# Site Occupancy and Cavity Use by the Northern Flying Squirrel in the Boreal Forest

> Where does a flying squirrel sleep when it is -40 °C outside? Radio-tracking squirrels through a Quebec winter, researchers found them in woodpecker holes, in nests built in conifer branches and even in burrows up to a metre underground. Males favoured branch nests in the cold and females favoured ground nests, so tree cavities turned out not to be the limiting resource many expected.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2011-trudeau-imbeau-site-occupancy-cavity-use/

## Citation and access

- **Authors:** Caroline Trudeau, Louis Imbeau, Pierre Drapeau and Marc J. Mazerolle
- **Year published:** 2011
- **Journal or source:** The Journal of Wildlife Management
- **Full citation:** Trudeau, C., Imbeau, L., Drapeau, P. and Mazerolle, M. J. 2011. Site occupancy and cavity use by the northern flying squirrel in the boreal forest. The Journal of Wildlife Management 75(7): 1646–1656.
- **DOI:** 10.1002/jwmg.224
- **Access status:** Hosted (freely available elsewhere)
- **PDF:** https://www.originalwisdom.com/wp-content/uploads/2026/11/2011_Trudeau_and_Imbeau_etal_site-occupancy-and-cavity-use-by-the-northern-flying-squirrel-in-the-boreal-forest.pdf

## Tags

- **Species:** Mammals > Northern flying squirrel
- **Region:** North America > Canada
- **Behaviour and ecology:** Behaviour; Den site selection; Habitat selection; Nesting; Predation
- **Methods and technology:** Akaike Information Criterion (AIC); Bootstrapping; Chi-square test; Cover boards; Detectability; Logistic regression; Occupancy modelling; Radio telemetry
- **Data type:** New field data
- **Observer qualification:** Not needed
- **Discipline:** Behaviour and ecology
- **Year published:** 2011

## Detailed keywords

- **Species:** Studied: northern flying squirrel (Glaucomys sabrinus). Cavity excavators seen at the study sites: northern flicker (Colaptes auratus), yellow-bellied sapsucker (Sphyrapicus varius), downy woodpecker (Picoides pubescens), hairy woodpecker (Picoides villosus) and pileated woodpecker (Dryocopus pileatus). Nest trees and forest trees: trembling aspen (Populus tremuloides), balsam poplar (Populus balsamifera), white or paper birch (Betula papyrifera), white spruce (Picea glauca), black spruce (Picea mariana), jack pine (Pinus banksiana), balsam fir (Abies balsamea), black ash (in Figure 1 only). Discussed but not studied: southern flying squirrel (Glaucomys volans), spotted owl (Strix occidentalis), great horned owl (Bubo virginianus), snowshoe hare (Lepus americanus), Arctic ground squirrel (Spermophilus parryii), weasels; dwarf mistletoe (Arceuthobium spp.) and spruce rust (Chrysomyxa spp.), which cause witches’ brooms.
- **Location and habitat:** 59 sites within 50 km of Rouyn-Noranda (48°18’N, 79°05’W), Abitibi, northwestern Quebec, Canada. Even-aged deciduous stands dominated by trembling aspen, 10 to 70 years old, in the balsam fir-white birch domain of the boreal forest; subarctic climate (mean annual temperature 1.2 °C, 914 mm precipitation). Weather data from the Val-d’Or station. Some sites were near homes, and 3 nests were inside the walls of private houses.
- **Topics:** tree cavities as a limiting resource; secondary cavity users and nest webs; site occupancy; detection probability; trap height; weather and trapping success; winter nest choice; tree cavities, external (branch) nests, witches’ brooms and ground nests; underground nests beneath snow; thermal insulation and thermoregulation; sex differences in nest use; predation in underground nests; cavity tree size and status (alive or dead); coarse woody debris and fungi as food; lateral cover and predator avoidance; forest management for cavity trees.
- **Statistics or models:** single-season site occupancy models in program PRESENCE 3.1 (11 candidate models, 5 hypotheses); Akaike’s Information Criterion corrected for small samples (AICc), model weights and multimodel inference (model averaging) in R; 10,000 bootstrap samples for goodness of fit; t-tests; chi-square tests; mixed logistic regressions with individual as a random effect (lme4 glmer), as a stand-in for a mixed multinomial model; parametric bootstrap (1,000 data sets) on the sum of squared errors for model fit. Internal inconsistencies: ground nests are 35% of locations in the abstract but 36% in the Discussion; tree cavities are 26% in the abstract and 25% in the Management Implications (24% for males and 25% for females in the Discussion); cavities were “90% trembling aspen” in one sentence and 88.9% in another; used cavity trees are described as having a “greater” diameter than available ones although the test was not significant at the usual level (P = 0.053). The 87 nests include the 3 house nests that were left out of the analysis (31 cavities + 33 external + 20 ground = 84).
- **Data type:** New field data. Live trapping 10 September to 5 December 2008; radio-tracking 18 October 2008 to 13 April 2009 (twice a week from January to April 2009); habitat measurements May and June 2009.

