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Lemieux, M., V. Lamarre, and J. A. Tremblay. 2025. Hemlock looper outbreak: new insight about how Black-backed Woodpecker (Picoides arcticus) respond to resource pulses in eastern Canada. Avian Conservation and Ecology 20(1):4.ABSTRACT
Black-backed Woodpecker (Picoides arcticus) is known to benefit from pulse-resource disturbances in boreal forest for example, by colonizing recently burned habitats. Although insect outbreaks are ubiquitous in some parts of the eastern boreal forest, the opportunities offered by these natural disturbances for the Black-backed Woodpecker remain poorly understood. Between 2012 and 2014, a small-scale hemlock looper (Lambdina fiscellaria) outbreak occurred in central Québec (Canada) within the eastern boreal forest. Using global positioning system (GPS) tags, we documented home range sizes and habitat selections of Black-backed Woodpeckers at different scales and assessed nest survival 2–3 years post-outbreak. We tracked 5 birds and found 13 active nests. Mean home range size was 368 ± 134 ha. A negative relationship was observed between home range size and the proportion of area affected by hemlock looper-induced mortality. Mortality stands (> 75% tree mortality or severe defoliation, 70–100% foliage loss), were selected at both the landscape and home range scales, while light to moderate defoliated stands (1–69% foliage loss), were selected only at the landscape scale. At home range scale, nest site selection was predicted by the volume of early-decayed dead wood. The probability of nest sites being selected was greater than 50% when the average volume of early-decayed dead wood was greater than 61 m³/ha. At the tree scale, nest-tree selection was predicted by diameter at breast height (dbh) and tree type. The probability of nest-tree selection was greater than 50% when mean dbh was higher than 17.1 cm (mean nest dbh: 31.4 ± 1.7 cm) and woodpeckers preferred deciduous trees for nesting. We did not detect any temporal nor habitat variable effects on daily nest survival rate. The daily survival rate was 0.985 ± 0.007 and the nest success rate was 0.589 ± 0.147. Our results highlight that Black-backed Woodpeckers can benefit from pulse resources in stands affected by the hemlock looper 2–3 years after the outbreak. They are able to establish their home ranges, and successfully nest, despite logging operations that salvaged 38% of the affected stands.
RÉSUMÉ
Le Pic à dos noir (Picoides arcticus) bénéficie des perturbations naturelles qui créent des pulses de ressources dans la forêt boréale, par exemple en colonisant les habitats récemment brûlés. Bien que les épidémies d’insectes soient omniprésentes dans certaines parties de la forêt boréale orientale, les opportunités qu’elles offrent pour le Pic à dos noir demeurent mal comprises. Entre 2012-2014, une épidémie d’arpenteuse de la pruche (Lambdina fiscellaria) s’est produite dans le centre du Québec (Canada) au sein de la forêt boréale de l’Est. À l’aide d’émetteurs GPS (Global Positioning System), nous avons évalué la taille des domaines vitaux, la sélection d’habitats du Pic à dos noir à différentes échelles et la survie des nids 2–3 ans après l’épidémie. Nous avons suivi 5 individus et trouvé 13 nids actifs. La taille moyenne du domaine vital était de 368 ± 134 ha. Une relation négative a été observée entre la taille du domaine vital et la proportion de peuplements morts à la suite de l’épidémie. Les peuplements de mortalité (> 75% de mortalité ou une sévère défoliation (70–100% de perte de feuillage) ont été sélectionnés à la fois à l’échelle du paysage et du domaine vital, alors que les peuplements légèrement à modérément défoliés (1–69% de perte de feuillage) n’ont été sélectionnés qu’à l’échelle du paysage. La sélection des sites de nidification a été favorisée par le volume de bois mort en début de décomposition. La probabilité de sélection d’un site de nidification était supérieure à 50% lorsque ce volume était supérieur à 61 m³/ha. La sélection de l’arbre de nidification a été favorisée par le diamètre à hauteur de poitrine (dhp) et le type d’arbre. La probabilité de sélection de l’arbre de nidification était supérieure à 50% lorsque le dhp moyen était supérieur à 17,1 cm (dhp moyen du nid : 31,4 ± 1,7 cm) et les pics ont préféré les arbres feuillus pour la nidification. Nous n’avons pas détecté d’effets des variables temporelles ni d’habitat sur le taux de survie quotidien des nids. Le taux de survie quotidien était de 0,985 ± 0,007 et le taux de succès de nidification était de 0,589 ± 0,147. Nos résultats montrent que le pic à dos noir peut tirer profit de ressources épisodiques provenant de peuplements 2–3 ans après l’épidémie d’arpenteuse de la pruche. Les pics y ont établi leurs domaines vitaux et ont niché avec succès, malgré les opérations de récolte qui ont récupéré 38% des peuplements affectés.
