Tracking animals can be an elusive endeavor because so many things can alter the condition of the tracks. Have recent flurries erased the details of an imprint? Has the sun melted and enlarged a track? Was every toe registering? Did wind-blown snow cause the tracks to vanish into thin air? Was the animal walking, loping or tunneling or a combination of all three?
The reason you use more than just one track to gather information, such as the stride of the animal and the width of its trail, is that sometimes the individual tracks defy the hard and fast rules of some tracking guides. A commonly accepted generality is that Deer and White-footed Mouse tails leave drag marks, and Meadow Voles’ shorter tails don’t. However, in the right conditions, even a vole’s one-inch tail can drag (see photo), though not creating as long a line as a mouse’s tail would. The Meadow Vole whose tracks are in this photograph was loping along when it suddenly decided to seek cover under the snow and began to (try to) tunnel. Perhaps a predator instigated this behavior.
With snow on the ground, the season of stories in the snow has begun. Many of the animals that remain active in New England during the winter are nocturnal so we rarely get a glimpse of them. But, more than at any other time of year, we are privilege to their activities due to the tracks and traces they leave in and on the snow during the night.
Much information can be gathered from these signs. Often at a kill site, the identity of the predator as well as the prey can be determined by shape, form or measurement. One can see from this photograph that a bird of prey (measurements indicate a barred owl) swooped down on a small rodent (judging from tracks, probably a meadow vole) and was successful in capturing it.
Even though a kill scene such as this, or any other wildlife activity recorded in the snow, may reveal the probable identity of the characters in the story, there are always more questions than answers, which is what gets us out in this frigid weather, day after day. The main question I’m left with after viewing this scene is why do voles and mice risk their lives by travelling on the surface of the snow at times, when they just as well could have covered the same ground in the maze of tunnels they’ve created deep in the subnivean layer between the ground and the snow (where they would be out of sight of hungry predators)? (Thanks to Rob Foote for photo.)
Frequently you find a hole about an inch wide in the snow in the middle of a field, with no tracks going in or coming out of it. Logic tells you it leads to the subnivean layer – where the snow, warmed by the ground, sublimates into water vapor, creating a small space between the surface of the ground and the snow where the temperatures is relatively stable at 32 F. It is here that small rodents such as mice and voles create a maze of tunnels through which they travel from their sleeping quarters to feeding stations, undetected by many predators.
However, the lack of tracks into and out of this hole indicates that it is not an exit or entrance to the subnivean layer, but rather, it is a vent leading from the subnivean tunnels to the surface of the snow. Carbon dioxide from animal respiration as well as carbon dioxide released from the ground builds up to an unhealthy level in these tunnels. The holes we see in the snow are ventilation shafts, allowing the carbon dioxide to escape from the tunnels. The formation of crystals around the edge of the pictured vent indicates that warm, moist air from rodent lungs is rising up out of it.
Under perfect conditions, with no predators, no deaths and abundant food, a pair of meadow voles (Microtus pennsylvanicus) could produce a million descendants in a single year. Because they are prey for nearly every meat-eating animal that exists, however, their population, while large, is nowhere near this.
Even though the number of meadow voles is relatively high and they are active day and night, year round, it is unusual to actually set eyes on one. What we do find, especially this time of year, are meadow vole signs in the snow: mazes of runways on the surface of the ground that are exposed as snow starts to melt, air-exchange holes originating in their tunnels and extending to the surface of the snow, tracks and entrance/exit holes to their tunnels.
The social behavior of meadow voles is about to undergo a seasonal change. During the winter, when they are not breeding, meadow voles are more social and commonly share their nests, probably to conserve heat. In another month, however, as breeding begins, females become fiercely territorial towards other females, and males are aggressively establishing dominance over each other. The peaceable subnivean meadow vole kingdom is about to come to an end. (Thanks to Susan and Dean Greenberg for photo op.)
Meadow voles (Microtus pennsylvanicus), tiny mouse-like rodents, spend a majority of their time on the surface of the ground, particularly in moist fields filled with grasses and sedges. They have elaborate runways through the vegetation – well-worn trails about the width of a garden hose which they keep mowed down with their incisors. Latrines of small brownish-green pellets can be found intermittently along the trails.
Most meadow vole nests are constructed out of dried grasses, also on the surface of the ground, although they are sometimes built at the end of shallow burrows. When above ground, the nests are often located in the center of a grass tussock, where they are less apt to be flooded. When there is snow on the ground they are a bit easier to find, as the heat of the voles inside the nest melts away the snow, forming a chimney that sometimes reveals the nest below.
Owls swallow small prey, such as mice and voles, whole, while larger prey is torn into smaller pieces before being swallowed. Once eaten, prey goes directly into the owl’s stomach, as owls have no crop, and thus no ability to store food for later consumption.
Like other birds, owls have a stomach with two chambers — one is the glandular stomach, or proventriculus, (yesterday’s mystery photo) which produces enzymes, acids and mucus and begins the process of digestion. (Because the acids are weak, only the soft tissues are digested.) The second stomach is the muscular stomach, or gizzard, also called the ventriculus. The gizzard lacks digestive glands – it serves as a filter, holding back bones, fur, teeth and feathers that are difficult to digest. The soft parts of the food are ground by the gizzard’s muscular contractions, and allowed to pass through to the rest of the digestive system.
Several hours after an owl has eaten, the indigestible parts remaining in the gizzard are compressed into a pellet the same shape as the gizzard. The pellet travels back to the proventriculs and remains there for up to ten hours before being regurgitated. Because the stored pellet partially blocks the owl’s digestive system, new prey cannot be swallowed until the pellet is ejected. If more than one prey is eaten within several hours, the remains are consolidated into one pellet. (In this case, one very large pellet!)
Update: I left the contents of the deceased Barred Owl’s proventriculus outside last night, and a resident Barred Owl recycled the Meadow Vole and Masked Shrew.
Mice and voles are commonly lumped together, probably because the differences between them are so slight. Both are small, furry rodents, but mice generally have large eyes, large ears and long tails (close to or greater than the length of their bodies). Voles have smaller eyes, smaller ears (often concealed in their fur), and shorter tails. Voles tend to be active day and night, whereas mice are mainly nocturnal. ( Meadow voles are commonly referred to as “field mice,” which tends to add to the confusion regarding these two groups of mammals!) There are five species of mice in New England (white-footed, deer, house, meadow jumping and woodland jumping), and four species of voles (meadow, southern red-backed, rock and woodland).