The following is a Guest post by Ross Stevens, on behalf of Total Ecology who undertake surveys for water voles and otters at http://www.totalecology.com/mammal-survey/. Thanks Ross!
Water Voles don’t hibernate. They remain active through out the cold months, however they spend most of their time underground within their nest chambers. In winter the colony will often contract, so that adults that were previously territorial now live side-by-side within the same burrow system. They normally choose a place where they will be safe from flooding, where they can dig burrows higher up the bank sides away from the waters edge. If winter grounds are not chosen wisely then a severe winter flood can result in a whole colony of water voles being flooded out and lost. In this situation, if they don’t actually drown, then they are usually picked off by one of the many animals that feed on them. The most famous water vole predator is the American Mink, which escaped or was released from fur farms during last century and is now widespread throughout the country.
Flooding is not the only problem a colony will face over winter. Less vegetation means less cover to protect them from predators, but more importantly it means less food. Water voles will eat a wide variety of plants, however tree bark, especially willow, provides an important food source in winter. It’s at this time of year that water voles will also turn to the roots and bulbs of plants in order to survive the winter months and in autumn fallen fruits are a favourite.
A whole colony living in one place makes them far more susceptible to persecution. Water voles are far larger then field voles, bank voles and mice. They are only slightly smaller then a rat, so are far too often mistaken for one. Colonies of water voles are often dug out, or poisoned, or killed by terriers. Thought to have undergone a 94% decline, the water vole is now one of the most threatened mammals in the country. This harmless vegetarian does not spread diseases like rats do, so please think carefully before putting poison next to a watercourse or any water body.
It is thought that a water vole needs to obtain a weight of at least 170g in order to survive the winter. This means that those born earlier in the year have better chance of survival. A water vole rarely survives more then one winter, but very occasionally they may survive 2 or even three. All in all, it seems that winter is a hard time for a water vole. It is actually thought that up to 70% of a colony is lost over winter, leaving just a few individuals to set up breeding territories again in March.