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Old Friday 31st March 2017, 14:01   #1
peter.jones
You may say I'm a dreamer.. but I'm not the only one

 
Join Date: Jun 2010
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Unusual bat recorded over house (Hampshire, Eng)

Hi, anyone have any thoughts on this bat which passed over my house (Test Valley, Hampshire) just before midnight last night (30th March, 2017), during a remote recording session. (Time expansion recording)

Fairly slow repetition rate (~6 per second)
Peak frequency 35-36KHz,
signal length ~6ms

My initial thoughts on anything in the mid 30's locally would be Barbastelle (previously unheard of in my garden / immediate area, but recorded once a mile or so away, and many records within 10 miles), but on closer analysis the signal is pretty much constant frequency, I would guess a Pipistrelle flying in open space.
The Peak Frequency would point to something extremely rare.
(Bat Explorer software is claiming Savi's Pipistrelle).

One disclaimer would be that I have no idea which direction or distance the bat was in relation to the detector. Therefore weak, and deflected signals can cause confusion, but this one looks pretty sharp especially the power signal, albeit with a quite faint sonogram.

Any idea would be welcome, including any experts who I could forward the wav file to for a closer look.
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Old Friday 31st March 2017, 19:45   #2
peter.jones
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I'm pretty sure i have solved this one now.. A bit of a stretch to get there, but my assumptions are feasible, and a lot more likely than a pipistrelle species peaking at 36khz going over my house. (Even given the recent Vagrant Emperor Dragonflies, a small number of overshooting migrant birds seen around the south of England today, and at least one Cruise liner recently arriving at Southampton!).

I believe the bat was a fairly distant Barbastelle, and due to attenuation, the type "a" calls of this species were all undetected. This explains the very slow repetition rate.

Secondly, the FM component of the visible type b calls didn't get picked up by the detector, due to the distance; or got partially picked up but mixed in with the cf giving the slightly high peak frequency.

Thirdly, the signals bounced off the house wall giving each signal an echo / longer duration. Many of the signals have a small break in the middle.

If I reduce the call duration in the bat explorer software, and change the signal type from CF to FM.. out pops a Barbastelle.

Assuming this is the case, I now have two Barbastelle records close to home in the past year. None in the previous 10 years.

It also confirms that total reliance on artificial intelligence will always be less accurate than the human brain in cases where the data quality is less than perfect.

Sadly, there are very few opportunities for me to leave a bat detector unattended, pointing across open spaces, and expect it to still be there the following morning. So walls, trees, fences, all complicate the picture, and distort the data to some degree.
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Old Sunday 3rd December 2017, 09:37   #3
peter.jones
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I had the chance to run this past my county bat recorder, and it isn't a Barbastelle.. there are a few clues, the most obvious being a hockey shaped call as the bat fades away, which I had missed originally, concentrating on the stronger signals emitted.
So it was one of the rarer Pipistrelles, Most likely Nathusius's, but who knows!
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