The Balance of Voles

The barn owls will be nesting very soon.  They've already started the process of dating, the males bringing Valentine's gifts of dead voles to their prospective mates and spending the daylight hours together tucked safely away in a dark corner somewhere.  If the female likes the gifts and the company, she'll soon begin laying her white eggs, counting on the male to provide all the food for her and then the entire family for weeks to come.  Hundreds of voles will be consumed in the process.

Some years ago, I provided some dark corners in the form of nest boxes to see if barn owls might help the local farmers with the ebb and flow of wild voles, which can cause thousands of dollars per farm in crop damage in a peak year.  I put out eighty boxes spread between two counties, in strings of three to five boxes per fence row.  The study finished several years ago, the funding gone, the results inconclusive, but the boxes are still standing.  To help out both the owls and the farmers who gave me access to their lands, I go out each December and maintain the boxes.

Barn owl nest box.  Photo: J. A. Gervais

Farming must be the most difficult form of legalized gambling, where those who farm have to work very hard, in often physically demanding and dangerous conditions, in hopes of producing a crop that can then be sold at a profit.  Almost nothing in this process seems to be in the farmer's control beyond the calculated decision of what to plant.  More so than any other livelihood directly linked to natural resources, farming is a long-term gamble, a balance of good weather and not too many pests, then favorable market conditions.

Barn owls rely on small rodents for their living.  The females will lay up to thirteen eggs, although the average clutch is closer to five eggs.  They hedge their bets by incubating during laying, so the eggs hatch over several days and the young owlets range in size.  The biggest owlets eat first.  In a pinch, they'll eat their younger siblings if the parents do not bring enough voles.  In the best of years, the adults will raise a second brood.

The farmers I work with like wildlife, and do what they can to accommodate the owls while they also pursue a precarious living.  Far more than most professions, farming reminds us that nature is ultimately in control, and we all rely on good weather, healthy soil, and clean water far more than most of us remember.  For a farmer, it is each year's bottom line, the balance between inputs and outputs tipping precariously on the edge of the appetites of voles.  Unfortunately, the appetites of owls are not great enough to swing the balance on the bad years, but they help.  Barn owls nesting in a barn make a mess but are usually tolerated.  Barn owls in a nest box are universally welcomed.

Cleaning out a nest box.  Photo: J. A. Gervais

The necessary maintenance of a barn owl box requires cleaning out the detritus of the past year, occasionally making repairs to the flap, or reinforcing the connection point between box and post.  Although natural cavities may fill with nesting material and prey remains over the years without much problem, the weight of all that material in a box that leaks a bit is much too great to be allowed to build up over time.  Armed with a gardening hand rake and a face shield, I open the flap, reach inside, and haul out all manner of rubbish into the dim December light.  It is a messy act of renewal.

My assistants are frequently showered with starling nests, great wads of sticks and straw that nearly fill the generous cavity.  It is usually raining, so the rain gear we wear serves a dual purpose.  Other unknown inhabitants bring bits of lichen and moss, or a few strands of grass.  Kestrels leave almost no traces of their nests, and a box used as a roost by barn owls may give no hint of its sometime occupants.  In a year with abundant voles, sometimes a vole carcass or two comes cascading out of the box, often with little else; we put those back, as they were put there by someone as a hedge against hard times.  Sometimes deer mice take up residence, bringing a light fluffy mass of shredded bark and leaves inside.  Others are empty except for a the abandoned comb of the paper wasp.  A barn owl nest, however, is an entirely different story.

Inside a barn owl nest box.  Last year's abandoned egg sits on the remains

 of hundreds of voles.  Photo: D. K. Rosenberg.

For those, I whack away with my hand rake at the thick, compacted mat of fur and bones and nameless dried goo, cemented in layers if the box is in the lee of some trees, a wet sodden mass if the rain can get inside.  Sometimes we find the remains of a young owl that did not survive the nestling period, or an unhatched egg now dessicated and filthy.  There is some satisfaction to getting all that mess out of the living quarters, leaving things ready for another year's accumulation.  Farmers, meanwhile, maintain tractors and other equipment, and soon they too will be getting down to the business of the year in the fields.

It is hard to leave a warm house on a cold, wet December morning before dawn to spend the day plodding along the fence lines in the rain carrying a ladder for miles.  There are compensations.  Sometimes we're given beautiful views of light and cloud, the song of a flock of swans, the calls of killdeer or snipe, and occasionally, the silent explosion of a white and fawn barn owl, as it exits the box just as I raise the flap.  There's time for the owl to get over the fright before the decision of where to lay must be made.  There is still time to plan out the new year, room for hoping for a good harvest and the right balance between voles and owls and people, all counting on the green grass in the fields.

