Can bat detectors disturb bats and cause them to leave their roost?

[rsk]

This is something that's bothered me for a while and the recent moquito thread reminded me of it.

As I understand it, there are various types of bat detectors but the simplest kind works by emitting its own frequency close to that of the bat, then generating sum and difference frequencies - the difference frequency being within our hearing range.

If it's generating a frequency close to that of the bat, then I assume the bat can detect this. And it'll be a continuous tone rather than the pulsed tone of the bat. Wouldn't this be distressing for the bats?

A few years back we discovered we had bats living in the wall of my mam's house. We spent several evenings watching them fly off to roost at dusk. Then we contacted a local bat protection group to ask if there was anything we should or should not do and they sent a woman round to see. She arrived one evening with her bat detector, counted the bats as the left. But They Never Came Back. Not that night, not the next, or the next, we never saw them again. This was not at the end of the season, she reckoned there'd be baby bats in the roost and that in a few weeks they babies would come out as well and their mothers would teach them to fly.

I've wondered since then about the coincidence of them disappearing after this Pied Piper woman visited, but only relatively recently started to think about the detector.

Anyone know any more about this?

(Not really sure whether this should go in general physics or here, so it's here for now...)

 

[.Scott]

If it's generating a frequency close to that of the bat's, the detector will be using that electronic frequency internally. It will not be converting it to sounds waves, so the bats will not hear it.

 

[rsk]

Ah! That's reassuring. So if I use a bat detector I'm not risking scaring them off?
Phew.

(When we go back to school I'll set it up with the oscilloscope and have a look but obvs don't have one of those lying around at home.)

 

[rsk]

A follow up then, still thinking about the coincidence of them leaving the day after Mrs PP visited.

She had with her a baby bat which she'd been called to rescue just before she came to see us. It was in her top pocket and I wondered if it was scared, emitting some kind of distressed call and whether that had anything to do with frightening the others off. Any bat experts out there?

 

[.Scott]

Here are two articles related to bat distress calls:
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0136146#
https://www.sciencedirect.com/science/article/pii/S0003347297906655?via=ihub

Both talk about "mobbing". This is a common bird behavior where several smaller birds will descend on a larger bird in flight to pester that bird into leaving.
For example, it is common for black birds to mob a hawk for the purpose of keeping their neighborhood safe.

In the articles about, there is discussion over whether bats really do mob - or if they are simply investigating the source of distress for their own individual purposes. In both articles, other bats did not appear to be "scared away".

 

[rsk]

Thank you! That's very helpful and informative. I can stop worrying that we did anything wrong then.

Coincidentally, there are bats where I am now as well. They roost in one of the nearby buildings (I live in the very old centre of the town) and I sit up on the roof to watch them leave at nightfall. It makes up in a way for the disappointment of them having disappeared before.