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Tracking bats by Blagdon Lake

Updated Tuesday 30th October 2012

Daniel Hargreaves takes the Saving Species team for a nighttime visit to the bats of Blagdon.

Handling endangered bats is a gentle task - and one for which you have to be properly licenced.

Daniel explains some of the noises bats make, and why sometimes bats might have other things than food on their mind...




[Sound of bat]

Interviewer:  Daniel, why is there such a mystery surrounding these nathusius bats?

Daniel Hargreaves:  Now the nathusius’ pipistrelle - that’s actually one calling there that we heard - it’s a bat that’s been found across the UK, but it’s been very rarely recorded or understood from its ecology.  So nathusius’ pipistrelle, generally on a bat detector, people are picking them up all the way from Cornwall up to the north of Scotland, but people actually don’t know where the maternity roosts are, where the males are advertising to mate.  It’s quite a rare bat to actually find in the hand.

Interviewer:  Now, you know where they are here, because we’re on the shores of BlagdonLake and - what’s that?

Daniel Hargreaves:  That’s a soprano pipistrelle; it’s a different species.

Interviewer:  Well, now the lake is a large body of water really, and we’ve watched the sun go down here and we’re just standing now and it’s dusk, it’s a very clear night isn’t it, so perhaps a good bat watching night?

Daniel Hargreaves:  Quite a good bat watching night.  It’s quite cold, so the insect numbers are going to be low tonight.  So we’re having a flurry of activity now and then it’ll probably go quiet later on.  BlagdonLake, we came here three years ago with the aim of doing a survey -

Interviewer:  What’s that?

Daniel Hargreaves:  That’s a soprano pipistrelle again.  With the aim of doing a survey to look for nathusius’ pipistrelles using bat detectors, and when we were on the lake we – that’s a soprano pipistrelle again – we actually checked some bat boxes and we found a male nathusius’ pipistrelle roosting inside a bat box on the lake.  And after that we decided to start investigating why nathusius’ pipistrelles are at BlagdonLake.  And our initial thoughts were, we had lots of nathusius’ pipistrelles, and we would have a large maternity colony of females and they’d be happily breeding around the lake, and two years ago we started trying to capture bats to identify if they were males or females, and so far we’ve only ever caught male nathusius’ pipistrelles at BlagdonLake.  And we think what’s happening, we have bats all year round here, nathusius’ pipistrelles all the way throughout the year, and we’ve caught 25 male bats now and no females, and we’ve caught females of 11 other species, so it’s very strange that we’re not catching these female bats.  And we think the males are holding territory on this large water body and they’re waiting for females to arrive, and it’s an obvious migration route, or it’s a stopover point from bats moving through the area.

Interviewer:  Yes, because what’s exciting about these bats coming to Britain are that they are migrating here aren’t they, and most of our other bats don’t migrate.

Daniel Hargreaves:  Yeah, that’s true.  We know very little about bat migration in the UK.  They’re very very difficult animals to study and trying to get long range or long range movements of bats is extremely difficult.

Interviewer:  And where are these nathusius’ bats coming from?

Daniel Hargreaves:  Well, we know that there’s records from the North Sea on oil rigs and on ships of bats landing on the North Sea, in the middle of the North Sea.  So they’re obviously records from Norway, Scandinavia, bats that are moving across.  We also know there’s a lot of maternity colonies in Northern Ireland, so maybe there’s movement -

Interviewer:  What’s that?

Daniel Hargreaves:  That’s a soprano pipistrelle again.  They’re a lot more common species on BlagdonLake.  So we know that bats each year are moving in from different areas in the continent, maybe some from Northern Ireland, maybe some from Scandinavia.  And if we look across Europe there’s other European countries that have done a lot more work on nathusius’ pipistrelles, and they’ve shown obvious migration paths from Russia, Lithuania moving in a south west direction down towards France and down towards where we are at Blagdon Lake.

Interviewer:  Now, you’ve got a very small, it’s almost the size of a sort of mobile phone really, a little bit bigger, and you can pick up the echo locating calls of bats.  That one that we’ve just heard, how do you know it’s a nathusius?