INTRODUCTION
Fire is a predominant natural disturbance in circumboreal forests (Bergeron et al. 2001, Gauthier et al. 2015), whereas windthrow and insect outbreaks are considered secondary disturbances that can be regionally dominant, especially in long fire return regions of eastern Canada (De Grandpré et al. 2018). Owing to their damaging effects on wood quality and quantity (MacLean 1990), significant efforts have been made to prevent or mitigate the impacts of natural disturbances through strategies such as fire suppression, post-fire salvage logging, insecticide spraying, and sanitation logging during insect infestations (Nappi et al. 2004, Hutto 2006, Thorn et al. 2018). Current applications of salvage logging mostly consist of clear-cutting all merchantable timber in the recently affected areas (Barrette et al. 2015). However, insect and fire disturbances create resource pulses on which many species may benefit for foraging and nesting (Hannon and Drapeau 2005, Schieck and Song 2006). For instance, wildfires create resources for wood-boring (Cerambycidae and Buprestidae) and bark (Scolytidae) beetles, which depend on early-decayed dead wood for their life cycle (Saint-Germain et al. 2004, Boulanger and Sirois 2007). Similarly, a greater abundance of birds (ground nesters, aerial foragers, and cavity nesters) has been documented in recently burned forests (Nappi et al. 2004, Hannon and Drapeau 2005, Schieck and Song 2006, Hutto et al. 2020). Likewise, spruce budworm (Choristoneura fumiferana Clem.) outbreaks affect different communities of forest birds, either directly by affecting food supply (spruce budworm larvae), or indirectly by changing the vegetation composition of forest canopies (Drever et al. 2018, Germain et al. 2021, Moisan Perrier et al. 2021).
Woodpeckers (family Picidae) are highly reactive to natural disturbances and forest management practices, making them indicators for detecting rapid changes in forest health (Drever et al. 2009, Drever and Martin 2010, Czeszczewik et al. 2013). They are often considered keystone species because they create cavities that are reused by non-excavating species that require cavities for nesting or roosting, thus providing access to resources that would otherwise be unavailable (Mikusiński et al. 2001, Virkkala 2006, Cockle et al. 2011, Cadieux and Drapeau 2017, Hardin et al. 2021, Cadieux et al. 2023). Owing to their positive impacts on other species, woodpeckers are widely considered reliable indicators of overall bird community health. Consequently, they are often considered focal species in forest management decisions, and their occurrence may serve as an indicator of ecological community well-being (Cheveau 2015, Tremblay et al. 2015a, Lemieux 2024).
The Black-backed Woodpecker (Picoides arcticus Swainson) breeds in undisturbed boreal forests of Canada (Setterington et al. 2000, Tremblay et al. 2009, 2015a, 2015b, Craig et al. 2019, Lemieux 2024), where they forage mainly on early-decayed dead wood (i.e., snags) in older stands (Tremblay et al. 2009, 2015a, Lemieux 2024). In the last decade, studies have reported Black-backed Woodpecker breeding in undisturbed forests in the western United States of America (Fogg et al. 2014, Verschuyl et al. 2021, Kerstens and Rivers 2023). The species is also known to take advantage of resource pulses created by disturbances throughout its range (Tremblay et al. 2020). For example, Black-backed Woodpeckers thrive in recently burned forests (Powell 2000, Dudley and Saab 2007, Saab et al. 2007, Nappi and Drapeau 2009, Stillman et al. 2019, Tingley et al. 2020) and prescribed burns (Rota et al. 2014a) where dead trees are created and used to forage and nest. Black-backed Woodpeckers also respond to the higher food abundance and foraging opportunities created by mountain pine beetle (Dendroctonus ponderosae, hereafter MPB) outbreaks in the western part of its range (Goggans et al. 1989, Bonnot et al. 2008, 2009, Edworthy et al. 2011, Rota et al. 2014a, 2014b, 2015, Saab et al. 2019). To a lesser extent, the species also uses dying trees in bark beetle-infested forests following drought episodes in California (Tingley et al. 2020). However, it is unclear whether the Black-backed Woodpecker also benefits from insect outbreaks for nesting in the eastern part of its range.
In eastern North America, hemlock looper (Lambdina fiscellaria) ranks second among the most important defoliators of coniferous trees, after the spruce budworm (Hébert and Brodeur 2013). In Canada, its range includes Saskatchewan to Newfoundland and Labrador and coincides with balsam fir and eastern hemlock ranges; however, in Québec, defoliations have exclusively occurred in fir stands (Jobin 1973). The larvae feeds on foliage of all age classes without necessarily eating it entirely (Watson 1934, Carroll 1956), resulting in mortality of severely defoliated trees the following year (Dobesberger 1989, MacLean and Ebert 1999). When defoliation is partial, affected trees grow more slowly and are more vulnerable to other insects and diseases (Watson 1934, MacLean and Ebert 1999), such as Pissodes similis and P. striatulus known as secondary pests attacking weakened or recently killed trees (Béland et al. 2019). In Newfoundland, hemlock looper outbreaks occurred at intervals ranging from 7 to 18 years and typically lasted between 6 and 9 years between 1910 and 1972. Their impacts also varied greatly, affecting areas ranging from 8300 ha to 830,300 ha during that period (Otvos et al. 1979).
