The Mountain Has Voles!

Mary Beth James-Thibodeaux is a longtime volunteer at the Mission Blue Nursery, joining in 2011 which was early in the nursery's development. She has also been a regular and dedicated steward, making the Mountain and the Mission Blue Nursery a part of her life. In this Glimpse From The Field she reflects on California voles and the part they play in the Mountain's ecology. Be sure to check out her previous post Where are the Bees?


I often volunteer with the South San Francisco Weed Warriors, one arm of the stewardship program run by San Bruno Mountain Watch. The restoration work on the south slopes is led and managed by Chuck Heimstadt and Loretta Brooks who have done wonders restoring the south slopes of the Mountain.

One workday as I approached the section where the Weed Warriors planned to clear some non-native thistle, I noticed small holes in the ground. Loretta noted, somewhat like a mother discussing a beloved but wayward child, that they were the burrows of voles whose feeding and nesting mowed down the grasses. Microtus californicus, also known as Meadow mice, look like fat mice or gray hamsters. They are crepuscular (active at dusk or dawn) or nocturnal, like our familiar skunks and raccoons. Darkness protects them from their many predators, including birds of prey, snakes, coyotes, skunks and even the great egret.

Not to be confused with moles, these small rodents do dig burrows but they don’t eat worms and they spend much of their time out of their burrows collecting their favorite diet of grasses and grass seeds.  When I read that their favorite grasses are the imported European weed grasses: wild oats, wild rye, and ripgut brome, I was thrilled. The fact that the Meadow mice eat these grasses should be reason enough to love them, plus when voles thrive, their predators thrive too – and that’s good!

How the voles developed their taste for annual non-native grasses is lost to evolutionary history but it may be as simple as availability – the native grasses are diminishing and the non-native grasses are flourishing. Many successful animals adapt to changing habitats. Similar to voles’ adaptation to a new, non-native food source, the Anise Swallowtail butterfly (common on San Bruno Mountain) now primarily uses introduced sweet fennel or anise (Foeniculum vulgare) as the host plant and food source for their caterpillars. The introduced fennel is abundant and is also in the same plant family as the original native host plants (in the Lomatium genus of the Apiacea/Carrot plant family).

juvenile red-tailed hawk with vole

juvenile red-tailed hawk with vole

As I continued weeding on a steep portion of the south-facing hillside, I straightened up to observe at least 15 burrow entrances. Voles obviously prefer a southern exposure. They are not present every year and biologists have spent decades studying what causes their populations to cycle, with no clear results. However, they are a critical food source and other animal populations change in response to the vole population – it’s all about food! We, and their predators, will enjoy them this year and see them again some time in the future.

Join the SSF Weed Warriors on the Mountain’s south slopes this Friday March 31st and/or this Saturday April 1st, 2017. The south slopes are erupting into spring blooms, so don't miss this spectacular display and at the same time contribute to these restoration efforts!

Check Upcoming Events for future dates of their monthly outings.

the Mountain's south slopes in bloom

the Mountain's south slopes in bloom

(photographs of the California vole: © Ron Wolf 2009)