James K. Swingle

Distribution and Abundance of Tree Voles in the Northern Coast Ranges of Oregon

Abstract We sampled 86 randomly selected survey plots to evaluate the distribution and abundance of Red Tree Voles (Arborimus longicaudus) on the Tillamook and Clatsop State Forests in the northern Coast Ranges of Oregon in 2011–2013. We conducted surveys by visually searching for tree vole nests while walking along 500 m of transect in each plot. Trees with potential nest structures were climbed to determine if the structures were tree vole nests. We found tree vole nests in only 4 random plots, all of which were located near the coast, at the western edge of the Tillamook State Forest. Of 33 tree vole nests located, 20 (61%) were in old forests (≥80 y old), and 13 (39%) were in 1 plot in a young forest that was 65 y old. We concluded that tree voles were absent from most of the Tillamook and Clatsop State Forests, probably due to the fact that most of the area was either logged or burned in the early 1900s, and subsequently intensively managed on short rotations. We also suggest that remnant stands of old forest on Bureau of Land Management and state lands are acting as source populations from which tree voles populate adjacent young forests. Cuttings found in nests provided evidence that most of the voles were feeding on needles of Western Hemlock (Tsuga heterophylla) or Sitka Spruce (Picea sitchensis).

Behavior of Red Tree Voles (Arborimus longicaudus) Based on Continuous Video Monitoring of Nests

Abstract We used video cameras to observe the activity patterns and behavior of three female red tree voles (Arborimus longicaudus) and their young in arboreal nests in western Oregon. Observation periods at the three nests were 63, 103 and 148 days. All three voles were primarily nocturnal, but occasionally foraged for brief periods during the day when they had large young in the nest. The median time when voles began and ended activity was 95 min after sunset and 168 min before sunrise, respectively. The median amount of time spent outside the nest at night was 27 min (range = 0–712 min), and most of this time was spent foraging. Nocturnal activity consisted primarily of long periods inside the nest interspersed with short periods of intensive foraging, during which voles ran out of the nest, harvested cuttings and hauled them back to the nest. Cuttings were stored inside the nest or on top of the nest. The mean number of foraging bouts per night was 3.4 ± 0.1 (range = 1–13), and the mean number of cuttings harvested per night was 20.4 ± 3.5 (range = 0–75). Females harvested more cuttings when they had large young in the nest and there was no evidence that they ate anything but the needles and bark of the cuttings that they brought to their nests. All three females produced litters while we observed them (1, 2, and 3 litters, respectively). Juveniles first began to explore outside the nest when they were 30–35 days old and dispersed when they were 47–54 days old. During the last two weeks before they dispersed the juveniles spent considerable time outside the nest at night, becoming increasingly adept climbers and gradually beginning to harvest their own food. Dispersal of siblings occurred on the same night and appeared to be precipitated by female aggression towards the young.

Red Tree Voles in the Columbia River Gorge and Hood River Basin, Oregon

Abstract In 2003 to 2008, we conducted surveys to document the eastern and northern range limits of Red Tree Voles (Arborimus longicaudus) in the Columbia River Gorge and Hood River basin, Oregon. Our survey indicated the current range of the vole includes the area from Wahkeena Creek, 20 km east of Troutdale to Seneca Fouts State Park, 6 km west of Hood River. We also found a concentration of Red Tree Vole nests in the headwaters of the Lake Branch of Hood River, 27 km southwest of the town of Hood River. Of 70 vole nests located, 36 (52%) were old unoccupied nests with no evidence of recent use, 24 (34%) were unoccupied but had evidence of recent use as indicated by the presence of fresh green resin ducts or cuttings in the nest, and 10 (14%) were occupied by voles. Of the 70 vole nests, 74% were in forests dominated by mature or old-growth trees and 26% were in stands 25- to 60-y-old. This survey is the first to document the existence of Red Tree Voles in the upper Columbia River Gorge east of Cascade Locks. It also documents the occurrence of Red Tree Voles on the east slope of the Cascade Range in the headwaters of the Lake Branch of Hood River.

Survival, Mortality, and Predators of Red Tree Voles (Arborimus longicaudus)

Abstract Although estimations of vital rates are important to understand population dynamics of small mammals, there is little information on survival rates and causes of mortality for many species. In 2002–2003, we estimated monthly and annual survival of 50 radiocollared red tree voles (Arborimus longicaudus) during a study of movements and diel activity patterns in western Oregon. Estimated annual survival for both sexes combined was 0.15 (95% CI = 0.06 to 0.31) and was influenced little by mass at initial capture. In the analysis of explanatory variables, we did not find strong effects of gender, vole age, or forest age on survival. We suspect this may have been due to small sample size and low power to detect effects, because some of the point estimates were suggestive of large differences among groups. Most mortality was due to predation, with 15 of 25 deaths attributed to weasels (Mustela spp.). Weasels preyed upon significantly more females than males (14:1, respectively). Other confirmed or suspected predators were owls (n = 3), a gopher snake (Pituophis catenifer), and a domestic dog (Canis familiaris). Although our results did not support the hypothesis that survival of tree voles was higher in old forests than in young forests, we caution that our sample for this comparison was small and recommend that more definitive studies with larger samples be conducted to better elucidate relationships between vital rates of tree voles and forest age and structure.

Response of arboreal rodents to increased availability of nest substrates in young forests

Many forest-dependent animals require complex branch and bole structural features as substrates for nesting, and these features may take decades or centuries to develop. In young forests, lack of suitable nest substrates may limit occurrence and abundance of arboreal rodents. To test this hypothesis, we measured the response of arboreal rodents to installation of 429 artificial nest platforms at 17 young forest sites in the Coast Range of Oregon, United States. We compared the percentage of 100-m2 plots at sites containing nests before and after installation of nest platforms, and examined 5 a priori hypotheses of spatial patterns of use using logistic regression. One year after installation, we observed a 5.8-fold increase (95% CI = 2.4–9.2) in plots containing nests of red tree voles (Arborimus longicaudus) and a 2.9-fold increase (95% CI = 1.3–4.4) of tree squirrels (Glaucomys oregonensis, Tamiasciurius douglasii). In addition, we captured 37 adult red tree voles (30 females, 7 males). Presence of conspecific nests < 75 m away increased the odds of use by red tree voles but not by tree squirrels. Our results support the hypothesis that lack of suitable nest substrates limits occurrence of red tree voles in young forests, but results were ambiguous for tree squirrels. Increasing the availability of nest substrates via installation of nest platforms may increase abundance of red tree voles in young forests. However, we do not know if this will allow red tree voles to persist in young forests that are subjected to repeated commercial thinning and clear-cut harvesting.