In today post I want to illustrate how a variety of techniques is somewhat needed to verify the presence of a species on a site. My case study for this is the Harvest Mouse. Harvest Mice are the UKs smallest rodent weighing only 6g when adult and not a species I expected to find on my land.
The habitat is suitable, thick brambles and stands of tall
grasses and reeds are separated from agricultural land by a hedgerow. When I
first started to manage my patch, I did wonder if Harvest Mice occupied the
site and put out some cut of tennis balls to see if any would nest in them,
this was a complete failure a) because even with GPS recording them, I lost them
in the dense vegetation and b) because the frequent winter floods swept them
away. I saw no signs of their presence although I have looked for nests and
whilst I have seen bundles of grass, they were never distinct enough for me to
rest an identification on.
I assumed Harvest Mice if present were restricted to the
field which predominantly grew wheat and corn, that was until this year.
I blogged earlier in the year about my purchase of a static
batlogger and my success with it. From June I have been putting it out to
record the bat species. I am still getting a handle on interpreting that data
and in my June post I explained that I had detected Harvest Mouse calls.
Harvest Mice are known to vocalise in ultrasonic pulses
between 20 and 70 kHz, with four notable call types that are distinct from
other rodents. The data I collect with the Batlogger is analysed through the
BTO pipeline. This outputs the results of all recorded sounds with an assigned
identification, this identification is qualified by a probability that the
sound file matches the sonograms known for that species. It is generally that
any probability lower than 0.5 is discarded. This value seems very low to me,
and I had decided only to accept probabilities above 0.75 and 0.9. The spread
of the data can be seen in the boxplots below.
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As you can see the average probability is around 0.6 and the number of identified files is very low. For this reason and the thinking that the detectors’ location did not ideally line up with suitable habitat I decided to not count Harvest Mice as a resident species at present but to keep an eye, or rather, sensor out.
Given their tiny size it is very difficult to see a Harvest
Mouse in the wild, specific surveys needed to be conducted. I doubted that my
three trail cameras would yield much given their locations and the fact that I
have always considered Harvest Mice to be a species that doesn’t come down to
the ground much and spends much of its time in the vegetation.
I was proved wrong on the 19th August. A single
image and 10 second clip from my Mostela camera revealed what seemed to be a
harvest mouse.
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The individual in shot was tiny, its tail thick, its ears small and its nose blunter, but the image was black and white, and I wanted data to back up my gut. I posted the image to the Mammal Society Facebook group to garner opinions and set about researching. My Mostela is regularly visited by Wood Mice and shrews, and this individual had none of the ‘vibe’ of those species. As you can see from the images below the mouse was about 2.265 inches long (Head and body length) this equates to 57mm right at the lower end of the expected range of Harvest Mice and well below that of the Wood Mouse.
In the image below I show a Wood Mouse so that you can see the size difference.
Research indicates that Harvest Mice do travel along the ground. At this time of the year, they should be breeding and be using breeding nests in dense reeds, grasses, brambles or cereals. The field by the mostelas was sown with wheat which due to the hot weather and the variety grew little higher than 40cm and was harvested on the 5th July. Maybe the nest was destroyed then and the mice dispersed into my patch.
What I like about this story is the way in which the
technology and research work in tandem to identify a hitherto unconfirmed species.
This identification used two remote recording devices, a host of Mammal experts
on Facebook, and deep dives into two books (Sound Identification of Terrestrial
Mammals of Britain and Ireland, and Mammals of the British Isles)
It gives me a new drive to investigate this species to
establish if they are a resident under reported species. I will continue to use
the Batlogger relocating it closer to the mostelas and conducting a proper
winter survey looking for old nests. I ll report back here as and when I find out more.
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