Between 2012 and 2014, a hemlock looper outbreak was detected in balsam fir stands located 70 km north of Québec City, Canada (Arsenault et al. 2014, Ministère des Forêts, de la Faune et des Parcs 2014a). This outbreak occurred > 650 km away from the nearest only other known outbreak in the province, which offered a unique opportunity to investigate the response of Black-backed Woodpecker to the resource pulses of early-decayed snags from a new disturbance agent. Hence, the main objectives of our study were to determine home range size, evaluate habitat selection at three scales (i.e., landscape, home range, and tree), and assess nest survival rates of Black-backed Woodpeckers in balsam fir stands 2–3 years after the hemlock looper outbreak. Given the pulse in food resources resulting from rapid stand mortality following the passage of hemlock looper, we hypothesized that tree mortality engendered by the outbreak would influence woodpecker habitat selection. First, we predicted that (i) Black-backed Woodpecker home range size would decrease as the proportion of the study area affected by hemlock looper induced mortality, which includes both stands with > 75% tree mortality and those with severe defoliation (70–100 % foliage loss), decreases. Along the same lines, at the landscape scale (ii), woodpeckers would establish their home ranges in hemlock looper induced mortality stands. At the home range scale (iii), they would select mortality stands for foraging activities and nest site selection would be associated with the availability of food near the nest. At the tree scale (iv), large trees with a mid-decay stage would dictate nest-tree selection. Finally (v), we expected nest survival rates to be associated with dead wood abundance.
METHODS
Study area
The present study was conducted from May to August 2016–2017 at the Forêt Montmorency (47°15′N, 71°10′W), a teaching and research station at Université Laval, located approximately 70 km north of Québec City, Canada (Fig. 1). The study area is located on the southern edge of the boreal forest and is part of the eastern balsam fir-white birch bioclimatic domain (Bouliane et al. 2014). Balsam fir (Abies balsamea (L.) Mill.) is the primary commercial species, and companion species include white spruce (Picea glauca Moench.), white birch (Betula papyrifera Marsh.), and to a lesser extent, black spruce (Picea mariana (Mill.) B.S.P.) and trembling aspen (Populus tremuloides Michx.; Bouliane et al. 2014). The two main natural disturbances that have shaped the landscape over time, creating a mosaic of stands of varying ages, are spruce budworm outbreaks, and windthrows (Bouliane et al. 2014).
The first mention of a hemlock looper outbreak was reported in our study area in 2012 (Ministère des Ressources naturelles et de la Faune 2012, Bouliane et al. 2014). It caused extensive balsam fir mortality throughout the Forêt Montmorency for three years (2012–2014; Bouliane et al. 2014; P. Pineault 2015, unpublished data). Aerial surveys indicated that over 2800 ha of forest were affected by the outbreak during that period within the limits of Forêt Montmorency (Ministère des Ressources naturelles et des Forêts 2017a). The defoliation classes were defined as follows: (i) light defoliation (1–34 %) affecting foliage in the upper third of some crowns in a stand; (ii) moderate defoliation (35–69 %) comprising foliage loss in the upper third of most tree crowns and on the full length of the crown in some trees; (iii) severe defoliation (70–100 %) comprising a complete loss of foliage along the full crown length in most trees; and (iv) stands with > 75 % tree mortality (Ministère des Forêts, de la Faune et des Parcs 2014b, Béland et al. 2019). Salvage logging operations were conducted in the study area and clear-cut of about 38% (474 ha out of 1249 ha) of stands with moderate to severe defoliation (classes ii and iii), targeting coniferous tree species while retaining live and dead deciduous tree species.
We conducted our study over an area of 85 km², located in the southwest part of the Forêt Montmorency, in the core of the outbreak (Fig. 1). The boundaries of our study area consisted of a 2.5 km buffer for every playback station (see section Capture and telemetry data), as woodpeckers were detectable up to approximately 1 km away from the radio receiver, and we were able to cover between 1 and 1.5 km in the forest during telemetry monitoring.
Habitat classes were established using 1:250 000 forest maps, published by the Québec Ministry of Natural Resources and Forests in 2017 (Ministère des Ressources naturelles et des Forêts 2017a, 2017b). We did not distinguish deciduous stands from coniferous stands because 99.7% of the stands were coniferous or coniferous dominated in the study area. Habitat classes were grouped according to age structure as follows: salvage logging, regeneration areas (6–20 years), young forests (21–40 years), mature forests (41–80 years), and old-growth forests (> 80 years). We also included two different habitat classes for the hemlock looper outbreak related to different beetle assemblages obtained by Béland et al. (2019). Light and moderate defoliation stands (category i and ii) were combined into a class named “defoliated stands.” Severe defoliation and stands with a mortality rate of over 75% (category iii and iv) were combined into a class named “mortality stands.” Finally, unproductive habitats, including dry and wet barrens, alder stands, lakes, rivers, and roads, were not considered in the analysis because they did not represent significant habitats for woodpeckers.
Capture and telemetry data
Black-backed Woodpeckers were captured between 29 May and 20 June 2017. We broadcasted playback calls and drumming to attract Black-Backed Woodpeckers and used mist-nets to capture individuals. Male and female woodpeckers were fitted with PinPoint-VHF-50 tags (Lotek Wireless Inc.). The tags were attached to the central tail feathers using a polyethylene braided microfilament fishing line, which is resistant to abrasion. The knots were solidified with cyanoacrylate glue (Tremblay et al. 2009). The tags used acquired GPS positions and allowed woodpeckers to be tracked by telemetry (VHF). To track individuals, birds were located using the “homing” method (Mech 1983) with radio receivers (SRX-800; Lotek Wireless Inc.) and a three-element flexible yagi antenna. Woodpeckers were tracked for the duration of their active nesting period (extending from the egg-laying period until fledging or nest failure). Periods of GPS location acquisition, VHF telemetry tracking, and remote data download were scheduled daily to optimize transmitter battery life (Table A1). Nests were found either by direct observation of unmarked individuals, by telemetry tracking, or by prospecting areas with clusters of GPS locations acquired during remote data download.
