The 119th Audubon Christmas Bird Count is now underway, continuing until January 5. I’ll report on the highlights of Maine counts in the next few posts.
The Christmas Bird Count (CBC) was conceived by Frank Chapman, an ornithologist at the American Museum of Natural History, shortly before the turn of the 20th century. In the magazine Bird Lore (now Audubon magazine), Chapman proposed an alternative to the barbaric side hunts that we common in the late 1800s. In a side hunt, villagers divided up into teams and then scoured the countryside, shooting any bird seen. The carcasses would be piled up and the team with the most birds killed would be declared the winner of the side hunt. You can imagine the carnage.
Chapman suggested people might better put their energy into counting rather than killing birds. Some Bird Lore readers agreed and 27 people conducted the first CBCs in 1900. The CBC program took off and continues to grow.
A count is standardized. A group counts all the birds seen on one day between the middle of December and early January within a circle 15 miles in diameter. The number of observers is recorded so that the counts can be adjusted for effort.
Over 70,000 counters participate now in nearly 3000 count circles. Counts are done in a number of countries around the world. The standardized protocol means that counts can be meaningfully compared among count circles as well as between years.
The CBC has become a valuable tool to track changes in the distribution and abundance of the birds of North America and beyond. The National Audubon Society makes the data available to researchers. Dozens of scientific articles have been published based on this rich database. I’ll describe three of my own papers as examples of the value of CBC data.
The practice of bird feeding has been on the increase since 1970. We know that bird feeding can facilitate range expansions and alter habitat preferences. Some have argued that the increase in bird feeders is allowing irruptive finches to overwinter at higher latitudes if supplemental food is available, courtesy of humans. I used CBC data to test for this short-stopping effect.
I analyzed the CBC data from eastern North America between 1970 and 1994. Although I found that the southern extent of irruptions for six species of northern finches was further early in the study period, I was unable to show an increase in the number of birds wintering at higher latitudes as the short-stopping hypothesis requires. My analysis indicates that short-stopping is a weak effect at best.
Using CBC data from 1960 until 2014, my wife Bets Brown and I, analyzed the abundance data for Red-breasted Nuthatch to examine the irruptions of this species on both geographic and temporal scales. We found that this widely distributed species showed broad synchrony in irruptions across North America. Irruptions west of the Rocky Mountains were generally accompanied by irruptions in the Midwest and along the eastern seaboard. At finer geographic scales, we found great variability. Abundance might be high in one state or even CBC circle but low in an adjacent state or CBC circle. Their winter abundance is a patchwork of high and low densities.
We compared the nuthatch pattern to that of three irruptive finches: Purple Finch, Pine Siskin and Common Redpoll. We found no relationship between Red-breasted Nuthatch irruptions and the irruptions of the three finches.
One of my students, Paul Dougherty, and I used CBC data to test for synchrony of the irruptions of northern finches. We know that Common Redpolls commonly show a biennial pattern to their irruptions. We found that American Goldfinches in the northern parts show a biennial pattern opposite that of the redpolls. In the north, redpolls and goldfinches alternate their irruptions.
If interested, you can download copies of these papers at: http://web.colby.edu/whwilson/publications/