What’s for Dinner? Voles Top the List for Raptors

By JOE EATON Special to the Planet
Friday February 06, 2004

It’s been quite a year for voles. The evidence for this is indirect: high numbers of hawks, from the pastures of Point Reyes to the farmlands of Solano County. Word seems to get around that there’s a bumper crop of tasty rodents. 

I don’t recall that I’ve ever seen a California vole—a six-to-eight-inch-long mouse with dark brown fur, small ears, and a short tail—that wasn’t being eaten by something, or about to be. They’re an important prey item for raptors, composing 90 percent of the diet of the white-tailed kite. Northern harriers, also known as marsh hawks, will snatch a vole nest out of the grass, give it a good shake to dislodge the occupant, and snag the mouse as it falls. 

Herons and egrets, normally thought of as fish-eaters, also go for voles. I’ve watched a great blue heron dunk a vole like a furry doughnut until it stopped struggling, then gulp it down. In peak years, opportunists like ravens and Western gulls get into the act. 

It’s not just birds, either. Voles also fall victim to coyotes, foxes, cats (feral and domestic), weasels, and snakes. Weasels have been known to move into voles’ underground homes after eating the occupants. 

Pressure from predators tends to favor some kind of defensive adaptation among prey: speed, cryptic coloration, protective armor, noxious chemicals. Voles have none of those. As a group, voles survive—and they’ve done quite well at it, with 44 species worldwide, 17 in North America—by being fecund in the extreme. The vole’s defense is its ability to produce lots of new voles. 

In terms of reproductive biology, voles are about the closest thing to Star Trek’s tribbles that this planet has to offer. (I don’t know how they would feel about Klingons). A female vole is fertile at the age of three weeks. Weaning takes place at two weeks, and some females can even become pregnant at that point. With a gestation period of 21 days, an average litter size of five (maximum 11), and four or five litters in a good season—well, do the math. 

This potential for explosive reproduction pays off when voles have the opportunity to colonize a new area. Brooks Island is a 55-acre chunk of grassland off the Richmond shore, once managed by a gun club for pheasant hunters but now maintained as a reserve by the East Bay Regional Park District. California voles managed to reach the island in the summer of 1958. Within six months, they had occupied all the available habitat. 

We know a lot about the voles of Brooks Island thanks to William Z. Lidicker, Jr., a UC Berkeley emeritus professor. The island population was ideal for long-term study of the rodent’s social structure and population dynamics. They seem to be testy little creatures. Males will fight each other at the slightest provocation; females are peaceful unless an adult male is present. They sort themselves out into monogamous pairs, although males can be polygynous when conditions are favorable. Males have glands on their hips which they appear to use to scent-mark their runways in a behavior that Lidicker describes as “swaggering.” 

In a typical year, the Brooks Island voles started breeding a few weeks after the fall rains began. Their numbers built up through the spring, then dipped in summer when the vegetation dried out. 

As populations grow, California voles can reach densities of up to 400 per acre. Females tolerate daughters that stick around the parental territory, but mom’s pheromones suppress the daughters’ maturation. Take this far enough and you get the hive societies of the naked mole-rat, a creature that frankly gives me the willies. The pheromones of strange males cause females to abort their litters, giving the males a fresh opportunity to spread their own genes around. 

With an increase in density, greater numbers of large (for voles) males appear. Vole researchers used to believe the supersized males were a genetically determined type that was favored at high population densities. This was christened the Chitty Effect, after volologist Dennis Chitty. But Lidicker found that the big males often occupied resource-poor territories and were no more likely than males of normal size to be paired with fertile females. His Big Wimp Hypothesis posited that these guys were just standard-model voles that had grown to unusual sizes because of abundant food. 

Lidicker also discovered an interesting distinction between his island subjects and their mainland counterparts. Voles, like their close relatives the lemmings, are subject to cyclic population booms and busts. In mainland voles, a three- to five-year cycle is typical. But the Brooks Island voles demonstrated only a weak two-year cycle. Lidicker thought the absence of mammalian predators on the island accounted for the difference. (The only other mammals there were house mice, which the voles crowded into extinction, and rats that scavenged dead voles but did not prey on live ones. ) It’s not just a matter of the rodents depleting their food base, or of behavioral changes at high density slowing population growth; predation also seems to regulate their numbers. 

Voles are important enough as prey to be considered a keystone species by ecologists—a species without which an ecosystem would collapse. And there’s one recent study that suggests they also promote plant diversity: James Bartolome, a professor of Ecosystem Sciences at UC-Berkeley, found more plant species around the entrances of California vole burrows than in similar voleless areas. Voles may be small and obscure, but they play an ecological role out of proportion to their size.