Woodpeckers and other excavators build a legacy of diversity

This blog post is provided by Kurt Trzcinski and Andrea Norris and tells the #StoryBehindThePaper for the paper “Woodpeckers and other excavators maintain the diversity of cavity-nesting vertebrates”, which was recently published in the Journal of Animal Ecology. Featured image of a female downy woodpecker by Andrea Norris.

In 1995, Prof. Kathy Martin and a handful of students launched a study to examine the effects of forest management activities on forest birds and small mammals, with a special emphasis on the tree cavity-nesting community near Riske Creek, British Columbia, Canada. For the next 16 years, students and post-docs would spend their summer waking before dawn to follow the same carefully placed point count stations through the dewy mixed deciduous-coniferous forest sites. Late mornings and afternoons were spent finding and following cavity-nesting species in search of nests, which were marked and monitored until the end of the breeding season.

Each year Kathy, new and old students, and a growing number of collaborators (54 people over 17 years) would return to Riske Creek, conduct point counts, find new nests, monitor the use of previously used cavities, and document changes in habitat. The hard work of following cavity-nesters to a nest tree, climbing a ladder, adjusting a mirror and flashlight at the perfect angle, being careful not to fog the mirror with labored breathing after hauling a ladder through the forest was rewarded with the excitement of finding a clutch of eggs or chicks.

We made many discoveries along the way, ranging from the detailed nesting ecology of the cavity nesters in the region, to the characteristics of their host trees and the architecture of their cavities. We found both differences and similarities between species. Many species showed differences in their preference for cavity size, but showed significant overlap in tree species preference. In short, the cavity nesting community shows complex interactions among its members when it comes to the creation of nest sites by excavators (woodpeckers and nuthatches), competition for nest sites among secondary cavity nesters (SCNs: species that do not excavate cavities but need them for reproduction), and predation, as some species (e.g., owls, raptors, and squirrels) feed on others.

What links this community together is the creation of tree cavities and their subsequent use. For a given cavity, species can be linked from tree host species, to excavator, to secondary cavity nesters (Fig. 1).

Fig. 1. Diagram of the creation of cavity #231 and its subsequent use at our study area in central British Columbia, Canada

After years of working in fisheries science, Kurt drew a link between fisheries stocks and the production of tree cavities and the diversity of SCNs. He capitalized on the detailed information on tree cavity creation and the “survival” of cavities until a tree blows down or the cavity becomes unusable, to conceptualize tree cavities as a “stock” whose dynamics can be modeled (Fig. 2). Following this analogy, cavity creation (“recruitment” of new cavities) can be modeled as a function of excavator abundance (Fig. 3), which has direct implications for SCN diversity and abundance.

Fig. 2. A conceptional model of cavity production, loss and use by secondary cavity nesters. Photo credit Stephen Shunk.
Fig. 3. On average, the more excavators at a site the larger the standing stock of tree cavities, showing that excavators leave a diverse “legacy” of cavities at a site.

It has long been debated whether cavity availability limits populations of secondary cavity-nesters. To test the hypothesis that cavities limit populations of SCNs in our study system, we compiled longitudinal data on nesting within cavities over 16 years, and calculated the number of cavities created by excavators (woodpeckers and nuthatches) and the subsequent diversity and abundance of SCNs in the following years at 25 sites in central British Columbia, Canada. We found strong evidence for cavity limitation and were able to separate the effect of cavities from significant effects of several environmental factors that also affected SCN numbers, such as food, habitat features, and host tree abundance. The importance of excavators to the overall cavity using vertebrate community were evident to researchers in the field within the first two years of the study. We quickly learned that returning to cavities made by woodpeckers and nuthatches in the following year would lead us to our daily jackpot of new nests. Our long-term analyses showed that excavators create a “legacy” of diversity at a study site, which was consistent over the 17 years of the study and persisted through major disturbances such as forest insect outbreaks, fires, and forest harvesting. Just like the excavators leaving a legacy of nest sites, we passed down our tips and tricks for navigating sites and finding nests, leaving our own legacy of nest-finding skills to new field crew members.

For conservation practitioners in ecosystems in which excavated cavities account for the greatest proportion of nesting holes (e.g., North America), these results demonstrate that the maintenance of the whole excavator community is critical in the conservation of resilient secondary cavity nester communities. Whether these findings can be extrapolated to other ecosystems and communities in which excavated cavities account for a much lower proportion of nesting holes (e.g. South America) remains to be seen, but nevertheless our study will be of great interest to researchers studying cavity-nesting birds, and as a study of population limitation, to population biologists broadly.  

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