2016/17 - the 'sweeoo' winter

As discussed here, chiffchaffs can make more than one call - collybita birds have a standard 'hweet' but occasionally we get large numbers of birds making an alternate call that can be described as 'sweeoo'. The winter of 2016/17 was certainly one of those occasions.

Before go on to discuss the birds occurring this autumn and winter, I should direct people who are interested in these things to this paper in Caluta, which summarises the sweeoo situation very neatly, describes previous 'irruptions' into eastern Europe, and gives a nice account of the variety of sweeoo calls that can be encountered. It also very conveniently lays out the questions that still need to be answered!

So what of this autumn and winter then. I encountered sweeoo birds on several occasions...

I spent a week on Ouessant in mid October, and like a moron did not record a single note. However, I can clearly recall hearing a large proportion of sweeoo calling chiffchaffs - from memory well over 50% of the birds I heard calling were sweeoo - and as unscientific as this sounds, they all looked pretty standard to me.

I'm also lucky enough to spend a lot of time in SW France, a part of the world where wintering chiffchaffs are not particularly uncommon. This winter chiffchaffs were even more numerous than in previous winters, and I estimated that approximately 95% of the calls I heard coming from chiffchaffs were more consistent with 'sweeoo' than they were with hweet. Luckily I was more interested in recording them than I was on Ouessant (many fewer distracting rares...)

Xeno-canto also backs this up to some degree. Of 15 chiffchaff calls recorded in western Europe during this period, 11 are classifiable as sweeoo types. Without getting the calculator out, that's about 70%. The ratios for previous years are below (chiffs recorded in Dec or Jan, in western Europe)

2016/17 - 15 calls, 11 sweeoo
2015/16 - 8 calls, 0 sweeoo
2014/15 - 8 calls, 2 sweeoo
2013/14 - 6 calls, 3 sweeoo
2012/13 - 2 calls, 0 sweeoo
2011/12 - 1 call, 0 sweeoo
2010/11 - 1 call, 0 sweeoo
2009/10 - 1 call, 0 sweeoo

What this shows is that the winter of 2016/17 has a particularly high proportion of recorded sweeoo callers in western Europe compared to other years. I did consider that there might be some bias here, insomuch as that people might be more likely to record birds they encounter giving these alternate calls - but of course if this is true for one year then it would be true for the rest.

And finally, other people have noticed them too - a few have been confused by them in Britain over the autumn, and I've also been sent recordings of birds from Mallorca, where again, the proportion of 'sweeoo' calling birds was said to be very high.

There appear to be at least 2 different motifs being used. One was particularly prominent, a call that was sometimes audibly trisyllabic. All show the same consistent differences from standard hweet calls. Calls like these have been recorded in both the UK and Mallorca this winter, and there are plenty in Xeno Canto, for example:

http://www.xeno-canto.org/341782
http://www.xeno-canto.org/350654

What do these calls look like then? They're all consistently different from a collybita hweet in having at least two (sometimes three) clear syllables, being higher pitched, and lacking the strong harmonic that a hweet has.



The second 'variant' sweeoo motif seems less common, which might be a result of it being less obvious. There are some obvious similarities between it and the hweet call, with its dominant upsweeping sound, and what appears on the sonogram to be a strong harmonic.



However, what looks initially like a strong harmonic is actually a separate, audible part of the call - so what you're hearing here is less sweeoo (steep upsweep followed immediately by steep downsweep) and more like swee-wee, with a steep upsweep followed immediately by another, lower pitched steep upsweep. In the sonogram below, you can see how one part of this call starts after the other has finished, rather than being a higher pitched (harmonic) version of the lowest part of the call (fundamental) as it is with a standard collybita hweet.



This call does sound different to the sweeoo described above, with the two notes sometimes clearly sounding like separate entities. A comparison between the three can be heard here (this is the track that is depicted further above with hweet, sweeoo, and variant sweeoo labelled - so the first three calls you here are hweet, the second three sweeoo, and the last 4 the variant sweeoo)

http://www.xeno-canto.org/355798 

All that waffling on doesn't get us any closer to the answer of the big question though, does it? The big question is 'why do these birds sound like this'?

