The bank vole is Finland’s most abundant mammal with a population estimated at about 300 million during peak years.
Bank voles live in forest and wetland edges and clear-cut forest. You can also find bank voles in gardens.
What makes the bank vole interesting to science is the regular fluctuation in population size. This follows a four-to-five year cycle where the population slowly increases to very high densities followed by a rapid population crash to such a degree that it can be hard to
find any bank voles in a certain area.
The photos in this gallery were taken in the winter of 2018-2019 when the population was near its peak. In the winter of 2019-2020 bank voles were initially easy to find, but rarely seen in 2020 and the population appears to have crashed.
In a few more years there will be an abundance of bank voles again.
The bank vole has been studied greatly in Finland, with much of the research linked to the cause of the population fluctuations. As is often the case in ecology, there is no single cause, but it is most likely a combination of disease, food competition and predation.
The yellow-necked mouse is Finland’s largest mouse. It is another rodent that lives in forest and forest edges. It will also come to gardens, especially if you are feeding birds in winter.
The population of the yellow-necked mouse also fluctuates, but interestingly this fluctuation is separate from that of the bank vole, even though it shares the same habitat and food as the bank vole.
Yellow-necked mice are mostly nocturnal, but can sometimes be seen during the day and
they are also active during the winter when you can see them during the day.
This gallery of forest rodents comes from the lens of Espoo-based wildlife photographer Paul Stevens.