To obtain a precise set of location data, we retained only GPS locations with a horizontal dilution of precision (HDOP) < 5 and those with > 4 satellites (Forrest et al. 2022). We then removed the first 24 h after capture to avoid relocation bias that could result from capture. We also discarded successive locations less than 100 m apart (Tremblay et al. 2009). The breeding cavity was considered as a single independent location, and to avoid undue influence on home-range size, no other location data points were taken within a 100 m radius of the nest tree (Tremblay et al. 2009).
Nest monitoring
We monitored the nests every four to five days. Nest content was inspected by using a wireless cavity inspection camera (https://www.ibwo.org/index.php). During each visit, we recorded the date and stage of the nest (egg-laying, incubation, or nestling). We assumed an 11-day incubation period (beginning on the day the last egg was laid) and a 24-day nestling period beginning on the day the first egg hatched (Tremblay et al. 2020). We used the characteristics of plumage development of nestlings to estimate the age of the nests that were found after hatching (Boily 2006, Tremblay et al. 2020). Monitoring was performed until fledging or nest failure. Nest success was defined when at least one nestling fledged. We inferred success when we observed well-developed nestlings within 2–3 days of fledging on one visit and found an empty nest on the next visit. When possible, a trail camera (Spypoint IR-6; https://www.spypoint.com/en) was placed on a tree facing the cavity to record predation events or fledging dates to reduce the unknown fate period concerning nest survival between visits. A nest was considered predated if all eggs or nestlings were missing, or if damage to eggs was evident (e.g., eggs or eggshell fragments nearby). Abandonment was presumed if the adults stopped frequenting the nest even if its content remained intact.
Home range estimation
We estimated home range size during the nesting period using the MCP method (Hayne 1949) with 100% of the filtered GPS locations with the “adehabitatHR” package (version 0.4.21; Calenge 2023). We used “stats” package (version 4.3.2; R Core Team 2023) to test the linear relationship between home range size and the proportion of each habitat class in the home range.
Analysis
We used a hierarchical design to account for Black-backed Woodpecker habitat use at three nested scales. At the landscape scale (second-order selection of Johnson 1980), we examined home range selection using selection ratios (Manly et al. 2002). We compared the proportion of each habitat class within the home range of individual woodpeckers to the proportion of available habitat classes within the study area (Design II analysis; Manly et al. 2002). At the home range scale (third-order selection of Johnson 1980), we examined foraging and nest site selection. For foraging site selection, we assumed that each GPS location was related to foraging activities. Thus, we examined foraging site selection by comparing used habitat (habitat class associated with individual telemetry locations) to habitat availability in individual home ranges (design III analysis; Manly et al. 2002). A habitat class is “preferred” when the 95% confidence interval (CI) is > 1 (i.e., used more than its availability), is considered “avoided” when the 95% CI is < 1 (i.e., used less than its availability), and is used proportionally to its availability when the 95% CI includes 1 (Manly et al. 2002). We computed Manly’s selectivity indices using “adehabitatHS” package (version 0.3.17; Calenge and Basille 2023) and estimated significance of mean and individual habitat selection by log-likelihood χ² tests.
At the home range scale, we also assessed nest site selection by comparing stand attributes in a 400 m² circular plot centered on the nest to one to three available plots located 200 m from each nest. For this analysis, we combined data from woodpecker nests found in 2016 with those found in 2017. Owing to labor constraints in the field, the number of available plots varied from 1 (n = 7), 2 (n = 3), and 3 (n = 2) per nest. No available plots were sampled for 1 nest, which was not considered for nest site selection. For each plot centered on the nest and its associated available plot, trees and snags with a diameter at breast height (dbh) ≥ 9 cm, species, decay stage following the classification used by Tremblay et al. (2010; Table A2), dbh, and height were noted. The number of fallen logs was counted along three 20-m transects starting 1 m from the plot center (Böhl and Brändli 2007) and oriented at 60°, 180°, and 300°. Species, length, decay class, and diameter at the intersection were recorded for logs with a diameter ≥ 5 cm at the transect line/log intersection. An angle of 45° was used to distinguish between a snag and a fallen log (Harmon and Sexton 1996). Fallen log volume was calculated according to the equations of Böhl and Brändli (2007).
We used conditional logistic regression with our used vs. available datasets to assess nest site selection. We formulated models based on three biological hypotheses: food availability, nest site availability, and anti-predation strategy. The model for food availability was evaluated based on the volume of early-decayed dead wood: snags and fallen logs combined (m³/ha). Nest site availability was assessed based on the density of large trees (decaying trees ≥ 20 cm dbh). We used stump basal areas to support the hypothesis that woodpeckers establish their nests in harvested forests, open areas, as a strategy against predation.
Next, at the tree scale, we analyzed nest-tree selection using conditional logistic regression. We compared a tree used for nesting with those in the 400 m² plot centered on the nest. The models for nest-tree selection included all additive combinations of three variables: tree type (deciduous or conifer), dbh, and status (alive or dead).