One thing that has occurred to me during it all is that the idea of delayed call development hits a bit of a snag when you consider that these calls have been heard throughout western Europe from at least mid October (my Ouessant birds) to at least mid January (a recording made on Mallorca kindly supplied by Jason Moss). Shouldn't they have crystallised into hweets over this period? Also, if you got that far in the Caluta paper you may have noted that the only genetic analysis performed on a sweeoo calling bird returned an abietinus. Could some combination of the two proposed answers to this question approach the real reason we get occasional influxes of sweeoo birds? Or does autumnal immersion in sweeoo calls lead to crystallisation of a sweeoo call rather than a hweet? I think the only way to really answer the big question is to trap a load of these birds and do some lab work - either stable isotopes or genetics. Maybe then I can stop writing about it!

Nocturnal bird recording - how I do it

If this appears familiar to anyone, it'll be because it is largely lifted from a thread on Birdforum that I started last year. With my first recording session of 2017 completed a couple of nights ago, it's time to get the ball rolling with getting some details onto here.

A few recording sessions in September last year revealed a few interesting results, such as greenshank and dunlin, goldcrest, and regular passage of terns (both Sandwich and common). I'm looking forward to seeing how numbers of night migrants and variety of species changes through the year.

My kit:

I'm using a Roland R5 recorder attached to a Rode NTG2 directional microphone. I use Audacity to visually scan through a spectogram of my recording, to try and isolate calls. I didn't alter the settings from what I'd normally have it on for standard field use, which means it's probably set to a fairly high resolution, and I could probably reduce the size of my sound files by dropping that a bit. I can't remember what the settings are exactly, but you should definitely make sure you're recording in MP3 rather than WAV, as an MP3 file is going to be a lot smaller. Not sure what size the card is, but I can record at least 1.07 GB and not run out of space.

The only issue with file size for me is that it made Audacity a bit clunky when I had 1 GB open in it. When I've analysed the recording, I archive all of the identifiable stuff I get but I dont keep every recording so a nights recording of 1 GB will reduce down to just over 1 minute of useable stuff in the end.

My set up:

While some people leave lovely parabolic reflectors set on tripods, pointing skyward over night, I have a slightly different set up. We have a shared garden and in this neighbourhood there is no way I am going to leave anything worth money accessible in the garden! Instead, I get an old handbag and place the mic in it, pointing upward. I hang this out of the window, trying to make sure the mic is pointing upward but away from the house a little (otherwise I'd just be getting crystal clear recordings of the gutter). I've bought a long cable so the recorder can easily be kept on the inside of the window while it is running. One advantage of this is that the mic probably gets a little more shelter from wind than if it was sitting in the middle of the garden.  

My location:

I'm based on the outskirts of Aberdeen, next to the harbour - a surprisingly quiet place all things considered.  It's certainly a decent place for flyover birds in the day and I've had whimbrels, whoopers, pinkfeet etc going over during the night. I do record the odd bit of ambient noise, traffic, trains, drunk people singing etc, but not enough to hinder hearing more interesting things.

My analysis:

This is the time consuming but fun bit. Once the recording has opened in Audacity (this can take a while) I reduce the size of the file by splitting the stereo track into two mono tracks and deleting one of them. I then zoom in to the recording (magnifying glass button in the toolbar) so that the screen width covers about 5 minutes worth of time - the x axis of the spectogram. Be aware that with a big file, the zooming in will not respond immediately so you might have to wait ten seconds or so for Audacity to respond. And then I press play! Obviously I'm not going to sit there and listen to 7 hours worth of recording, so I skip through looking for obvious noises. You can click anywhere in the spectogram, and pressing play will start your recording playing at that point. If I have a window that is obviously blank, I just click at the far right of the screen and press play - a few seconds later the window will have scrolled onto a new part of the recording and I can start looking for obvious visual signals again. While the file is playing, you can also skip through both forward and back, using the cursor. All the while I'm listening to whats going on through headphones, so my other half can watch 'Eastenders' in peace... 

