Yesterday I was lucky enough to spend a day out at Ouse Fen with the wardens, helping them with their annual water vole survey.

Despite being widespread in the UK, water voles have been identified as one of Britain’s fastest declining mammals. There are an estimated 220,000 water voles in the UK – this may sound like a lot but, prior to the 1960’s, there were thought to be around eight million!

 Water vole (Image credit: Ben Andrew, rspb-images)

The two main threats to water voles are habitat loss and predation by the introduced American mink. Minks first escaped from fur farms in the 1950s and 1960s and have rapidly spread across the country.

In order to monitor the health of water vole populations, we carry out annual surveys at Ouse Fen. At present, Ouse Fen is organised into so-called ‘cells’ – basically, discrete reedbeds where the water level can be controlled independently from other areas. We carried out surveys in four of these cells.

Rather than estimating the numbers of water voles, we were just looking for evidence that they are present on the reserve. Although it is difficult to spot the voles themselves, it is not hard to find signs that they have been around – if you know what to look for!

The first thing we were looking for in the reedbed were feeding stations. Water voles often bring food items to favoured feeding locations and, here, they leave behind neat piles of vegetation. These are usually about 8cm long and have 45° cuts to their ends.

 Water vole feeding station

The presence of latrines is the other way we know water voles are present. Latrines are basically piles of droppings, which are shaped a bit like a tic-tac! The droppings themselves have no scent and so voles will often rub their hind feet on scent glands they have on their sides and then stamp on the droppings. These trampled latrines are a good indication that breeding is taking place. 

 Water vole latrine

Having spent the past six months working at Fen Drayton Lakes, I am pretty familiar with what a reed bed looks like - from the outside that is. As we scrambled down a steep bank and into the reed bed I was so surprised by how dense the reed bed was. Previous water vole surveys I have done have been around ditches and rivers and it has always been fairly easy to spot the piles of green vegetation. It was a different story in the reed bed - the feeding stations blended in with the reed bed litter making them very hard to spot!

 A vole's eye view of the reedbed!

 Warden Hannah searches for signs of water voles

 Spot the Assistant Warden...

Happily, we found signs of water voles on all four of the cells that we surveyed. As Ouse Fen continues to increase in size over the coming years it is hoped that the reserve will be a home to even more water voles, as well as a plethora of other species.