Red squirrels: Little critters, complex lives

We might not notice them but these small mammals are industrious

Posted: Sunday, December 03, 2006

The red squirrel is a common resident of forests in our area, although lots of folks may tend to ignore it - except when an enterprising individual decides to nest in an attic, pull out some pink fiberglass insulation to improve its own shelter or raid bird feeders.

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This diminutive denizen of our forests scampers acrobatically up and down the trees and through the canopy, but it can also be seen basking on a sunny branch on a cool day.

Red squirrels are excellent swimmers - naturalists often report seeing them crossing lakes and rivers, even climbing into canoes to rest, and I once saw one swimming across a wide channel in the Beardslee Islands in Glacier Bay.

Red squirrels are fiercely territorial, defending an area of one to three acres that contains their yearly food supply. The main items on the menu are the seeds of conifer trees, although the squirrels also eat other seeds, pollen, fruit, fungi, willow sap, mice, and the eggs and nestlings of birds.

Territory size is related to food supply: high densities of food permit smaller territories, and therefore higher densities of squirrels. The food supply is generally greatest in mature conifer forest, because large trees produce good cone crops more frequently. Cone-crop failures can result in reduced or failed reproduction of the squirrels.

Both male and female hold individual territories, although a female may yield a portion of her territory to a daughter. A female usually defends her territory vigorously, except for the one day a year when she is sexually receptive.

On that day, all the available males in the neighborhood invade her territory, and a receptive female may be chased by as many as 10 males. When the excitement is over, she rears a litter of up to eight young, with larger litters in years of good food supply.

Nests may be in tree cavities or balls of vegetation placed in a tree. Here in Southeast, I have not seen tree nests, perhaps because the tree canopy is both dense and high above the ground, but I also suspect that many nests may be underground. Gestation lasts about six weeks, and the young are weaned after about 8-11 weeks.

Most of the juveniles disperse, usually not more than a few hundred yards from their natal territory. However, in Glacier Bay, small, isolated stands of Sitka spruce, several kilometers from continuous conifer forest, often harbor a resident squirrel, indicating that dispersal can occur over longer distances.

Because the spruce stands are small, these squirrels often forage in surrounding alder thickets, gathering alder cones. Dispersal is a dangerous time for juvenile squirrels, mostly because the risk of predation is greater when they are in unfamiliar areas. Major predators include hawks, owls, and marten.

Cone harvest occurs in summer and fall. Squirrels cruise about the treetops, cutting off cones and letting them fall to the ground for subsequent collection. Male squirrels start cone harvests earlier than females, which usually delay their cone harvest until after the young are weaned.

Harvested cones are stored in heaps between tree roots or fallen logs, or underground, typically in damp places where the moisture helps keep the cones from opening.

Every squirrel has favorite perches where it peels off the cone scales to extract the seeds. This creates piles of empty scales that are clear indications of a resident squirrel. The heaps of unopened cones and the piles of cone scales are both called middens.

Red squirrels are major predators of conifer seeds and therefore conifers have an array of adaptations that help protect their seeds from excessive consumption. These may include heavier cone scales that cover the seeds, cones that are more difficult to detach from the branch, fewer seeds per cone, more copious sticky resin, and cone crops that vary markedly in size from year to year.

Red squirrels are one of the most important predators of bird eggs and nestlings in our forests. My observations in Glacier Bay suggest that songbirds may avoid nesting in deciduous forest around isolated spruce stands inhabited by squirrels; however, birds nesting in the continuous conifer forest cannot avoid nesting near resident squirrels.

These squirrels are also important consumers of subterranean fungi such as truffles; the squirrels digest only the fleshy part of the fungus, passing the intact spores through the digestive tract and depositing the spores in their scat, thus spreading the fungi to new sites.

Most of these fungi are mutualistic with many of the forest plants: they use some of the carbohydrates synthesized by the plant and pass soil nutrients to the plants in return.

Red squirrels are widespread in North America but did not occur naturally on some islands (including some in the Alexander Archipelago of Southeast Alaska).

Under the largely erroneous assumption that red squirrels are the favorite prey of pine marten (which actually seem to prefer voles, at least in Southeast Alaska), red squirrels were introduced to several of these islands where apparently they did not occur naturally.

In Newfoundland, introduced red squirrels ate up the conifer seeds that were also the main food of local crossbills, with the results that the distinctive Newfoundland population of crossbills is extinct.

Closer to home, the introduction of red squirrels to Chichagof and Baranof islands must have had a serious (but unmeasured) impact on nesting songbirds.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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