Finally, we used a logistic exposure model with a binomial distribution to estimate daily nest survival (DSR; Shaffer 2004), which was assessed using two temporal factors (nest age and initiation date) and three habitat variables (volume of early-decayed dead wood, density of trees > 20 cm, and stump basal area). Regression coefficients (estimate ± 95% CI) were used to estimate the DSR of nests. We calculated the probability that a nest will survive from incubation to fledging (i.e., nesting survival rate; NSR) as the product of the DSR for each day of the nesting period (Shaffer et al. 2007). Productivity was computed as the product of NSR and the mean number of young per successful nest. We performed DSR on nests that had been monitored for more than one visit.
We compared and ranked nest sites, nest trees, and nest survival models using Akaike’s Information Criterion corrected for small sample sizes (AICc) and Akaike weights (wi; Burnham and Anderson 2002). In cases in which more than one model could explain the selection (ΔAICc < 2), we performed model averaging using the “AICcmodavg” package (version 2.3-3; Mazerolle 2023). All analyses were performed using R statistical software version 4.3.2 (R Development Core Team 2023)
RESULTS
Of the seven adult Black-backed Woodpeckers radio-marked in 2017, six were breeders, and only five provided sufficient GPS locations to estimate home range size (Table 1). Mean home range was 368 ± 134 ha (mean ± se) and tended to decrease with the proportion of the study area affected by hemlock looper induced mortality (β = -18.23, p < 0.05).
At the landscape scale, the woodpeckers selected stands with hemlock looper induced defoliation and tree mortality but avoided young and mature forests (Fig. 2). Young forests represented the highest percentage of available habitat in the study area (26 %) whereas salvage logged areas represented the least (6%; Table A3). At the home range scale, for the foraging site, Black-backed Woodpeckers selected mortality stands, whereas old-growth forests and regeneration areas were avoided (Fig. 2). Stands with hemlock looper induced defoliation represented the highest percentage of available habitats within the woodpeckers’ home range (30 ± 2%) whereas regeneration area the least (5 ± 4%; Table A4).
We included a total of 12 nests (n = 7 in 2016 and n = 5 in 2017) for nest site selection; seven (58%) were found in salvage logging areas and five (42%) in defoliated stands by hemlock looper (Fig. 3). Nest site selection was mostly supported by the volume of early-decayed dead wood, with no other competing models (ΔAICc < 2; Table 2). The probability of Black-backed Woodpeckers selecting a nest site strongly increased with the volume of early-decayed dead wood (β = 1.03, 95% CI: 1.00–1.06), and was greater than 50% when this volume exceeded 61 m³/ha. The mean volume of early-decayed dead wood around Black-backed Woodpecker nests was 97 ± 62 m³/ha, with snags representing 55% of this volume (Table A5).
We included a total of 13 nests (n = 7 in 2016 and n = 6 in 2017) for nest-tree selection. Ten (77%) Black-backed Woodpecker nests were in dead trees (n = 3/10, decay class 5, and n = 7/10, decay class 6), and three (23%) were alive trees (decay class 2). Eight (62%) nests were found in deciduous trees and five (38%) were found in conifers. The mean dbh of the nesting tree was 31.4 ± 1.7 cm. For nest-tree selection by Black-backed Woodpecker, there was strong support in favor of the dbh and tree type model (Table 2). This model ranked first and was 2.68 times more likely than the second-ranked model, which included dbh, tree type, and tree status (Table 2). Together, these two models had a cumulative Akaike’s weight of 0.98. Black-backed Woodpeckers tended to select trees with a larger dbh (model-averaged βdbh = 0.15, 95% CI: 0.07–0.23) and preferred deciduous trees (model-averaged βdeciduous = 2.31, 95% CI: 0.56–4.06) to establish their nests. The probability of nest tree being selected was greater than 50% when the mean nest dbh was larger than 17.1 cm. There was no evidence for an effect of decay status (βdead = 0.25, 95% CI: -1.63–2.13).
We included a total of 12 nests (n = 7 in 2016 and n = 5 in 2017) for survival analysis. A total of four nest failures (25%) occurred over the two years of nest monitoring. One nest was abandoned during the incubation period and then was predated, whereas the other three were depredated during the nestling stage. Cameras allowed us to identify two nest predators, the red squirrel (Tamiasciurus hudsonicus) and the American marten (Martes americana). DSR was not explained by any variable, and the intercept-only (null) model was equivalent (ΔAICc < 2) to the top model (initiation date; Table 2). Mean DSR and NSR estimates based on the intercept-only was 0.985 ± 0.007 and 0.589 ± 0147, respectively (n = 12). Mean number of fledglings per successful nest was 2.75 ± 0.18 and mean productivity per nest was 1.6 ± 0.4 (n = 8).
DISCUSSION
In the eastern part of its range, the role of insect outbreaks in Black-backed Woodpecker ecology is poorly documented. We found that Black-backed Woodpeckers colonized the hemlock looper outbreak, where they established home ranges and successfully bred within three years post-outbreak despite salvage logging operations.