I probably miss a few birds like this, especially early in the morning when the local robins start singing, and the sonogram is full of 'visual signals' that make finding night migrants a lot more difficult. However, I've found some good stuff using this method, including a huge surprise on my first session this year - A coot flying over at about 00:50! I've only seen two coots locally before so it was well worth all of the sifting! I'm sure there are plenty of other surprises in store too.

A coot over the house at night. 

the weak hweet part two

In this post I described how a bird that ticked most of the plumage boxes for a siberian chiffchaff threw us a curve ball by making some rather untristis noises - and I finished by suggesting that it would be great to get some more recordings. It's far from being the first time that tristis looking birds have made unusual calls - there is plenty to read about it here and here, for example

Well, fortunately for me, an excellent opportunity came along.

There have been several chiffchaffs on my local patch this winter (perhaps 7 all together - a remarkable number of wintering birds for a scottish site) and a local ringer expressed an interest in doing a study to see how long they lingered on their wintering grounds and ultimately, whether any remained to hold territory. I knew that some of these birds looked like tristis and some of them sounded like tristis, so this presented me with an opportunity to have a look at some of these birds in the hand, record any noises they made, and ultimately to be able to link calls with plumage types.

After an excellent first session during which 5 different birds were caught, we were delighted to get an unringed bird in the nets almost immediately on our second session.

Plumage wise it was a bit of a puzzler, appearing quite dark and olive but with nice whitish underparts, dark bare parts, decent super and reduced impact eye-ring. I suspect that in better light it would have looked paler - this picture was taken pretty much at first light, and I reckon that a certain coldness can still be detected in the mantle - still, it's a confusingly green looking bird! While we admired it and puzzled over it's origins it began to call...

While it could basically be described as sounding like 'hweet', there are obvious differences between it and the standard collybita call. The differences can be seen on the sonogram in the prior post linked at the top of this one, but there are audible differences too. You can hear the call here:

http://www.xeno-canto.org/356004

To me this sounds subtly, but clearly higher pitched, thinner, and weaker. It's also damn near identical to the calls recorded on that first encounter - quite probably near enough to say that it's the same bird - and definitely near enough to say it's the same call type.



This call was the only one given while the bird was in the hand, and I was cautious that it might not be doing its 'normal' call under the duress of being handled. We had the initial (awful) recordings of course, but it was nice to get more confirmation that this was a call that was regularly being used. Below is a recording made in the field of a chiffchaff that despite being seen briefly, looked pretty good for a tristis. Unfortunately I didn't get to see whether it was ringed or not.



Below, some of the calls above can be seen in comparison with the in-hand calls and those from the initial encounter, and the field recording can be heard here:

http://www.xeno-canto.org/356005



Unfortunately on this occasion the bird did not shed any feathers. Luckily though it did on a later date so hopefully we will be able to get a genetic confirmation of what this bird is. Personally I have no idea but I guess I'd be a little surprised if it didn't come back from the lab as a tristis. Whatever it comes back as, it'll hopefully go some way towards answering the general confusion over what these variant calling birds are.

I'd welcome any feedback on this birds appearance or any other encounters with chiffs that call like this.

Refining your sonograms with Window size and type

Tucked away in the Audacity menu, hiding in the preferences tab, are two ways of refining the appearance of your sonograms. These involve changing the size of the window (it doesn't actually change the size of the window as it appears on the screen) or changing the window type. This changes the way in which the sonogram is generated and can therefore have some effect on its appearance. 

