The Relationship Between Cavity Nesters and Forest Fires in the Yukon Boreal Wilderness
Species of woodpecker such as the Black-backed (Picoides arcticus), the American Three-toed (Picoides dorsalis), Hairy (Leuconotopicus villosus), and Yellow-bellied Sapsucker (Sphyrapicus varius) breed in recent forest fire burns in the Yukon boreal. In these stretches of blackened, mostly dead pine, spruce and aspen stands, woodpecker activity also encourages other bird species to rear young. I spent the summer as a Field Technician for Wildlife Conservation Society’s Northern Boreal Mountain Program in the Yukon researching their unique ecological niche. The field work entailed data gathering as part of a sustainable forestry research project to highlight the wildlife value of dead wood in recent burns.
An important component of the fire burn ecosystem is the presence of insects, especially the beetles (e.g., bark beetles, wood-boring beetles) that move into burn areas, sometimes even when the forest is still smouldering (Cooke et al., 2019). Most nights, we fell asleep to the sounds of incessant rhythmic chewing right outside our tents. This mass of beetle protein sustains Black-backed, Hairy and American Three-toed woodpeckers over the short breeding season in the boreal forest. Data sheets from 351 trees showed that the most common trees selected for beetle foraging were black spruce (Picea mariana), white spruce (Picea glauca) and lodgepole pine (Pinus contorta). Some Yellow-bellied Sapsucker foraging was also found on mature quaking or trembling aspen (Populus tremuloides). Black-backed woodpeckers were observed foraging on fallen spruce and pine trees within large areas of debris, strengthening the wildlife value of fresh burnt tree-downfall. Dead wood is usually salvaged for fuel but leaving some behind will support woodpeckers during the high energy-demand spring and summer periods.
The nest cavities that woodpeckers drill into dead or broken-top trees are reused by a myriad of secondary cavity nesters, such as swallows, bluebirds and even rodents like flying squirrels. Secondary cavity nesters are unable to form their own cavities within the hard pine exterior, so woodpecker abundance directly impacts their population size. During the critical bird-breeding period in late May, when we started our field season, we saw this in action in the form of the Mountain Bluebird (Sialia currucoides) and Tree Swallow (Tachycineta bicolor) tussling over limited availability of cavities.
In a large patch of high-severity burned spruce and pine, a pair of Yellow-bellied Sapsuckers had excavated a cavity into a mature and dead, broken-top aspen which had many fungal fruiting bodies. Fungus, like heart rot, is common in burns as it takes advantage of the plentiful dead trees (Horvath 2020). Its presence shows that the interior of the tree is soft and therefore easier to drill. Interestingly, we observed that the Northern Flickers (Colaptes auratus) also selected rotted mature aspen to create their cavities, whereas Black-backed and American Three-toed woodpeckers selected the more resistant spruces and pines. Fungus and the excavation of cavities by woodpeckers also aid the decomposition process by breaking down wood and releasing nutrients back into the soil.
As well as woodpeckers, aerial insectivores (birds that feed on insects while flying) are especially attracted to recent forest burns. Common Nighthawks (Chordeiles minor) are one example of aerial insectivores who are declining throughout North America. The reasons for their decreasing numbers are unknown, but it is possibly because of human disturbance. Reassuringly, we saw and heard this species multiple times in the scorched forest stands although reportedly the nighthawks are common throughout the North. The forest burn ecosystems can attract Common Nighthawks because they provide open areas for nesting and large populations of flying insects for food, both common traits of recently burned forests. Generally, forest burns have not been treated with pesticides or herbicides, providing opportunity for the Common Nighthawk populations to recover (Environment Canada 2016).
We used different techniques to survey woodpeckers and their cavities. In late May, a FoxPro (a piece of hunting equipment that plays various wildlife game calls) projected woodpecker drumming and calling into the woodland in the hope of receiving a reply. Woodpeckers are active at this time of year, drumming regularly to stake out their territories against rivals. The drawback in using audio is that it can coax a pair of woodpeckers away from the cavity we are attempting to find. To listen for woodpeckers without disturbing them, we walk slowly and quietly through the wood, listening for the tell-tale ‘picking’ sounds of a woodpecker tapping at tree bark in the search for insects. Once found, we watch and follow the woodpecker in hopes that it would fly to their cavity.
From observations in the field, Yukon trees are consistently smaller and thinner than trees in southern coastal BC, likely because of the shorter growing season and cooler climate. Consequently, cavities are lower, sometimes only half a metre above the ground, which aided our search for nest cavities. A round, blackened tree nub viewed from a distance sometimes tricked us into thinking there was a cavity.
Once the woodpecker nestlings became more vocal around early to mid June, cavity hunting was more straightforward. Kicking a tree or scratching the bark gently near the cavity entrance will inspire a euphony of ragged squeaking from hungry chicks inside and a scolding from the parent nearby. Often, a parent would fly to a tree close to us and start ‘pik-pik’-ing aggressively, indicating that there was a cavity in the vicinity. We would then quickly scan the bases of nearby tree trunks to search for tell-tale piles of fresh wood chippings covering the roots of cavity trees. By early July, the nestlings grew wise enough to stay quiet when they heard movement outside the cavity, so nest-searching became more difficult.
To locate woodpeckers, we searched for clear signs of foraging. Foraging marks in the form of flakes of bark on the ground around the base of a tree and deep excavation holes in the trunk revealed if woodpeckers frequented a pocket of woodland. Some trees had almost all their bark chipped away while others had only a couple of small-chipped patches. Courtship feeding, when an adult bird feeds the other, was also seen regularly in the Black-backed and American Three-toed. In these species, this behaviour appeared as the male foraging for insects under the bark and feeding them to the female perching close by.
To gather data for habitat preference analysis, we took the following measurements of cavity trees and the surrounding trees: diameter, species identification (ID), burn severity and decay severity, as well as woodpecker species ID and nest status (excavating, incubating, or nestlings). In the data gathering process, we came across a few natural obstacles. Clouds of mosquitoes, chest-high fireweed (Chamaenerion angustifolium) obscuring several layers of logs and the worry of running into an overly protective Grizzly with her cubs (Ursus arctos horribilis), as well as rain, hail and thunderstorms made navigating the unstable terrain an interesting task.
Woodpecker-watching does not always have to be this intense. An adaptable family of species, woodpeckers thrive throughout urban and rural environments across North America, and they can be seen regularly in forests and parks throughout the Lower Mainland.
References
Cooke, H. A., Morissette, J., Cobb, T., and Reid, D., 2019. Fire and Insects: Managing Naturally Disturbed Forests to Conserve Ecological Values. Wildlife Conservation Society Canada Conservation Report No. 12. Toronto, Ontario, Canada.
Environment Canada. 2016. Recovery Strategy for the Common Nighthawk (Chordeiles minor) in Canada. Species at Risk Act Recovery Strategy Series. Environment Canada, Ottawa.
Horvath, W. 2020. Introduction to Forest Pathology - Sporophore Identification. Lecture Notes. Natural Resources Measurements RENR 2110.