Black-backed Woodpecker home range sizes varied considerably (mean: 368 ha, range: 140–813 ha). The mean home range sizes within two years since fire range between 88 and 270 ha in South Dakota and California (Rota et al. 2014b, Tingley et al. 2014). However, mean home range sizes in older burned habitats (i.e., ≥ 3 years post-fire) were generally similar in range (333–458 ha; Dudley and Saab 2007, Rota et al. 2014b, Tingley et al. 2014) to our three years post-outbreak. Also, our home range sizes are comparable with those of the MPB outbreak in South Dakota (Rota et al. 2014b) but seem to be larger than those observed in unburned forests in eastern Canada (Tremblay et al. 2009, Lemieux 2024). It might be expected that the new technology used in the study (i.e., PinPoint tag that records GPS locations automatically) would have an impact on home range sizes compared with a traditional method where bird locations are obtained by “homing” method (Mech 1983). Yet, Lemieux (2024) determined home ranges using the same PinPoint technology and obtained home range sizes similar to those obtained by Tremblay et al. (2009), who used the “homing” method within the same region of central Québec. Home range sizes observed in our study present a large variability. Indeed, a large mean home range size can be explained by two individuals appearing to have extended their home ranges to take advantage of induced mortality stands further away (at the edge of their home ranges; Fig. 1), whereas their home range exhibited a high proportion of younger forests in their home range. Together, these two individuals had an average home range size of 678 ± 139 ha compared to an average size of 162 ± 12 ha for the remaining three individuals (Table 1). Young forests are generally unattractive foraging habitats for the Black-backed Woodpecker because they tend to contain a low density of preferred foraging substrates (i.e., large early-decayed snags; Tremblay et al. 2010). Another factor that may explain the expanded home ranges of these two individuals could be competition for food resources. Indeed, more than one nest was found within their home ranges in 2016 and 2017 (see Fig. 1), and territoriality may have driven them to search for food resources farther from their core areas.
In our study, home ranges size showed a negative relationship with the proportion of mortality stands within Black-Backed Woodpecker home range. This led to the belief that the Black-backed Woodpecker took advantage of the resource pulses for up to three years after the outbreak of hemlock looper. Previous studies have highlighted that species have smaller home ranges when food availability near the nest is abundant, as suggested by both Dudley and Saab (2007) and Rota et al. (2014b), who found that Black-backed Woodpeckers increased their home range size in older burned habitats, likely induced by the decrease in food availability over time post-fire. Likewise, Tingley et al. (2014) found that the variation in Black-backed Woodpeckers’ home range size in burned habitats was explained by the average basal area of snags, while Tremblay et al. (2009) reported that home range size tended to increase with distance between old coniferous habitat patches and the foraging habitat in unburned forests.
We observed consistent patterns of Black-backed Woodpecker habitat selection at landscape and home range scales. At both scales, woodpeckers selected stands with hemlock looper induced tree mortality. At the landscape scale, woodpeckers also selected stands defoliated by hemlock looper. This suggests that the entire disturbance (all areas affected by the hemlock looper) attracted Black-backed Woodpeckers to the area for nesting but that mortality stands likely offered the best foraging opportunities. Similarly, Edworthy et al. (2011) obtained a strong numerical response from multiple woodpecker species to a large-scale MPB outbreak in British Columbia. Furthermore, the index of wood-boring larvae abundance was the most important predictor of Black-backed Woodpecker in MPB outbreak stands in South Dakota, suggesting that food resources drive home range establishment rather than nest site availability (Bonnot et al. 2009). The Black-backed Woodpecker is known to colonize recently burned forests to feed on the high abundance of Cerambycid larvae in early-decayed snags (Tremblay et al. 2020). However, its diet can be broader than previously reported, as the species seems to be more opportunistic and its diet may vary depending on habitat type (Stillman et al. 2022, Tremblay et al. 2023). Thus, selection of stands with hemlock looper induced mortality at the home range scale is likely related to insect succession following this disturbance. Indeed, within the same hemlock looper outbreak, Béland et al. (2019) observed a rapid succession of saproxylic insects with massive colonization of the striped ambrosia beetle (Trypodendron lineatum) in stands severely defoliated during the first two years, followed by Sirex/Serropalpus in 2016 (unpublished data). Our results support that Black-backed Woodpeckers can benefit from resource pulses offered by natural disturbances that create early-decayed dead wood, and that the species is not restricted to colonizing recently burned habitats (sensu Tremblay et al. 2020). It is worth mentioning that the species was barely present before the beginning of the outbreak (personal communication), and likely colonized by juvenile dispersion during the first years of the outbreak (Siegel et al. 2016, Dumas et al. 2024). In addition, the study site appeared to have reached its maximum capacity to support nesting pairs of Black-backed Woodpeckers, given that a similar number of nests was found in 2016 and 2017.
We found no evidence of selection for old-growth forest at either scale. Old-growth forests (> 80 years) are scarce and limited to small patches in the Forêt Montmorency. The management strategy at the time aimed to standardize age classes by prioritizing the harvesting of declining stands or those more susceptible to natural disturbances. The first management plan (Côté 1966) targeted the remaining old-growth forests stands on the territory, which may explain why Black-backed Woodpeckers were rarely reported in the study area prior the outbreak. However, since 2014, the management strategy has shifted, focusing on the restoration of irregular old-growth forests (Bouliane et al. 2014).
