We have undertaken a Harvest Mouse nest search each year since 2013.

We have surveyed the same two areas in the same way every year. The survey involves ten x 10-minute searches of 10m sections of two 100m transects... yes that sounds more complicated than it is. This year we also surveyed a new area.

Normally we do the survey in December with Lark Rise volunteer monitors, but the Cambridgeshire Mammal Group requested a visit to do this survey in National Mammal Week as there was a particular interest in Harvest Mice from the Mammal Society.

The surveys must be done after the mice have finished breeding as the nests are used for their litters, but it is advisable to do the search before the vegetation gets completely bedraggled by winter weather. It was already fairly bedraggled by Nov 1st this year!

Transect A and B

In Telegraph field is 100m long, divided into ten 10m sections. Volunteers search each 10m section for 10 minutes, looking in amongst the tussocks of grass for the old nests of Harvest mice, across a search width of 2m.

If a nest is found that section is 'positive' and if not, it is 'negative'. We can then see how many sections out of 10 are positive and give that transect a score between 0 and 10.

We did the same in Wood Field and then get a total out of 20 for the whole farm.

These two sites are over 1km apart at Lark Rise, so the surveys give us a good idea of the status of Harvest Mice across the farm on an annual basis.

We add the two transect totals together and then get a total out of 20 for the whole farm.

An official '0' this year but we did find nests…

Although it is disappointing to get 0 this year, we did spend a short period of time before doing the first survey searching a likely patch of habitat. This was to give the volunteers their first experience of searching for nests and to show them what one might look like if we could find one.

Amazingly we did find two nests during this practice session, so the species is still present at Lark Rise Farm. All small mammals have highly fluctuating population sizes according to weather, predators, habitat, and food supply. They can quickly build numbers back up again as they breed prolifically in good years.

The habitat that Harvest Mice use to nest in is temporary - they like rough grassland and even arable crops. They weave a nest onto grass stems above ground level. We searched in the grass margins which sometimes get mown (making them unsuitable for searching) and sometimes get overgrown by scrub (making them unsuitable for nesting).

The mice prefer the margins 2-3 years after being mown, as they are transient occupiers of each field margin. Farmer Tim Scott mows different margins in different years. It's likely that the optimum margins were elsewhere, and the mice will be moving around the farm finding their best places each year.

It is possible that they did not survive the wet winter 2019/20 terribly well, as this is such a tiny species weighing around 5g.

Westfield’s first Harvest Mouse Survey

After our standard survey, we had a lunch break then moved on to Westfield where we have never surveyed for Harvest Mice before. Instead of doing a formal transect we undertook targeted searches of the best bits of habitat at different locations around the farm, just to see if the species was present or not.

At Lark Rise, Harvest Mice were introduced over 20 years ago, but we don't know if they would have been there naturally or not. At Westfield, we wanted to see if the species were present (which would be good news) or absent (in which case, we might consider another introduction scheme).

Ten of us spent about an hour and a half searching and one Harvest Mouse nest was found - a new site record and confirmation that if you create the right habitat, threatened species can find it and take advantage of it.

It seems 2020 is a poor year for Harvest Mice in general

The Mammal Society is planning to reinstate some sort of national monitoring scheme which means that in future we can compare our trends to a national picture.

When we started this survey in 2013, it was following a proposed national survey method, but shortly afterwards, that survey stopped operating but we have continued at Lark Rise for our own interest. Our historic records will be sent to the Mammal Society as part of their data set for designing the new survey next year.

Cambridgeshire Mammal Group will be putting details on their Facebook page. Many thanks go to Peter Pilbeam from the CMG for proposing the survey and arranging for so many of their members to come and help - we had 10 of their group doing the searching while I just had to operate the stopwatch and show them around the farm!

Although I do take credit for finding the Westfield nest which came about partly by good fortune. This survey was ideal for a socially distanced operation as we all searched 10m apart as shown in the photo of the search operation!

Dr Vince Lea
Head of Wildlife Monitoring

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