Monday, 28 October 2013

Searching for the Micro-mouse

...can be the proverbial needle in a haystack. When managing a particular habitat, we need to bear in mind which species might be occurring there. Last year, while surveying grasslands in the North West Zone with ecologist Laurie Jackson, we came across a tiny nest.


It looked rather typical of Europe's smallest rodent: a Harvest Mouse (Micromys minutus). This then got Laurie to thinking!

Somewhere, out there, deep in the River Mole grasslands... 

Harvest Mice are listed as a Biodiversity Action Plan (BAP) species because their habitats are being reduced and populations almost certainly declining. Jim Jones and his colleagues at the Surrey Wildlife Trust are conducting a Harvest Mouse Project to boost the ecological data on these often overlooked rodents. Laurie Jackson belongs to the Sussex Mammal Group and, as Gatwick is on the boundary of two counties, I was fortunate to have two pros involved in Gatwick's first small mammal survey.

A Longworth mammal trap made up of two chambers; a tunnel at the front and a chamber at the back with bedding and food. Mounting it onto a stake is an idea plagiarised from the study at Thundry Meadows. 

Longworth traps are humane, weight-triggered traps, stuffed with enough hay and food (such as oats, mealworms and carrot) for an overnight mammal stay. We check them every 12 hours, meaning that the survey is rather intensive and takes some planning. As days go by, the mammals tend to acclimatise to the traps and our hit rate increases. Below are some photos from our first and pretty successful trapping sessions:

Jim and Laurie demonstrate to assistants how to carefully bag up and empty a closed trap

Our first species was this Common Shrew (Sorex araneus) 5-14g. One of the UK's smallest mammals

An adult Field Vole (Mircotus agrestis) 20-40g. Scruffing mammals is the gentlest way
 to hold them in order to determine gender and breeding condition

A juvenile Field Vole (Mircotus agrestis). At this age they can seem almost tame

Wood Mouse (Apodemus sylvaticus) 13-27g. This particular female was both lively and pregnant

Then a bonus find in the form of a juvenile Harvest Mouse (Micromys minutus)! 
Britain's smallest rodent tips the scales at around 4-6g

It was Jim who found Gatwick's first Micro-mouse on both mine and Laurie's day off, scoring 1-0 to Surrey. This of course was quite unacceptable, so another surveying effort was needed. This time we raised the traps off the ground on stakes, hoping to target the acrobatic Harvest Mice climbing up amongst the grass stalks instead of the anxious little Common Shrews.

Chumming for Harvest Mice -  a selection of porridge oats, mealworms, apple and 
peanut butter placed in each trap to hopefully generate a feeding frenzy

I didn't go as far as baiting traps with live blowfly larvae, but I did raise the game with peanut butter and apple chunks. After setting up in the morning, we hooked our first Harvest Mouse that very evening!

A glimpse of my first wild Micro-mouse

Scruffing these little mites certainly takes some skill

We trapped two different adult Harvest Mice on consecutive days, and not that it's a competition... but I do believe that makes 2-1 to Sussex! The survey came to an abrupt end as the stormy weather began to close in; on the final evening we had a very good turn out of mammologists and a rather poor turn out of small mammals.

20 longworth traps on stands, all un-triggered. You win some, you lose some

All in all the effort was very worthwhile, so we hope to have another crack at it in spring. Many thanks to Tom Simpson and Kevin from Gatwick Greenspace for their help with the trap stands, to Laurie for her precious time and sourcing equipment, to Jim for his help and good advice, and finally to Katie, Jamie, Heather, Anthony, Martyn, Row, Pete, Reka, Rachael, James and Sue for coming along to help out on a rather damp and intensive survey.

*NB: In the defence of Surrey, it was a bit weighted towards the Sussex side in terms of people. Jim has therefore demanded a rematch!