A static detector records ultrasound all night and software identifies each call from its shape, and a person checks the doubtful ones by eye on a spectrogram. At one barn it records greater mouse eared bat, a species with no colony in this country in thirty years, and the calls are textbook.
I am a licensed bat ecologist, and static detectors are how most of my surveys are done.
How the recording works.
A detector is a box with an ultrasonic microphone. You strap it to a post at dusk, it records every call above the human range through the night, and in the morning you take the card. Software sorts the calls into species by frequency and shape, and you check the ones it is unsure of by looking at the spectrogram, which is a picture of the call.
Species have signatures. A common pipistrelle peaks at about forty five kilohertz, a soprano at fifty five, and a noctule lower and louder. After a few seasons you know them on sight, as you would know a voice down a corridor.
Greater mouse eared bat has a signature too, and I learned it from recordings, because there are none to hear. The last colony in Britain went in 1990 and there has been one lone individual since, in one place a very long way from here.
At one barn on one farm, a static detector records greater mouse eared bat calls every night I put it there.
Between eleven and twenty passes a night. Not one doubtful call. The software calls it and I call it and two colleagues I sent the files to called it without being told what to look for.
The barn is an ordinary stone field barn in a county that has never had the species.
What has been checked.
The detector. Three different units over three years, two makes.
The microphone and the calibration. A tone through the microphone at a known frequency reads at that frequency, so the recordings are not shifted.
Other species. The obvious error is misidentifying a call, and greater mouse eared is distinctive enough that it is a hard mistake to make, and three of us have made it independently, which is not what a mistake looks like.
Playback. Somebody could in principle play a recording of the species near the detector, for reasons I cannot imagine. I have sat in the barn with a handheld detector, which lets you hear the calls converted down into your own hearing, on four nights.
I hear them. Through my own ears, live, converted from the air in front of me, the long dry calls of a bat I have only ever heard from a file.
What I have not been able to do.
Catch one, or see one.
You can trap bats under licence with a harp trap, and I have set one across the barn door on two of those nights. The detector recorded the passes. The trap caught two pipistrelles and nothing else.
You can see them, with a thermal scope, leaving a roost, and I have watched that barn at dusk with a scope. Pipistrelles leave it. Nothing the size of a mouse eared bat leaves it, and they are big, and you would not miss one.
The calls are there, in the air, in the barn, at the volume and rate of an animal hunting the length of the building, and there is nothing in the building doing it.
What is done about it.
I have not recorded it. That is a professional decision and I want to set it out, because a record of that species at that site would be the ecological news of the decade and would bring a great many people to a farmer's barn to find nothing.
The recordings are on a drive, labelled, with the metadata intact, and the two colleagues who checked them have agreed to say nothing until there is an animal.
The farmer knows. He said his father talked about big bats in that barn, and then he changed the subject.
What I do.
I survey the sites I am paid to survey, and I put a detector in that barn on my own time a few nights a year, because I cannot not.
The one change is that I stopped using the handheld.
The handheld converts the calls into sound you can hear and you stand in the dark and listen, and it is the best part of the job.
On the fourth night, in 2023, I was standing in the middle of the barn at about one in the morning with the handheld up, listening to a greater mouse eared bat work the length of the building, pass after pass, coming closer on each one the way they do when they are curious.
On the last pass it came down the barn towards me at head height, on the detector, and the call got louder and closer the way a real one does, and at the end of it, at the point where a real one would have gone over my head, the handheld gave the little flat click of something touching the microphone.