Daniel Hargreaves:  The nathusius’ pipistrelle has a peak frequency.  So when the bat’s calling we look at the frequency that it’s calling at in kilohertz.  So a nathusius’ pipistrelle calls between 35 and 40 kilohertz.  So I can listen on my detector and tune into that frequency, and if the pitch sounds right I know that it’s a nathusius’ pipistrelle.  With my bat detector I've also got a small computer and I can see the sonogram.  So this is, it’s a bit like a musical note sheet, if you will, and it’s for different frequencies that the bats are making.  A nathusius’ pipistrelle comes up quite clearly for me between 35 and 40 kilohertz.  We have three species of pipistrelle in the UK, and on a bat detector you can tell the differences between the frequencies.  So the soprano pipistrelle is 55 kilohertz because it’s a higher frequency, the common pipistrelle is 45 kilohertz, and then the nathusius is just below 40 kilohertz.

Interviewer:  One of the other things that you’ve done on the shores here of this lake - and it is wooded, there are pockets of woodland around the lake and farmland with hedges - you’ve set up some traps haven’t you to see if we can actually see one tonight.  So it would be really good if we can perhaps go off and do that too.

Daniel Hargreaves:  Yes.  We can do that, yes.

[Sound of bat]

Daniel Hargreaves:  Now there’s one bat in the net there.  I’ll just gently remove it.  I’m just lifting it out.  So you can see it’s very small.

Interviewer:  So delicate!  You have to have a licence to do this, don’t you?

Daniel Hargreaves:  You do, to handle bats yeah.  There he goes.  He’s out of the net.  So he wasn’t caught for very long.  Yeah, it’s very small.

Interviewer:  Oh it’s so tiny, and what is it?

Daniel Hargreaves:  It is a nathusius’ pipistrelle.

Interviewer:  No!  Oh I say.

Daniel Hargreaves:  So we’re very lucky, that’s the first bat that we’ve caught tonight and it’s a species that we’re trying to catch.

Interviewer:  Oh look at that.  Now that’s its little call isn’t it?

Daniel Hargreaves:  It is yeah.

Interviewer:  It’s not very happy in your hands really is it?

Daniel Hargreaves:  It’s, like any wild animal, if you catch a wild animal and you hold it, it’s going to try and call to make it sound larger and bigger than it actually is.  But once it’s in my hands you can see it’s quite placid and it’s calming down.

Interviewer:  Now just tell me what the difference is then between this nathusius’ pipistrelle and a normal common pipistrelle.

Daniel Hargreaves:  Okay, so it’s slightly larger than a normal common pipistrelle.

Interviewer:  And yet it’s tiny.

Daniel Hargreaves:  And yet it’s very very tiny.  You’ll see that the fur is very dense and long.

Interviewer:  Yes.

Daniel Hargreaves:  It’s got long shaggy fur.

Interviewer:  It’s a sort of brown, lovely brown colour.

Daniel Hargreaves:  Lovely brown colour, and if I turn it over –

Interviewer:  Aw.

Daniel Hargreaves:  - onto its back -

Interviewer:  Oh look, it’s so tiny.

Daniel Hargreaves:  - you can see that it’s got pale, paler hair on its ventral surface.  So it’s sort of brown on the top, very shaggy hair and then paler fur on the ventral surface.

Interviewer:  Oh it’s beautiful.

Daniel Hargreaves:  One of the ways to tell them apart from the other pipistrelles is if we look at the wing, the wing is much broader than a normal pipistrelle bat, and they have this venation in their wings.  It’s a pattern within the wing membrane.

Interviewer:  I can see it.

Daniel Hargreaves:  You can see, and this bat here, there’s a line coming up here and then there’s a line breaking it, which tells me that it’s a nathusius’ pipistrelle.  The common and the soprano pipistrelle have a slightly different pattern in the wing venation.  So what we’ll do now is take the bat back to where the equipment is, and we’ll just measure the bat and just confirm that it is a nathusius’ pipistrelle.

Interviewer:  Okay.  I don’t want to cause it any distress at all because it’s so rare.

[Sound of bat]

Okay, we’re at the back of the car now, and we’re just doing a little bit of paperwork and weighing.

Daniel Hargreaves:  Now that’s seven grams.

Interviewer:  Is that quite big for a nathusius?

Daniel Hargreaves:  It’s quite, for this time of year it’s quite small.  So this male has probably had other things on his mind than eating and foraging.  So I can imagine that he’s probably been looking for a mate, and it should be time that he’s spending looking for insects.  So you can just see its muzzle here and its nostrils and two tiny little eyes just either side of its muzzle towards the base of its ears.  It’s looking at my data sheet, so we’ll go and release him.  So we’ll just walk off the road, just back to where we were, and we’ll watch him fly away.

Interviewer:  Okay, so you’re just checking to see the wings are okay?

Daniel Hargreaves:  Yeah, I’m just stretching his wings a little bit, because it’s a cold night, and then I’ll just hold him in the palm of my hand, and he’ll start to flap his wings and away he goes.

