OtterCam often detects "non-target" species, ie things that aren't otters, most commonly a juvenile water rail. This time it detected a vole. When I reviewed the video on the camera's small replay screen I thought it was a brown rat but it was clear on the computer that it was a vole. The question is, which vole? Our three native voles (bank vole, field vole and water vole) are distinguished by size, fur colour and relative length of tail. On a black & white infrared video it is down to body size and length of tail but the problem is that there is no calibration. Here is a still from the video.
I found picture of a water shrew on an earlier recording that was more or less the same distance from the camera.
An adult water shrew is 6.3-9.6cm in length and the vole looks to be about twice as long as the shrew. A bank vole is 8-12cm, a field vole 9-11cm and a water vole 14-22cm. So if this was an adult water shrew the vole has to be a water vole. But the still image also shows that the vole's tail is relatively short, perhaps one third of its body length. Quoted relative tail lengths are about one third for a field vole, one half for a water vole and two thirds for a bank vole.
So I tried a different calibration. I know the diameter of the culvert is 75cm and using the diameter at the vole's position in the video gives a vole body length of 10-11cm. Coupled with the short tail I came to the conclusion that this must be a field vole and not a water vole. (I now presume the water shrew is a juvenile.)
Disappointing, as I would have liked to find a water vole but common things are common and field voles are certainly common - there are about 75,000,000 field voles in this country at the end of the breeding season, about one hundred times the number of water voles.
So my quest for a good trail camera picture of a water vole goes on. In the meantime here are the video images from the two cameras.
How fun! Bayes rules ok!
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