Changing the window size effects how much frequency detail is revealed. The general rule is the larger the number, the more frequency detail one gets at lower frequencies. The payoff is that there is less temporal resolution, but I find that this is less critical when viewing sonograms of birdcalls. How the window size effects the appearance of the sonogram can be illustrated nicely with this rather poor recording of a distant tawny owl (that has been ‘cleaned’ with a bit of noise reduction). To alter the size, open the 'Audacity' dropdown and select 'Preferences'. Select the 'Spectograms' tab, then, experiment with the window size options in the relevant dropdown.

The first sonogram shows a sonogram with a window size of 512, and the second with a window size of 2048 - see how the frequency detail is so much easier on the eye at 2048.

Changing the window type usually has a less radical effect on how your sonogram appears, but it can be worthwhile experimenting with different options if you have a sonogram that could look better. Again, choose the 'Prefefences' from the 'Audacity' menu, and experiment with the different window types in the 'Spectograms tab. Here, this firecrest call is displayed with the default window type, which is called ‘Rectangular’. 

Not very helpful! Changing it to the second option, called Bartlett, helps thins a little, but still, doesn’t really produce anything that you can really work with. 

Choosing the option called Gaussian(a=4.5) however, produces something much more useful. I don’t know of any hard and fast rules regarding how choosing different window types might improve your sonograms, so it’s always worth just experimenting with a few options in order to get the best results.

The weak 'hweet'

On 3rd January 2017, Ryan Irvine and I found a couple of chiffchaffs in Nigg Bay, Aberdeen. This has become a regular wintering site for chiffs over the last few years (possibly the only one in NE Scotland) and has hosted the odd Siberian chiffchaff too.

When we got the first few glimpses of the first bird, we assumed that we had stumbled into another 'tristis' bird, based on plumage features. Rather confusingly though, while we watched it, this bird called several times and consistently gave calls that to our ears sounded like a weak version of the standard collybita 'hweet'. At this point a proper Siberian chiffchaff call started up and we abandoned the original chiff to get views and recordings of the bona fide Sibe. The original bird is shown in the photos below.

It wasn't until a week later that I decided to listen back through all of the recordings I made while trying to record the Sibe. I was surprised and quite pleased to find that I'd accidentally managed to record the original bird calling twice. Running it through Audacity, it quickly became clear that what sounded to our ears like a weak version of a standard 'hweet' bore very little resemblance to the sonogram of the standard call. Both call types are compared in the sonogram below.

The two calls on the right are the bird in the photos above - totally different to a collybita 'hweet', and reassuringly similar to one another. I can't find any similar looking calls in Xeno-Canto (although I can't say I've checked all of the chiffchaff calls as there are hundreds...) and there's nothing similar looking (or transcribed) on Alan Deans website, so I'm none the wiser as to where this bird and its unusual call might be from. One possible explanation is that this is not the whole call - sometimes birds give truncated versions of their usual call (I've heard yellow-browed warblers do this, for example) - and perhaps we're hearing the first part of a 'sweeoo'. It was a dull day, and when I've heard YBW truncating their calls it's generally been cold, dull or early.

It's worth noting that the photos over emphasize the coldness and paleness of the bird very slightly - in the field, there was a touch more olive/green in the mantle, too much yellow in the underparts, and even signs of streaking on the chest. Many people have suggested that it could still be tristis - with the calls being within known variations for this taxa. Particularly, it has been suggested that the calls of this bird look very similar to a variation in the Sound Approaches brilliant 'Catching the Bug'. 

While the shapes of this birds calls look very similar to those on page 164 of CtB, there are a few differences that are worth noting. The horrendous sonogram below compares the CtB variants with the bird above, and shows that these variant tristis calls cover a much wider frequency range, and are stronger at the top end of the frequency range rather than the bottom.

However, I guess if Siberian chiffchaffs can make one variant call then they're capable of making a few, and could easily do something similar to the calls above. The thing that could dissuade me from considering this to be a variant call was that we heard this bird doing this call upwards of ten times, and each call sounded like this (rather than throwing in variations here and there among more normal calls). In other visits to the site this winter I've only ever heard tristis calls (although the birds are largely silent, infuriatingly!) rather than a mixture of 'peeping' and 'hweeting' birds.

