|fwg is a long-term project of the ottawa field-naturalists' club|
Chipmunks at the FWG
by Christine Hanrahan
The Sciuridae or squirrel family is well represented at the Fletcher Wildlife Garden (FWG) where the four common species of eastern Ontario can be found. The eastern chipmunk (Tamias striatus) is the smallest member of the family, recognized by its reddish fur and five dark stripes along its back. A common and abundant species across its range, this diurnal squirrel is very clever at avoiding detection when needs be. While we generally think of this species in connection with deciduous woodlands, they are also found in mixed forests and commonly in residential areas. The FWG provides good habitat for this little ground squirrel, with an abundant food supply and numerous places to build their protective burrows.
Many species of chipmunks occur across North America, all bearing a striking resemblance to each other in terms of colouration and pattern. Our eastern chipmunk is the largest species, with an average weight of about 100 g and average length of 27 cm (Woods 1980, Forsyth 1985).
While not as vocal as the red squirrel, chipmunks do emit a bird-like "chip" (from which they take their name), as well as squeaks or squeals when agitated. It might be easy enough at first to confuse the sounds made by red squirrels and chipmunks, but with a little familiarity, the differences are soon apparent. FWG is a good place to learn these different vocalizations because chipmunks and red squirrels are usually quite noticeable here.
No doubt many of us have noticed at one time or another that chipmunks seem to vanish in midsummer. I was puzzled the first time I became aware of this and wondered if they had all been killed off. Yet by fall they were much in evidence again. In the past, authorities have wondered whether chipmunks enter a sort of estivation during the hottest time of the year. This has now been discounted and biologists believe that their seeming absence is in part due to the females being busy giving birth to their second brood. Since those first days when I thought weíd lost our entire population of chipmunks, Iíve found that they are more difficult to find because they stick to heavy shade, and they also seem to be more active, and therefore more visible, earlier and later in the day - but they are certainly present.
Although very efficient at climbing trees, chipmunks are almost entirely dependent on an underground network of burrows. These burrows are important to chipmunks year round, not only as natal sites, but for sleeping, food storage, protection from the elements, and hibernation. Most authorities agree that chipmunks will maintain the same burrow for their entire lifespan. Forsyth (1985) says, "It enlarges its tunnel system each year so that by the end of its two-to-five year life, it has dug an extensive network of side tunnels in addition to the main one."
Chipmunks are far less aggressive than many species and tolerate others of their kind in very close proximity. Woods (1980) notes that the home range for a male is roughly 0.2 ha and for a female 0.1 ha. Population density depends on many variables including food supply and competition from other species. Woods suggests that an "ideal habitat" may contain 25 adults per hectare. At FWG, this would mean an astonishing 125 animals on our 5-hectare site. Competition for food (with other species), the relatively high disturbance levels from humans and dogs, and the presence of predators (dogs, raptors, foxes) all combine to keep populations relatively low at the garden. However, no real census of this species has been done at the FWG, and efforts to track numbers may reveal a higher density than previously thought.
It appears that there may be 6 to 10 chipmunks living at FWG at any given time. Numbers vary of course, and there will be a temporary increase when youngsters are born, but the mortality rate is high and as far as I can determine, we usually end up with the same number of animals by summerís end.
Chipmunks, almost more than any of the other squirrels, seem pre-occupied with the business of gathering food. As with many rodents, vegetable matter forms a big part of their diet. Seeds are the preferred food, but nuts, bulbs, berries, fruit, frogs, salamanders and invertebrates are important food sources. I remember reading somewhere that they will also take birdís eggs and nestlings.
Although usually encountered running over the ground, along fallen logs, on top of fences and brush piles, chipmunks are also highly arboreal, adept at climbing trees to reach food sources.
At the FWG I have often watched them climbing our many crabapple trees (Malus spp.) in the fall to feed heavily on the fruit. They are skillful at separating the fruit from the seeds, which then get stored in their cheek pouch and eventually taken to their underground food cache. I have also watched them eating the seeds of Manitoba maple (Acer negundo) and sumac (Rhus typhina), acorns of red oak (Quercus rubra), and of course seeds that have fallen from the two bird feeders at the FWG.