Salvage logging negatively affects woodpecker abundance and reproduction (Basile et al. 2023). Indeed, salvage logging practiced in burned (Hutto and Gallo 2006) and insect-defoliated forests (Bonnot et al. 2009) tends to reduce woodpecker abundance despite sufficient availability of nest trees, suggesting that the impact of the reduction in foraging substrates is more detrimental. Our results revealed that the probability of a nest site being selected increases with the volume of early-decayed dead wood, whereas neither the density of large trees nor the basal area of stumps supported this hypothesis. This is in line with other studies that observed nest site selection based on food availability (Hutto and Gallo 2006, Tremblay et al. 2015a, Stillman et al. 2019). In addition, we noted that the volume of early-decayed dead wood around nests (97 ± 62 m³/ha; all habitat types confounded) was higher than that reported in the home ranges of Black-backed Woodpeckers in old-growth forests (Tremblay et al. 2009, Lemieux 2024). Black-backed Woodpecker nests that were in salvaged stands (58%) also contained a high volume of early-decayed dead wood (84 ± 61 m³/ha; Table A5), suggesting that a large quantity of foraging substrate was maintained within those harvested areas. Hence, it appears that the amount of retention of affected stands with the outbreak extent (38% of the stands with moderate and severe defoliation were salvaged logged) were sufficient to support seven nesting pairs of Black-backed Woodpeckers at the maximum.
Nest-tree selection was influenced by tree diameter and tree type, with a clear preference for white birch. Notably, our study found that white birch snags were used exclusively for nesting among deciduous species, while nests in coniferous trees were found in either live-declining or white spruce snags. Similarly, Nappi and Drapeau (2011) found that Black-backed Woodpeckers preferred nesting in deciduous trees within burned habitats while Tremblay et al. (2015a) and Craig et al. (2019) report 100% and 96% of their nest trees in coniferous trees, respectively, in unburned managed forests. In the western parts of its range, Bonnot et al. (2009) reported that 24% of Black-backed Woodpecker nests were in live and dead aspen. The preference for white birch in our study area may have been influenced by salvage logging practices that selectively harvested coniferous trees (i.e., white spruce and balsam fir), which are more economically valuable.
Despite the effect of food resources on nest site selection, no habitat or temporal variables were predictors of the daily survival rate. Daily survival rate was 0.985 and NSR at 59%. These results are lower than estimates reported in recently burned habitats where NSR ranged from 73% to 84% (1–2 years post-fire; Nappi and Drapeau 2009), 72% (1–5 years post-fire; Rota et al. 2014a), and 85 % (2–10 years post-fire; Stillman et al. 2019). Nevertheless, we found equivalent results with NSR obtained by Tremblay et al. (2015b) in burned forests (61%; up to three years post-disturbance). NSR tends to decrease with time since fire (Bonnot et al. 2008, Vierling et al. 2008, Nappi and Drapeau 2009, Rota et al. 2014a). Thus, the lower DSR and NSR obtained vs recently burned habitats could be related to the fact that our study took place two to three years post-outbreak. Then, our NSR (59%) was similar to those observed during ongoing MPB outbreaks (63% Goggans et al. 1989; 44-78% Bonnot et al. 2008; 60% Rota et al. 2014a), as well as those in undisturbed forest of eastern Canada (59% Tremblay et al. 2015b; 63% Craig et al. 2019). In addition, our productivity (1.6 ± 0.4) was very close to that of undisturbed forests (1.5 ± 0.4; Tremblay et al. (2015b) and MPB outbreak (2.0 ± 0.3 and 1.4 ± 0.3; Bonnot et al. 2008). This suggests that woodpeckers nesting in areas affected by hemlock looper outbreaks could face similar challenges and predation risks to those nesting in MPB outbreaks and undisturbed forests. Saab et al. (2011) found that nests that were farther from undisturbed areas were more successful. The hemlock looper outbreak where our study took place is a small-scale disturbance and nests were generally close to undisturbed areas (50 m). Thus, predators may not have been constrained by the effect of open areas created by salvage logging in the study area because predation occurred in those habitats. Some studies have reported that woodpeckers may prefer nesting in retention trees in cut blocks because of the lower risk of nest predation (Tremblay et al. 2015a, Craig et al. 2019). Indeed, red squirrel and American marten were the two observed nest predators in our study, and they mainly occur in older forest stands and avoid cut blocks (Holloway and Malcolm 2006, Herbers and Klenner 2007, Cheveau et al. 2013). Thus, most of our nests were in salvage logging areas, where 38% of the moderate and severely defoliated stands were harvested. Although this strategy may minimize the risk of predation, it does not necessarily lead to higher nest success rates. Woodpeckers nesting in salvage logging following a forest wildfire tend to have lower nesting success than those nesting in unharvested burned forests (Saab et al. 2011). Food delivery rate by Black-backed Woodpeckers is normally greater where the abundance of prey is high, like in recently burned habitats (Tremblay et al. 2016). Harvesting in those habitats could result in lower food abundance (i.e., early-decayed snags) and consequently may affect the food delivery rate and nest success.
