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The Christmas Bird Count: Why We Count Birds in the Winter

Updated: Dec 20, 2023

I recently signed up to be a leader for a block on the Christmas Bird Count (CBC) organized by Denver Field Ornithologists (DFO). This was my first time leading a CBC, as the dates never worked out for me previously with holiday travel conflicts. As I’ve gotten more serious about birding over the years, I’ve come to understand the importance of getting involved in annual bird surveys. But why pick one of the coldest months of the year, when many survey areas seem relatively depauperate in avian diversity?      

Christmas Bird Count volunteers Evan and Martha survey the roads in a rural block in the Rocky Mountain foothills southwest of Denver.


In the late 19th and early 20th centuries, North American hunters participated in what was known as the Christmas “side hunt.” This annual tradition required hunters to choose a team and compete against rivals to return with the greatest number of hunted animals possible. As unregulated hunting began to take its toll on wildlife, the conservation movement grew swiftly. In 1900, Frank Chapman, bird curator at the American Museum of Natural History and author of the magazine Bird-Lore, suggested that we replace the Christmas side hunt with a “Christmas Bird Census,” where birds are counted instead of hunted.

Frank Chapman asked the participants of the first Christmas Bird Counts to submit their observations to his regular publication, Bird-Lore.

What is the Christmas Bird Count?

The Christmas Bird Count, or CBC, is an annual survey that aims to record all bird species seen and heard within a given date, time, and location. It operates with the help of many unpaid volunteers and is considered the gold standard of citizen science projects to which all other citizen science projects are compared.

The standardized approach is to run the survey within one of several blocks within a 15-mile diameter circle. One 24-hour period is selected between December 14th and January 5th and the observers count by foot, car, boat, snowmobile, or by watching feeders or owling. All data are reported to compilers who consolidate them and enter them into a database. The data are later reviewed and confirmed by regional editors.

A Black-capped Chickadee alights on a branch near the roadside patch on our CBC survey.

The Christmas Bird Count Today


The CBC is the longest running wildlife census in the world. In addition to North America, it is conducted in the Caribbean, Central America, and South America. Many different conservation organizations participate in the CBC today, most notably the National Audubon Society and it local chapters. The data that are collected are used in addition to the annual Breeding Bird Survey (BBS) done in June to monitor the long-term status of bird populations around the country. These numbers help us determine the relative abundance – how many we find now compared to an earlier date – of different species.

We didn't find Steller's Jays on our CBC until things warmed up a little bit after 8:30 am. We wondered if the reduced activity in the morning had to do with the excessive cold temperatures.


One of the more exciting aspects of doing a bird count in the winter is the possibility of documenting irruptions of certain species. Last year in Denver, we experienced huge flocks of Bohemian Waxwings (Bombycilla garrulus), a bird whose winter range typically extends no farther south than Wyoming, Idaho, and northern Nebraska. When birds experience low food availability, they must travel farther for access to new food resources. There is research that shows that irruptions are more common in years with high reproductive success which can lead to more competition between first year birds and less availability to food. Before last year, the most recent irruption of Bohemian Waxwings in the Denver area was in 2008, according to eBird.


The Bohemian Waxwing has rich rufous-colored undertail coverts and is larger than the Cedar Waxwing, which has white undertail coverts (see photo below). I took this photo in the parking lot in front of my house when I heard a huge flock of birds randomly one afternoon last winter.


This Cedar Waxwing was mixed into the flock of Bohemians and American Robins right in front of my house in the Denver metro area.

Other species of birds that are known to irrupt some winters are both Evening and Pine Grosbeaks, Common and Hoary Redpolls, Pine Siskin, Black-capped and Boreal Chickadees, Red-breasted Nuthatch, and Clark’s Nutcracker. Among raptors, Rough-legged Hawk, American Goshawk, and Snowy, Great Horned, and Short-eared Owls are known to irrupt.

The Evening Grosbeak is a species well known for its irruptive tendencies. Some winters it can flock to feeders by the hundreds and other winters it is nowhere to be found.


CBC data are particularly helpful in teaching us about how birds are responding to climate change. After all, the value lies in what the data are telling us. Are certain species staying around longer? Are partially migratory species now becoming resident? Are altitudinal ranges changing for certain species?

CBC data have been used in several important reports, including Audubon’s 2014 Climate Change Report and the Environmental Protection Agency’s 2012 report. They are continually used by scientists to monitor ongoing changes in populations. For example, CBC data indicate that Bewick's Wren numbers have declined precipitously east of the Mississippi since 1976. Rusty Blackbird numbers have been declining 10% per year since 1965. In more positive news, CBC data shows a steady upward trend in Bald Eagle populations across the United States after the banning of DDT in 1972.

Analysis of four decades of CBC observations reveals that birds seen in North America in the first few weeks of winter are gradually moving farther north. In fact, significant northward movement occurred in 58% of observed species (157 of 305), with an average distance of 35 miles northward.


Even if it isn’t an irruption year and the birding is slow, the CBC is the perfect way to get to know other members of the surrounding birding community. Chances are, you will not know many of the people who volunteer to participate in your block. Use this as an opportunity to make friends and learn from people who may have a lot to offer in terms of their birding knowledge or photography skills. Many CBCs offer a compilation potluck or “tally rally” of some sort. I was unable to attend mine this year, but next year I will make an effort to be there.


So Why Winter?

No, it's not just meant to make you freeze your butt off outside in a time when the warmth of your home seems all too inviting. In this day in age when we are seeing dramatic bird declines, counting birds is a non-consumptive and sustainable way to enjoy our wildlife resources. It often gets us out to explore an area that is new and unfamiliar to us. It may give us an opportunity to see irruptive species that otherwise would not be present and it provides meaningful data that scientists use to make important management decisions. Most importantly, it brings the birding community closer together and allows for bonding that can kindle spirited friendships.

The Brown Creeper is a master of camouflage that can be found on winter bird surveys in Colorado.

How to Get Involved

The best way to get involved in a CBC is to contact your local chapter of the Audubon Society. From there, the team will get you connected to a nearby compiler who organizes a particular circle. Don't worry, you don't need to be an advanced birder to participate! This is a great opportunity to become a better birder as you set out alongside people with a wide variety of skill levels.


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