Interviewer:  And there he goes, right along the hedge, oh!

Daniel Hargreaves:  Flying away into the distance.

[Sound of bat]

Interviewer:  Because what’s exciting about these bats coming to Britain are that they are migrating here aren’t they, and where are these nathusius bats coming from?

Daniel Hargreaves:  Well, we know that there’s records from the North Sea on oil rigs and on ships of bats landing.  So they’re obviously records from Norway, Scandinavia, bats that are moving across.  So we know that bats each year are moving in from different areas in the continent, maybe some from Northern Ireland, maybe some from Scandinavia, and if we look across Europe, there’s other European countries that have done a lot more work on nathusius’ pipistrelles, and they’ve shown obvious migration paths from Russia, Lithuania, moving in a south west direction down towards France and down towards where we are at Blagdon Lake.

Interviewer:  So do you think they’re extending their range to the UK?

Daniel Hargreaves:  I think they’ve been here for a long time, and there’s a lot more people interested in bats now, and there’s a lot more people using bat detectors, so we’re finding more and more individuals.  I think we’ve still got a lot to learn about nathusius’ pipistrelles, it’s an obvious migrant species, and I think what could be happening is every time we record it on a detector, we presume it’s a male or a female or it’s a local colony, but we’ve actually got no idea; it could be bats that are moving in on a seasonal migration.  So I think what’s happening here on the lake is we’ve got male bats all year round, and the females between August and October are passing through here on the way to somewhere, or they’re using it as a wintering ground, the bats are mating, and then through the summer or early in the year the females disappear and the males stay on the lake.

Interviewer:  But as you say you’ve only recorded 25 males here.  Now there’s a small building just on the shore of the lake here, just behind us, which is a sort of disused waterworks building isn’t it, it’s an old Victorian building.  Now I think you’ve got some bats roosting in there haven’t you?

Daniel Hargreaves:  Yes, last year we did some radio tracking, so we put tiny transmitters on the back of the nathusius’ pipistrelles.  We caught three males and we put the transmitters on there, and one of the bats led us to this building.  And we’ve been watching this building now for several months, and on the 29th July we recorded a male advertising: he was singing to try and attract a mate.  And he was still singing two days ago when we looked at our recordings.  So from the end of July up until October as it is now there’s bats in this building singing trying to attract a female.

Interviewer:  And do you think we might hear them tonight?

Daniel Hargreaves:  It’s very possible, very possible.  They were here two days ago.  So if the temperature hasn’t dropped too much then there’s a good chance that we’ll hear them calling or advertising for a mate.

[Sound of bat]

That’s a nathusius.  That was a nathusius advertisement call that we just heard there.  And it sounded quite sharp and harsh on a bat detector, but if you listen to it on a computer it’s much more like a sound, it’s almost like a bird sound.  If you slow it down and lower the frequency, it’s got lots of tones in there and it actually sounds like an animal that’s calling this elaborate song.

Interviewer:  You have got a computer here haven’t you?

Daniel Hargreaves:  Yes.

Interviewer:  Do you think we could hear the sound that they make, sort of so you can decode it and we can hear it a little bit more easily?

Daniel Hargreaves:  Yes, we can do that.  We can play it in a time expansion mode.  So we lower the frequency so it’s audible to humans, and we slow the call down.  Because it’s a very fast call, so if we actually slow it down by ten times it sounds much more elaborate and you get a feel for what the bat’s actually singing.

[Sound of bat]

Interviewer:  Isn’t that beautiful?

Daniel Hargreaves:  It’s a beautiful sound, and if I play that to friends they often don’t realise that it’s a bat; they think it’s a bird or something.

Interviewer:  It sounds like a bird of paradise.

Daniel Hargreaves:  It’s an incredible song, and for me if there’s a female nathusius not attracted to that song then I feel sorry for the males.

Interviewer:  And are they the only species these nathusius that actually sing like this? 

Daniel Hargreaves:  No, many other bat species do sing.  There’s a lot of bat species that fly, they do what we call a song flight.  So they fly along a hedgerow or in woodland and they’re singing as they’re flying.  Nathusius are quite special that they do a lot of their calling from a static position, and they’re calling for such a long time.  From July to October, where we are now, it’s such a long time to be calling for.  So they’re quite famous for their courtship songs, if you will, and their songs that they’re singing this time of year in autumn.


Daniel Hargreaves was talking to Saving Species during the making of Saving Species Series 3, Episode 9


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