply don’t come.”

For avid birders like Mr. Keith, it all makes for a fascinating winter trek through the landscapes of the Vineyard. But as the years pass, the results don’t always bring good news. “Our bird populations have been steadily declining, and we just don’t have as many birds as we used to 50 years ago,” he said. He hesitated to use the term global warming because he said it carries too much baggage, but he and Mr. Pelikan both agreed that changing climate patterns, have had a noticeable impact on Island bird populations.

Still, some numbers in the count remain high on the Vineyard compared to other nearby count areas. “We get a lot more different kinds of birds than you would think,” said Mr. Pelikan. “Typically 120, or thereabout, different species.”

The Vineyard also generally has more participants than other areas in the state. Some take it a little more seriously than others, but the challenge to spot the largest number of species remains a strong motivation to all participants. And not surprisingly, the Vineyard maintains a friendly rivalry with Nantucket to bring back the longest list.

“They usually win,” said Mr. Keith with a laugh. “They import talent from the mainland. They have at least as many field people as we do, and maybe more . . . and Nantucket is just one of those places that attracts a lot of lingering birds in the wintertime.

“They usually find one or two more than we do. That’s just the way it is.”

Better luck this year.