Water voles are not the only small brown furry mammal found in the water ways of Oxfordshire - on many occasions it is more likely to be a brown rat.
They are so abundant it is hard to believe that they are not native, having originated in the far east, they were only first recorded in the UK in the 1970's.
One of the reasons they are so common is that they are prolific breeders, a female can have up to 10 litters per year (water voles have at most 4) and each litter can consist of up to 9 young (whilst a water vole has only 6 per litter) - hence (with the right conditions and sufficient food) a female rat can produce nearly 100 young in a year.
They are often considered a pest as they spread several diseases - salmonella & leptospirosis (Weil's disease) - hence it pays to be careful when doing water vole surveys.
Despite this, it always entertaining to watch a rat (they are very intelligent and can make good pets), even if - like this one - they are seen entering burrows, maybe seeking out young water voles, one of it's many sources of food - another thing which makes them so omnipresent