Every year since 1900, volunteers have censused birds in the early winter all across North America under the auspices of the National Audubon Society. At this writing, well over 1500 counts are conducted every year all over the U.S.A. and Canada and at a few other locations in the New World. To the author's knowledge, there is no other wildlife population study in the world with such a long history.
Compared to other census methods, the Christmas Bird Counts have a relatively informal structure:
The energy that drives this institution is not purely scientific: other motivations include the desire to find rarities and to socialize with other birders. Despite its many limitations, though, the Christmas Bird Count can still have great value as a barometer of overall population trends. Researches have found that population trends reflected in CBC data tend to correlate well with those from censuses taken by more stringent means.