Chipmunks are ingenious in their food storage. Iíve heard that they separate their sleeping quarters from their food storage area, or larder. However, some biologists believe that they build their leafy winter nests on top of their food supply (Woods 1980, Anderson and Stephens 2006). They spend a huge amount of time and energy stockpiling food and with good reason. A well-provisioned larder means the difference between surviving the winter or perishing from starvation. Although chipmunks hibernate (more about this below), they wake periodically to feed. According to Woods (1980), this periodic awakening is necessary as chipmunks are "ill-equipped to withstand a long cold siege without food as he doesnít have a heavy layer of fat like some of the other Sciuridae." Furthermore, they often emerge from hibernation before food becomes readily available and must rely on their larder to sustain them.
Unlike red and grey squirrels, which build large bulky nests or make very obvious use of cavities, chipmunk nests are built underground at the end of a long burrow. It can be difficult to find the entrance to such burrows because they are often hidden under brush piles, rock piles, shrubs or other places that afford protective cover. At the FWG I have found very few, but those I have encountered have been under brush piles, under downed logs, and under particularly dense shrubbery. More often than not, it is sheer luck that has allowed me to find the entrance to a burrow, rather than any systematic searching!
According to Forsyth (1985), chipmunks usually have two litters a year, one in the spring and one in mid-summer. With an average litter size of 4 or 5 young, it is easy to see how chipmunks could become one of the most abundant animals at FWG. However, as noted elsewhere, their numbers appear to be more or less stable (so far as we can determine) at 6 to 10 animals. I have seen very few young animals at FWG and those usually in early to mid-June. Like all young animals they are a delight to see.
The only way to get a look at a chipmunk nest would be to locate a burrow and dig up a wide area in order to try and expose the natal site. However, I donít recommend that and certainly would not do it myself. Nonetheless, some biologists have studied chipmunk burrows and nest sites and from them we get some idea of what they are like. Woods (1980) says that the tunnel descends sharply for the first 15 to 20 cm and then after a more gradual descent to a depth of about 65 cm, the tunnel "may continue parallel to the surface for up to 3 m before it terminates in an oval-shaped sleeping chamber," which is roughly 32 cm in diameter and lined with leaves.
Most rodents in the wild live very short lives. It has been estimated that chipmunks have an average life span of 3 years (Banfield 1974, Woods 1980). Chipmunks born in the spring are said to have a much better chance of surviving the winter than chipmunks born in late summer. This makes sense, for these latter animals would have far less time to prepare for winter.
By late summer, chipmunks are busier than ever gathering food in preparation for the coming winter. They seem less concerned with concealment and more concerned with getting the food into storage. Forsyth (1980) says that they will store about 6 litres of food which includes anything that stores well such as seeds, nuts, and plant tubers.
Chipmunks hibernate during the colder months, although this is not so much sleeping the winter away as entering a state of torpor from which they will periodically awaken in order to eat. Emerging from this torpid state would not be immediate, because their body functions slow down during this time. Banfield (1974) comments that when a chipmunk wakes from torpor "it trembles, stiffly unrolls, and staggers about, often with its eyes shut."
Unlike red and grey squirrels, they donít develop a thicker coat, and unlike many other animals who remain active year round, they do not fatten up for the winter. All their energy goes into ensuring their larder is full so that they can eat during the winter without having to expend valuable energy searching for food. On particularly warm winter days, you may even see the occasional chipmunk roaming around the garden, but this is by no means common.
You might think that being deep underground would guarantee safety, but weasels are known to enter chipmunk burrows and kill the animals, which would be very easy prey, particularly if still in torpor. Whether the short-tailed weasel we had around the garden a few years back ever did this is not known, but my guess would be that such a clever animal would make full use of all food sources!
By about mid-March, particularly if the winter has been fairly mild, we can expect to see chipmunks once again scampering around the FWG.