Here's the little youngster with the spot on its head. It's now getting the lighter- milk-chocolate coat of an adult.
The diary of a water vole colony
I've logged voles with white forehead spots several times on this blog, across several different locations, e.g. Spotty Voles I don't know what causes it, though several youngsters together with the same mark suggests it's genetic. It might also suggest how much water voles travel between different sites, keeping the populations healthy and preventing inbreeding. The photos above show two different youngsters, plus an adult, and only one of the juveniles has a spot. I don't know if any of them are related.
I think we'll find out in June whether this field will be built on and the water vole colony destroyed. It breaks my heart that these beautiful and supposedly legally-protected animals will disappear under the pressures of building work, habitat disturbance and domestic cats.