Whatever it is it's an interesting one, and I'd love to get a few more recordings of it!

Comparison of some chiffchaff calls

Chiffchaffs. Where do you start?

Well, very often, you start with the call, as it's frequently the first hint you get that there is a chiffchaff nearby.

We're used to chiffchaffs saying 'huit' in the UK, and when we're lucky, we get to hear Siberian chiffchaffs making their 'peep' calls (other phonetic renderings are available - such as 'eehp'). To complicate things, we sometimes encounter chiffchaffs that say what can loosely be described as 'sweeoo', and it's a bit of a mystery where these birds or their calls are coming from. For a while, they were ascribed to the various subspecies, with collybita saying 'huit' and the now near-mythical abietinus responsible for 'sweeoo'. We now understand this not to be the case, so there are two positions to start from.

a) these birds are from a distinct population.

b) these birds are the offspring of late breeders, so we hear juvenile, or less crystalised calls at a time when we're not expecting it.

I don't know what the answer is - but what makes the whole thing more interesting is that the 'sweeoo' calls are highly variable, and that the birds that make them seem to be 'irruptive'. I don't necessarily want that to mean the same as it would mean with, for example, waxwings - it's just a way of describing that they can be very numerous in some years, and very uncommon indeed in others. 2016 was definitely a 'sweeoo' year.

Before we get too deep into the nitty gritty of sweeoos, huits, and peeps, lets just have a look at the structures of the different calls. Lining the recordings up on top of one another allows a rough comparison, but putting the calls side by side (and zooming in a bit more, as this will allow) gives a much better opportunity to look at the differences between all of the calls.

The composite is easy to make. I opened a new track in Audacity (tracks menu -> add new -> mono track), and then simply copied (highlight relevent section -> edit menu -> copy) and pasted (click in new track -> edit menu -> paste) into the new track. These ones have had some of the time between calls cut out (scissors tool) so they all fit nicely onto the same page. The real beauty of this is that it lines everything up at the same time/frequency ratio, which allows you to make a valid comparison of frequencies and shapes of motifs. The rapidly upward rising 'huit' is obviously different from the flat topped call of the tristis type bird. The two 'sweeoo' calls are different too. The presumed collybita juv has two parts to the call, the upward inflection at the beginning followed by the downsweep. The winter 2016 birds sounded very different, with some calls showing a clear third component to the call, ending on a smaller, shorter upsweep. This could be heard in the field on occasion, with some calls sounding clearly tri-syllabic.

Lining them up together also allows a comparison of the frequencies the birds call at, such as the sonogram below.

Here it's clear to see how the tristis type call utilises a much narrower range of frequencies than all of the others - which explains why the call is uncomplicated, and sounds flat. I always took this familiar tristis description to mean flat as in the musical sense - but now when I hear it, I 'hear' a flat motif on a sonogram - or at least a 'flattened' one. 

It's also clear that the 2016 'sweeoo' birds reach some really high frequencies. They sound high in the field, but here it can be seen that they reach over 5.5 khz, over 1 khz higher than their 'huit' calling cousins. 

None of this helps with the conundrum of where the variable 'sweeoo' calls come from of course. But the more we recognise them in the field, or even get good, useable recordings of them, the better equipped we will be to come up with a reasonable answer.

Using the Equalisation tool

This tool is a great for reducing the levels of unwanted noise and enhancing the levels of the ones you want to keep, and it’s a great way to target certain frequencies. Again, it’s found in the Effects menu. 

Here I’m using a sonogram generated from a recording of a Siberian chiffchaff that I made with my iPhone. I’d left my recording kit at home as it was raining. It’s always the way…you find something interesting when you’re least prepared for it! Note how much darker the sonogram appear than in previous examples. I don’t know why for sure, but I suspect that this is because the mic is picking up more noise - not because it is more sensitive, but because it is not directional, so as well as the bird noise that you are trying to record, it’s also picking up background noise from all directions. 

