Spike Stories: Using Spike to Measure Woodpecker Cavities for Wildlife Studies

Ph.D. Candidate with the University of Idaho Measures Tree Cavity Dimensions with +/-1 cm Accuracy

Jessica M. Stitt, a Ph.D. Candidate with the Department of Fish & Wildlife Sciences at the University of Idaho, aspires for a career focused on the intersection between remote sensing and wildlife biodiversity. As part of her Ph.D. research, Stitt recently published an article in the February 2019 Wildlife Society Bulletin titled “Smartphone LIDAR Can Measure Tree Cavity Dimensions for Wildlife Studies”. In her Wildlife Society Bulletin article, Stitt shares data and results after testing Spike in the field to measure tree cavity dimensions created by woodpeckers in the Pacific Northwest of the United States. 

“I study biodiversity in forest ecosystems. In particular, my Ph.D. project is looking at woodpeckers, the standing dead trees that they put their nest cavities in, and the surrounding landscapes,” says Stitt. To collect the necessary data for her project, she started with airborne LiDAR. Then she went into the field to assess ground plots, count the number of dead trees and cavities, and conduct bird surveys to see if she could link the forest structure with the patterns of woodpecker use. As part of her field data collection, Stitt needed to measure woodpecker cavities.

“Woodpeckers are important because they are ecosystem indicators. They often nest in the tree cavities they make for just one year, and then make new cavities the next year. When a cavity is abandoned by woodpeckers, it can be taken over by a lot of other animals like owls, chipmunks, or bats. So the size of the cavity is crucial for understanding what secondary cavity users are out there in the forest,” adds Stitt.

The safest and most effective way to identify and measure cavities is from the ground. Stitt explains, “they are often in standing dead trees which are rotting from the inside out, and have been hollowed out by woodpeckers. Climbing them is not a good idea.” Determining the cavity opening size is also important because it can be a proxy for the type of woodpecker species.

Measuring the dimensions of these tree cavities is a big challenge from the ground. One option Stitt previously used is a tree ladder, which requires carrying several heavy sections and a harness, and then physically climbing the tree to measure the cavity entrance by hand. However, this solution has limitations, including safety risk and access to cavities that are higher off the ground. 

Another solution used in the industry is a cavity camera, which requires a bulky telescoping pole, making it hard to get the pole up and around the tree branches. Measurement is also tricky with the camera, because the diameter of these cavity entrances is within centimeters.

Stitt first learned about Spike in early 2016 from one of her advisors and co-authors, Dr. Lee Vierling, and she immediately purchased a Spike to pair with her iPhone. Before implementing Spike on her field data collection project, she wanted to run tests mimicking what she would see and experience in the forest. For example, in the field you can’t always get straight on to the cavity due to branches and other trees. It was this test that became the basis of her Wildlife Society Bulletin article. 

As part of her Spike tests, Stitt wanted to “see which extreme angles, heights, and distances allowed us to capture, with relatively high accuracy, the dimensions of a cavity”. Stitt worked with the logger sports club at the University of Idaho, because they had a tall climbing pole. She had the logging team climb up the pole and tape three sets of felt, dummy cavities with 4 different sizes at varying heights. 

She then captured photos and measurements with Spike from three different distances and three different oblique angles. In total, she measured 294 vertical and horizontal diameters of cavity entrances across all the combinations of distance, height, angle, and cavity size. 

Stitt found that “Spike was highly accurate, intuitive, and easy to use. I was able to measure openings as small as 3×3.5 centimeters, and measurement error for both vertical and horizontal diameters of cavity entrances was <1 centimeter on average.” 

After completing her Spike testing of the dummy cavities and seeing that Spike was robust enough for her field work, Stitt frequently brought Spike out into the field with her to collect true cavity dimensions. She got a side camera pack to carry Spike over her shoulder, and used Spike to collect data that is now part of the analysis that she’s working on for her dissertation.  

In addition to being accurate, Stitt found that Spike minimized the equipment she brought into the field, because she no longer needed the tree ladder, other measuring gear, or a harness. “Spike is still in my pack after 3 years” says Stitt. 

Some of the additional results that Stitt and her co-authors shared in their Wildlife Society Bulletin article on Spike include*:

• Spike proved to be a low-cost, portable technology that can noninvasively measure structures that are small and difficult to access.
• We consider Spike to be a precision tool, better applied to focusing in on specific structures rather than characterizing large areas. In fact, use of Spike may dovetail nicely with TLS or other laser-based or drone-based surveys, with the user able to supplement a data set with Spike photos of cavity entrances or other inaccessible structures accurately and opportunistically.
• Plus, as a non-invasive method of sampling, Spike could be safe for wildlife and other uses beyond the characterization of tree cavity entrances. The accuracy at small spatial scales (3-12 cm in length) suggests that tools like Spike could be applied to larger structures and obtain accurate results, proving useful for vegetation assessments, including the monitoring and measurement of additional habitat features.

*Source: Wildlife Society Bulletin by The Wildlife Society. “Smartphone LIDAR can measure tree cavity dimensions for wildlife studies”. First published 28 February 2019. Authors: Jessica M. Stitt, Leona K. Svancara, Lee A. Vierling, Kerri T. Vierling.