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Drilling Down

Red-headed woodpecker pair at the Cedar Creek Ecosystem Science Reserve.

Photo by Siah St. Clair.

It's hard to believe that we were wrapping up our 2020 red-headed Woodpecker field season just a few short weeks ago. We enjoyed some beautiful fall weather and spent the end of the season collecting data on nest tree density, or tree hardness. The data we collected will help us understand the role of wood hardness and decay in nest site selection by red-headed woodpeckers and whether the availability of decaying trees might be a limiting factor for nesting opportunities. Ultimately, we hope that this information can be used to guide land managers and private land owners working on red-headed woodpecker habitat restoration efforts.

Polypore fungi (Polyporaceae Family) at the Cedar Creek Ecosystem Science Reserve.

Photos by Elena West and Siah St. Clair.

Until fairly recently, methods for quantifying wood density have lagged behind methods for measuring external tree and habitat-level features that are used to understand nest site selection by woodpeckers and other cavity nesters. Past studies have largely used visual indications of wood decay, such as the presence of fungal conks, to classify stages of tree decay. However, these kinds of visual markers can be problematic because nest trees used by woodpeckers do not always show fungal conks, even when wood decay fungi are present. Research has also shown that tree decay classes do not always correlate with wood density. Woodpeckers and other cavity nesters appear to use a variety of decay classes, ranging from entirely live trees with no fungus or defects, to trees in advanced stages of decay, indicating that these types of visual markers are fairly unreliable indicators of nest site availability.

Researchers Kirsti Carr (on ladder) and April Strzelczyk collecting tree density data. Photos by Siah St. Clair.

To measure the hardness of trees we drill a small hole directly above or below a nest cavity. We then thread an increment borer into the drilled hole, and a torque wrench is used to turn the borer into the hole at one centimeter intervals. We then record the torque required to turn the increment borer at each interval. We also collect detailed measurements from each cavity, including dimensions of the entrance hole and cavity depth and information on tree species, the amount of bark present, and diameter of each tree. We collected the same tree-level data from a random selection of trees that were not used by woodpeckers for nesting. The data will be analyzed together so that we can better understand the factors that influence woodpecker nest site selection. Measurements taken from nest trees tended to be higher toward the outside of the cavity (indicating more dense wood), then decrease toward the middle (less dense wood), and then increase again toward the back of the cavity, whereas measurements from trees that were not used for nesting were consistently high along all increments of the drilled hole. Fascinating!

Adult (left) and juvenile (right) red-headed woodpeckers at the Cedar Creek Ecosystem Science Reserve, summer 2020. Photos by Siah St. Clair.

As of early October the majority of the red-headed woodpeckers at Cedar Creek had dispersed or migrated to other locations. This is the first year in the four years of our research that so many individuals, particularly adults, have left Cedar Creek for the winter. This has been an exciting development because it means that the 24 adult woodpeckers that we marked with GPS tracking devices earlier this summer may return next spring, and assuming they can be recaptured, we will be able to remove their tracking devices, download the data, and learn where they traveled during the fall and winter. Understanding more about red-headed woodpecker nonbreeding season movements has been a primary research goal for this project, but is has been particularly challenging to obtain sufficient data from year to year. Perhaps all of our hard work and patience will eventually pay off.

Trail camera placed above a red-headed woodpecker nest at the Cedar Creek Ecosystem Science Reserve, summer 2020. Photo by Siah St. Clair.

In other news, the nine trail cameras that we placed near red-headed woodpecker roosts and nests over the last year will continue to take videos at each cavity during the upcoming fall, winter, and spring. We are currently working to complete a Zooniverse project accessible to the public so that volunteers can help classify what's captured in these videos. The trail cameras have recorded thousands of videos—from red-headed woodpecker nesting behavior to interactions between other animals—and we hope that they will continue to provide interesting and novel insights into the inner-workings of the cavity ecosystem at Cedar Creek. We will be sure to keep you posted once that project is up so that you can participate - it should be an ideal winter activity!

Article by Elena West and Siah St. Clair.


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