This winter marks the 117th year of a scientific holiday tradition — the National Audubon Society’s annual Christmas Bird Count. Every year beginning on Dec. 14, thousands of birders around the world, organized into local bird-watching “circles,” head out to count and record millions of bird sightings. The count will end Jan. 5, after which the bird-sighting data will be compiled, edited and entered into the Christmas Bird Count database.

The count is an enormous undertaking in citizen science. Last year, about 60,000 people in the U.S. participated in nearly 2,000 circles, logging about 54.5 million bird sightings.

COUNT YEAR COUNT NUMBER OF CIRCLES NUMBER OF COUNTERS 1909-10 10th 117 183 1919-20 20th 147 319 1929-30 30th 185 605 1939-40 40th 258 1,801 1949-50 50th 392 4,399 1959-60 60th 557 7,475 1969-70 70th 821 13,049 1979-80 80th 1,180 29,920 1989-90 90th 1,377 30,397 1999-2000 100th 1,568 42,282 2009-10 110th 1,735 48,045 2015-16 116th 1,902 58,493 The number of bird counters in the U.S. has grown steadily Source: NATIONAL Audubon Society

And all of those birders aren’t simply wandering the countryside at random and counting birds willy-nilly; the count is more of a carefully orchestrated bird census. Each circle of participants counts within an area with a 15-mile diameter, determined by the longitude and latitude of an approved centerpoint. A circle cannot overlap or abut nearby circles, and once a circle is established and approved by the Audubon Society, it cannot change location from year to year. The Audubon Society recommends that there be at least 10 birders in each circle, counting for all daylight hours on one day during the three-week period (circles select their count day).

About 650 bird species were sighted in the U.S. in each count in the past decade. In every one of those years, the same bird has topped the count list: the red-winged blackbird. During the 2015-16 Christmas Bird Count, there were 8.9 million sightings of the red-winged blackbird; that’s more than twice the number of sightings of the runner-up, the snow goose.

COMMON NAME COUNT Red-winged blackbird 8.9m – Snow goose 4.2 – European starling 3.4 – Common grackle 2.4 – Canada goose 2.2 – Brown-headed cowbird 1.6 – Mallard 1.4 – American robin 1.1 – American crow 1.0 – Ring-billed gull 1.0 – Laysan albatross 0.9 – American coot 0.7 – House sparrow 0.5 – Rock pigeon (feral pigeon) 0.5 – Lesser scaup 0.4 – Most abundant U.S. birds in the 2015-16 Christmas Bird Count Source: NATIONAL Audubon Society

But a count is not the same thing as a population estimate. Red-winged blackbirds are one of the most populous birds in the U.S. (bird populations are difficult to determine; an estimate by Partners in Flight, a network of organizations interested in bird conservation, puts the red-winged blackbird population at 99 million in the U.S. ) But they are not as dominant as their No. 1 ranking in the Christmas Bird Count suggests. The American robin and the blue-gray gnatcatcher are estimated to have even larger U.S. population sizes (160 million and 110 million, respectively, according to Partners in Flight). The red-winged blackbird, however, has some advantages when it comes to showing up in the Christmas Bird Count: Their range covers the whole of the continental U.S., and they often flock by the thousands. Birds of a feather and all that.

COMMON NAME NUMBER OF TIMES IN TOP 15 American coot 10 American crow 10 American robin 10 Canada goose 10 Common grackle 10 European starling 10 Mallard 10 Red-winged blackbird 10 Ring-billed gull 10 Snow goose 10 Brown-headed cowbird 9 Rock pigeon (feral pigeon) 9 Laysan albatross 7 Northern pintail 7 House sparrow 5 Tree swallow 4 Lesser scaup 3 Brewer’s blackbird 2 Mourning dove 2 Great-tailed grackle 1 Long-tailed duck 1 Number of times a bird has been in the top 15 most-sighted in the past 10 Christmas Bird Counts in the U.S. Source: NATIONAL Audubon Society

The red-winged blackbird isn’t the only bird that regularly makes it onto the most-counted list in the U.S. Over the past 10 years, 21 bird species have made the top 15 in at least one year. Some, like the ubiquitous American robin, are known across the country, but others with high counts may be unfamiliar in many areas. The Laysan albatross, for example, has been in the top 15 in seven of the past 10 years but is found mostly off the California and Alaska coasts, with breeding grounds in Hawaii.

The Christmas Bird Count is remarkable not only for its size and organization but also for its longevity. Kathy Dale, who is the director of science technology for the National Audubon Society and helps manage the Christmas Bird Count database, said it was the first bird-related citizen science project of its kind in the U.S. Although a given year’s count can be affected by factors like weather and participation, the long-running nature of the count allows researchers to observe trends in the ranges and abundance of birds over time. Data from the count is used in research on climate change and conservation. And for many participants, it has become as much a part of their holiday traditions as gatherings with relatives or gift-giving. Dale has been taking part in the bird count for 35 years. “It brings people back year after year,” she said. “It is a very special event.”