An Ode to Voles

It's time to plant my peas, but I've been watching a series of holes appear around the foot of my garden's raised beds that suddenly began to appear in Februrary.  It looked like the lawn was undergoing a slow-motion boil, but in each spot where a bubble popped, a vole burst forth leaving a new burrow system behind it.  They've got their own plans for my vegetables, I'm afraid, because at least one new tunnel opened right into the bed itself among some overwintering beets.

Time to set some snap traps in the garden beds, and I've already sheathed our young orchard's tender trunks in hardware cloth.  Even as I take defensive action, though, I have to admit I admire these animals greatly.  An ounce or three of fur and digestive system, taken in the whole, has a tremendous impact on the ecosystem.

Gray-tailed vole, Microtus canicaudus. Photo by Jerry Wolff

Voles have fascinated ecologists for over a century with their high-latitude cycles of boom and bust; it's been a hundred years and literally thousands of scientific papers and we still don't really understand what drives those catastropic plunges in density that inspired the legend of mass suicide by drowning.  We are, however, making progress.  Regardless of what happens to the majority of them, a few voles survive the cataclysm, and the start of another wave builds from those survivors.

It turns out that the wave analogy is actually a very good one.  A number of organisms, from larch bud moths and pine beetles to voles and lynx, seem to display population dynamics that can be modeled as waves moving across the landscape.  There are a number of recent papers in the scientific literature by Jonathan Sherratt and others that apply the mathematics of periodic travelling waves to the population dynamics of animals.  Leaving the mathematics behind for now, imagine a wave of animals rising up on the landscape, their impact rising with their numbers, then abruptly dropping down as the peak of the wave moves on.  You can actually view a simulation of these waves on Dr. Sherratt's website.

When voles explode, they provide a ready protein source that is exploited by every carnivorous or omnivorous animal able to swallow them.  Some animals, such as the snowy owl and the least weasel, seem to be quite tied to the cycles of their prey; others are blatant opportunists.  I studied burrowing owls for many years.  The year the California vole populations were so dense I could sit on the cab of my truck and actually hear them grazing, the burrowing owls raised up to 11 young per pair.  The result: a wave of young predators spreads from the wave of available food, spreading concentric rings colliding and melding across the landscape.

Burrowing owl young. Photo: D. K. Rosenberg

Voles are small, but their numbers during a peak year can be impressive: estimates of voles' maxium densities run from 3,000 to 25,000 per hectare.  That works out to roughly three-quarters of a ton of small herbivore per acre.  Henry Howe and his colleagues put out exclosures in a prairie system and demonstrated that vole herbivory greatly affects the plant community.  What that study didn't consider is the dynamic nature of that herbivory as the populations crest and trough in any particular place.  So, imagine that wave sweeping over the landscape again, perhaps leaving behind distinct age classes in the vegetation.  I'm not sure anyone's looked for that yet.

Voles don't just eat and get eaten; they also dig and poop and pee.  Anyone who has ever looked after 750 pounds of herbivore living in a barn knows how much manure can be produced out the back end by non-ruminants especially, where so much of the bulk is passed undigested.  Voles dig, depending on the species; I've counted ten burrow entrances per square yard after a good outbreak year in western Oregon.  They create a vasculature of sorts, a series of pores and veins into the living soil which they then fertilize for good measure.  We've demonstrated that voles affect nitrogen in the soil profile.  We know the hollows in the soil allow both air and water to penetrate.  In western Oregon, the burrows may last longer than the voles that created them, in some cases up to a year or more before the burrows collapse again.  Even after the burrows have collapsed, the soil profile remains altered with pockets of less-dense soil.

Cross section of vole tunnel in soil sample.  Photo by J. A. Gervais

Imagine that wave again, flowing across a landscape, changing communities of plants and animals, changing the very properties of the soil, the foundation of the terrestrial ecosystem, and then abruptly receding.  Voles' impacts travel in both space and time, and after a century, we have only the most basic grasp of those dynamics and their consequences.

As global climate changes, so apparently are the cycles of the voles, and we have no idea what will happen to the ecosystems they inhabit when the voles no longer exert this incredible, dynamic force on the landscape.  The cycles may be disappearing in Scandanavia.  In other areas, the spikes in numbers may become greater or more frequent, heralding a different sort of change.  I don't know exactly what is driving the numbers of voles in my back yard, but I don't mind if they graze a few beets as long as they leave my peas alone, and they can help themselves to all the grass they can eat.  I'll be watching for young bobcats and barn owls later in the spring as the vole biomass (powered in part by my vegetable garden) works its way through the food web.