The Audubon Christmas Bird Counts were started in 1900. Conducted annually, these counts provide a tremendous resource for ornithologists and environmental scientists. The CBC program is the longest-running community science bird project in North America.

The CBC data are freely available on request to researchers. Hundreds of ornithological articles have used CBC data to advance our ornithological knowledge. Articles I have published on the winter movements of Red-breasted Nuthatches, Purple Finches, Pine Siskins, Common Redpolls and American Goldfinches would not have been possible without CBC data for all of North America.

The counts provide a way to assess changes in abundance of winter bird populations. To be sure, many species have increased or decreased over the past 122 years since the program began.

A recent paper published by National Audubon Society biologists, led by Sarah Saunders, takes a deeper dive into the North American CBC data. They begin by acknowledging that many species have shown significant increases or decreases. The biologists seek to learn why those changes have occurred.

The team analyzed the impacts of two human-related changes: global climate change and changes in land use/land cover. The former is the undeniable fact that temperature has been on an upward trajectory since the beginning of the Industrial Revolution. Changes in precipitation patterns are also occurring.

For land use changes, one does not need a long memory to realize how rapidly natural habitats are changing in our country. Human development eliminates natural habitats. Forests are cut and invasive plant species may find our altered landscapes to their liking.

Taking the longer view, agricultural land was at its maximum in Maine around 1900.  At least some of this former farmland has reverted to forest.

The authors expected that groups of species are responding to climate change and changes in land use in different ways.  We might expect forest birds to respond differently than waterbirds or grassland birds.

The authors strongly make the point that we need to understand how these two impacts affect birds if we are to develop effective conservation plans.  These plans should not merely address winter abundance but year-round abundance.

This study restricted CBC data to areas east of the 100th meridian (from the Great Plains eastward). CBC coverage west of the 100th meridian was sparse and would have resulted in large areas with no CBC circles. They also used only data since 1930 when CBC protocols were standardized. Altogether, 109 CBC count circles formed the grist for the mill, including several Maine counts. A total of 89 bird species were used. These birds were assigned to one of nine groups. For instance, woodpeckers, waterbirds, and large forest birds.

As a measure of climate change, the authors used the annual average minimum winter (October to December) temperature and cumulative precipitation from October to December for each year.

For measures of land use, the authors used the percentage of land within the circle devoted to urban or agricultural development and the percentage of preferred natural habitat for each species. So, for woodpeckers, the percentage of forest was used.

The statistical analysis showed that climate change affected the geographical distribution of all nine groups of birds.  Land use changes were even stronger for species with specific habitat requirements. Land use changes had twice the effect of climate change for these species.

I was a bit surprised by this result. Knowing that land use changes have stronger effects on abundance than climate change should inform conservation plans going forward, particularly for habitat specialists like grassland birds or waterbirds.

So, the take-home message is that climate change drives changes in abundance and in geographic distribution of the 89 species of birds. Land use changes have at least as strong an effect on generalist species but even stronger impacts for habitat specialists.

Maine is doing well for most groups of species except for waterfowl, which have declined between 1930 and 2020. Shrubland birds, woodpeckers, mixed-habitat birds and shrubland birds have all increased modestly.