Our recording of small mammals, at Red Cow in Cholsey, is almost entirely done by noting what lies beneath about 30 metal sheets. These are inspected once a day if and when time permits.
The Field Vole, or Short-tailed Vole has for years been our most often recorded small mammal. Then in 2004 the Bank Vole, that had always been lagging behind in second place, overtook our long time leader. This was attributed at the time to habitat change. Recently planted hedges and trees were growing taller, resulting in a small reduction in open space and a little more shading.
The following year, we noted that Bank Vole sightings remained remarkably constant, but that the Field Vole had further declined. By April 2006 we realised that our Field Vole population was in free fall. We recalled reading about vole population crashes somewhere – so reference books were quickly consulted. There it was – Field Vole populations were subject to periodic crashes. Also it seems the reasons for these three to five year fluctuations is not really understood.
A couple of months later we were reading in wildlife magazines and newspapers that 2006 was indeed experiencing a sharp decline in vole numbers. So feeling a bit smug that we had detected a change several months before reading about it, we decided to delve into our records to see what they told us.
As we delved, that smug feeling began to evaporate. The reason being, that the warning signs had been there long before we realised anything was afoot. Way back in early June 2005 was the last time we had seen more than one Field Vole together.
Prior to that it was not unusual to discover two together, or nests full of youngsters. So it seemed that for well over a year, the evidence suggested a failure to breed on the site. No surprise therefore that no Field Vole sighting has been made since 9th June
The area is still regularly patrolled by both Kestrels and Tawny Owls, so clearly they are not dependant upon Field Vole snacks. Perhaps our Grass Snakes are to blame, yet further examination of our records suggests otherwise. Surely when we had
discovered nests full of young Field Voles, it might be expected that a day or two later a snake with a full tummy would sometimes be found nearby. This was not the case. So no explanation is apparent, and we wait so see if Microtus agrestis returns.
Tony & Ro Rayner
A brief statistical summary of our sightings appears in the Vertebrates Recorder’s Report in this years’s Naturalist.