Still searching for Micromouse

We have conducted 3 consecutive seasons of mammal surveying now, targeting Harvest Mice for the Surrey Mammal Group genetics study. I can only describe the feeling as 'incredibly disappointed' when you do finally capture one of your target species, then it perishes on the survey.

Eurasian Harvest Mouse (Micromys minutus), deceased adult male

Ecological surveys help us to target our conservation efforts, providing important data for reports such as the State of Nature. We do not wish to put animals through undue stress, and it was unclear why this individual died in the Longworth trap. As per the best practice protocols, traps were generously stocked with bedding, food, sources of moisture and checked within the regulated times. Harvest Mice do however have very short life spans, so it could be this individual was just reaching the end of his natural life. 

And so, after this one and only capture of a micro-mouse plus several discussions with other Harvest Mouse surveyors, these are the things I have learned:

1. Harvest Mice are difficult to survey, despite finding plenty of nests on our site. The fluctuating weather, changing habitat use and population crashes could all be impacting on our struggle to catch these little critters.

2. Despite this unfortunate mortality, Harvest Mice are fairly resilient; after speaking with other surveyors it seems very few casualties occur with Longworth traps. The overnight temperatures were above the minimum recommended for surveys, so cold was unlikely to be a factor, but we still took the precaution of providing extra insulation for the traps.

Longworth trap on stand with bubble wrap plus layers of grass

3. Wood Mice are relentless. I've seen more Wood Mice this year than craneflies. Their adaptability and climbing skills are to be commended, and they are certainly not deterred by the experience of temporary captivity! In fact, they mostly seem to find the traps before the other mammals can get a look in...

4. Shrews are awesome and sometimes I wish we were studying Shrews instead.

Lucy Groves, keeper at British Wildlife Centre with a Pygmy Shrew

Shrews love these castors (fly pupae)

5. Tube traps are a faff and take about 3 times longer to clean than Longworth traps.

6. On this survey, the average occupancy of Longworth traps was 49.71% whereas tube traps were 42.33% 

Collating trap data

Adrian Ashley is a handy chap to know - he's both an ecologist and a bespoke jeweller! He has kindly serviced the Longworth trap mechanisms which were not working so well.

7. Traps stands can definitely help to reduce the number of Common Shrew captures, but do not deter Wood Mice one little bit!

Tom Simpson often raids the set of vampire films for us 

Traps off the ground are more likely to tempt in Harvest Mice and appeal less to Common Shrews

8. Let the record stand that Martyn Cooke has still never seen a live Harvest Mouse in the wild.

Our survey data ends up in 3 places; on the internal Gatwick database, online on iRecord (which feeds into the National Biodiversity Network Gateway), and with the Surrey and Sussex Mammal Groups. We will be discussing the merits of continuing these surveys and any new approaches we might take next year.

Below are a few of our mammal records from the past 2 weeks (photos by Martyn Cooke):

Pygmy Shrew (Sorex minutus)

Bank Vole (Myodes glareolus)

Wood Mouse (Apodemus sylvaticus)

Surprise! Not a mammal: Wren (Troglodytes troglodytes)

A huge thanks to all who have helped out these past two weeks of surveys, and to Jim, Lucy and Laurie for all your advice.