MANAGEMENT IMPLICATIONS
Our study highlights that Black-backed Woodpecker can benefit from pulse resources created by stands affected by the hemlock looper, in establishing their home ranges, and successfully nesting, despite salvaged logging operations that harvested 38% of the stands moderately and severely defoliated by the hemlock looper. The retention policy in our study area appears to have preserved sufficient early-decayed dead wood to support the nesting of up to seven woodpecker pairs in salvage-logged areas. According to our results, early-decayed snags is the most important driver of Black-backed Woodpecker nesting, likely as proxy for food abundance. Management practices that implement salvage logging operations that include both unharvested and salvage logged stands of defoliated/mortality stands can maintain Black-backed Woodpecker habitats after hemlock looper outbreaks. Based on our results, we suggest that salvage logged stands retaining a mean diameter at breast height larger than 17.1 cm and > 61 m³/ha of early-decayed dead wood will allow successful nesting opportunities for woodpeckers.
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ACKNOWLEDGMENTS
We thank Francis Lessard and Julien St-Amand for assistance during field work, staff for the Forêt Montmorency for providing logistical support, Michel Robert for the ATV loan, and André Desrochers for sharing bird monitoring data. This study was funded by Environment and Climate Change Canada.
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Fig. 1
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Fig. 1. Location of the study area in Québec (left), habitat classes, location of nests found in 2016 (blue) and 2017 (white), and home ranges of 5 nesting Black-back Woodpeckers (Picoides arcticus) in 2017.

Fig. 2
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Fig. 2. Results of Manly’s selection ratio at the landscape (yellow) and home range (blue) scales for the five nesting Black-backed Woodpecker (Picoides arcticus) home ranges. The horizontal broken line indicates the value 1.

Fig. 3
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Fig. 3. Photo of (a) a defoliated stand and (b) an active Black-backed Woodpecker (Picoides arcticus) nest in a hemlock looper (Lambdina fiscellaria) outbreak.

Table 1
Table 1. Number of GPS locations and home range sizes (ha) obtained by minimum convex polygons (MCP100 %) of adult Black-backed Woodpeckers (Picoides arcticus) in 2017. Mean (se) are computed only for TagID 44263 to 44267.
TagID | Sex | Incubation | Nestling | Total | |||||
Number of GPS locations | Number of GPS locations | Number of GPS locations | Home range size (ha) | ||||||
44263 | F | 35 | 35 | 140 | |||||
44264 | M | 26 | 118 | 144 | 180 | ||||
44267 | M | 31 | 31 | 62 | 166 | ||||
Mean (se) | 31 (3) | 75 (44) | 80 (33) | 162 (12) | |||||
44266 | F | 37 | 98 | 135 | 816 | ||||
44265 | M | 57 | 130 | 187 | 539 | ||||
Mean (se) | 47 (10) | 114 (16) | 161 (26) | 678 (139) | |||||
44268 | M | 6 | 6 | ||||||
Global mean (se) | 32 (7) | 94 (22) | 95 (29) | 368 (134) | |||||
Table 2
Table 2. Top-ranked models explaining nest site and nest-tree selections and daily nest survival of Black-backed Woodpecker (Picoides arcticus). Models are ranked based on Akaike’s Information Criterion for small sample size (AICc). Bold represents models within ΔAICc < 2.
Analysis | Model | K | AICc | ΔAICc | wi | Cum. Wt | 95% CI | ||
Nest site | Early-decayed dead wood volume | 1 | 14.79 | 0.00 | 0.77 | 0.77 | -6.32 | ||
Global | 3 | 17.48 | 2.47 | 0.20 | 0.97 | -5.26 | |||
Stump basal area | 1 | 22.26 | 7.47 | 0.02 | 0.98 | -10.06 | |||
Large trees density | 1 | 22.60 | 7.81 | 0.02 | 1.00 | -10.23 | |||
Nest tree | dbh + Tree type | 2 | 47.06 | 0.00 | 0.64 | 0.64 | -21.52 | ||
dbh + Tree type + Status | 3 | 49.04 | 1.97 | 0.24 | 0.88 | -21.49 | |||
dbh | 1 | 51.18 | 4.12 | 0.08 | 0.97 | -24.59 | |||
dbh + Status | 2 | 52.99 | 5.93 | 0.03 | 1.00 | -24.48 | |||
Tree type | 1 | 62.30 | 15.23 | 0.00 | 1.00 | -30.14 | |||
Status + Tree type | 2 | 64.07 | 17.01 | 0.00 | 1.00 | -30.02 | |||
Status | 1 | 72.25 | 25.18 | 0.00 | 1.00 | -35.12 | |||
Survival | Initiation date | 2 | 32.35 | 0.00 | 0.46 | 0.46 | -14.10 | ||
Intercept (Null) | 1 | 33.91 | 1.56 | 0.21 | 0.67 | -15.93 | |||
Early-decayed dead wood volume | 2 | 35.88 | 3.53 | 0.08 | 0.75 | -15.86 | |||
Large trees density | 2 | 35.95 | 3.60 | 0.08 | 0.83 | -15.90 | |||
Nest age | 2 | 35.96 | 3.61 | 0.08 | 0.90 | -15.90 | |||
Stump basal area | 2 | 36.00 | 3.65 | 0.07 | 0.98 | -15.92 | |||
Global | 7 | 38.34 | 5.99 | 0.02 | 1.00 | -11.37 | |||