## Synopsis

### 1. What they did

Tree cavities, mostly dug by woodpeckers, are thought to be the best winter shelter for small animals in cold places, and a shortage of cavities could limit how many animals can live there. The northern flying squirrel cannot dig their own cavities, so the authors asked whether cavity availability decides where these squirrels live in the boreal forest of Quebec, and which kind of nest they prefer on cold days.

- Trapping. At 59 aspen-dominated sites, they set 8 live traps along an 80 m line, alternating traps at 1.5 m and 4 m above the ground. Each site was trapped for 3 nights, twice, in autumn 2008 (2,880 trap-nights). Squirrels were ear-tagged, and 46 animals over 100 g were fitted with 4 g radio collars.

- Habitat. At each site they counted tree cavities with binoculars, measured well-rotted fallen logs (a possible sign of food, since the squirrels eat fungi), and measured lateral cover with a profile board (shelter from predators).

- Nest use. In winter they radio-tracked collared squirrels to their daytime nests, recording the nest type and the tree’s species, diameter and whether it was alive or dead.

- Analysis. They used occupancy models, which allow for animals being missed, to test whether habitat explained which sites were used, and whether weather or trap height affected the chance of catching a squirrel. They then tested how temperature affected each sex’s choice of nest.

### 2. What they found

Site occupancy

- They caught 85 squirrels (47 males, 38 females) and found squirrels at 34 of the 59 sites (58%), with up to 7 animals per site.

- None of the habitat measures (cavities, fallen logs, lateral cover) explained which sites were occupied.

- Squirrels were harder to catch in wet weather, and low traps caught more than high traps. Temperature made no difference to catches.

Nest use

- 26 squirrels (13 males, 13 females) were located alive in their nests 220 times, using 87 different nests. Each squirrel used 1 to 6 nests.

- Nests in trees: 31 in cavities (25 old woodpecker holes and 6 natural cavities), all in deciduous trees, mostly trembling aspen; and 33 external nests built on branches, all in live conifers, 2 of them inside witches’ brooms.

- 20 ground nests: 4 under logs or stumps beneath the snow, the rest below ground, up to 1 m deep, found by digging them out in 7 cases where a squirrel had been killed. 8 squirrels died in underground nests during the study.

- Across all locations, squirrels used tree cavities 26%, external nests 39% and ground nests 35% of the time.

- Used cavity trees were more often alive (50% versus 23.4%) than available cavity trees, and slightly larger (30.0 cm versus 26.5 cm diameter).

- Males mainly used external nests and females used more ground nests. As it got colder, males became much more likely to choose an external nest over a cavity (about 257 times more likely at -30 °C than at 0 °C), and females a ground nest over a cavity (about 79 times).

- The only 2 cavities used below -15 °C were in large trees (49 and 34 cm diameter).

### 3. What they concluded

- The idea that tree cavities limit northern flying squirrels in the boreal forest in winter was not supported. The squirrels are generalists and opportunists, using whatever nest types are available.

- External nests may insulate well in this dry climate, can be built close to food and can hold groups of squirrels. Ground nests under snow may be the warmest, and their use may be normal winter behaviour in a cold climate, though they may carry a higher risk of predation.

- The difference between the sexes may partly reflect small samples, because most collared males did not survive the winter.

- Cavities in large, live trees are probably scarce in their study area, and small trees may not insulate well enough for winter.

- Binocular counts probably overestimate usable cavities, and a stronger test would need experiments that change the number of cavities.

- Low traps catch more squirrels and are easier to set, so the authors recommend them.

### 6. Other technologies, skills or innovations

- Tomahawk live traps baited with apple and peanut butter, set at two heights.

- Radio collars (Holohil PD-2C, 4 g) tracked with a digital receiver and a hand-held Yagi antenna; squirrels were located during the day, when they stay in their nests.

- Vegetation profile board for lateral cover, and line-intersect sampling for fallen logs.

- Occupancy models that separate whether a site is used from whether a squirrel is detected.

### 7. Implications for conservation (from the paper)

- Keeping large live cavity trees, which insulate better, is likely to increase the squirrels’ use of tree cavities.

- As for any animal that uses old woodpecker holes, forestry that keeps conditions suitable for woodpeckers, such as partial cuts or leaving older trees and dead wood in clear-cuts, should be favoured so that suitable cavities stay available over the long term.

- The squirrels’ ability to use many kinds of nests may explain why they are common across such a wide range, reaching far north.

## Cite this summary

Lawrence, K. 2026. Summary of: Site Occupancy and Cavity Use by the Northern Flying Squirrel in the Boreal Forest (Trudeau and Imbeau et al., 2011). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2011-trudeau-imbeau-site-occupancy-cavity-use/