Before you do anything else, make sure you know roughly what frequencies you want to keep/enhance. Open up the Equalisation tool and select the radial button for ‘Graphic’ towards the bottom of the window, so that it looks like the picture below. Do you remember when they made Walkmans better by adding a graphic equaliser? Well this is exactly the same, but a little more sophisticated than just allowing you to adjust the bass and the treble etc. If the blue line does not cross the graph at 0 decibels just press ‘Flatten’ to reset it. 

You’ll see that there are a series of sliding scales at the bottom the window that relate to different frequencies (hover the cursor over the button and it will tell you its frequency). It’s now simply a case of decreasing the decibels for the sounds you don't want, and increasing the decibels for the ones you do. The effect this will have on your sonogram will be to make it darker (i.e. more noise) where you have increased the decibels, and paler where you’ve decreased them. Here, I want to keep, or maybe even enhance the area around 5 khz, but reduce the rest. Your sonogram should look something like the picture below.

Now, if you like you can further refine your equalisation by be being a lot more precise in reducing the decibels close to the ones you want to keep (you can see from the graph in one of the images above that I chose quite a bit of leeway around my pretty narrow window of interest of between 4.1 and 4.7 khz).

Alternatively, you can go back to some of the other ways of noise reduction that we’ve looked at already. Here, I used the ‘Noise reduction’ tool from the ‘Effects’ menu to clear up some more unwanted noise.

What we’re left with is a much cleaner recording, but some slightly ‘fuzzy’ looking Sibe chif calls! You can take some of this fuzziness away by going back into the Equalisation tool and reducing the ‘peaks’ you’ve set on the graph, like I’ve done in the picture below. Run this twice and you’ll end up with a nice clean looking sonogram! it will play quietly when you listen to it but you can increase the volume by turning up the ‘gain’ in the box to the left of your sonogram. 

When yellow-brows sound wrong

Sometimes Hume's warblers can be relatively easy to identify, but there are plenty of them that look pretty similar to good old fashioned yellow-browed warblers. Usually the really tricky ones can be done on plumage given good, prolonged views, but in the end with these birds, it's usually the call that finally nails the ID one way or the other, as with this bird described by Martin Garner, and this bird on Scilly, which to my eye looks very similar to a bog standard yellow-browed.

Much is made of the variation in the calls of Hume's warblers and the reliability of yellow-brows to say 'tsooweet', and I think that as a general rule, that's totally fine, However, when confronted with a tricky striped phyllosc in late autumn, it's worth remembering that not everything is set in stone. Yellow-brows frequently drop a syllable here and there among sequences of their standard calls, but how often do they give sustained bouts of consistent variant calls?

One morning in mid November 2014, Will Miles and I were birding the Nigg bay sewage works, enjoying a late fall of goldcrests and the odd chiffchaff. Andrew Whitehouse had reported a yellow-browed warbler at the site a few days previously, and it didn't take too long sifting through the crests to pick this bird up. While watching it, we were shocked to hear it give what consistently sounded like thin, high pitched monosyllabic calls. We agreed that these sounded much more like Hume's, and were audibly a decent match to a Hume's call I had on my phone. I managed to get some recordings of these while watching a bird that I considered to look like a yellow-browed warbler, with the understanding that I definitely hadn't seen it well enough to rule out something like the Scilly Hume's example above. We cautiously put the news out that the Nigg Bay YBW might be a Hume's, and sent the recording off to the Sound Approach, who also cautiously suggested that the bird might be more likely to be a Hume's warbler too.

I went back to the site at lunchtime, by now fine, sunny and warm, and picked up the same flock of crests at just about exactly the same spot. Almost instantaneously, a classic YBW call started to ring out from the flock. I was pretty sure that it was the same bird. The chances that a crest flock would host a single YBW, then a single anomolous bird, and then a single YBW again were much too slim for me, and also, based on what I'd seen, I was happy that the bird looked much better for a YBW.

Many features consistent with YBW can be seen in the photos above, including:

plenty of yellow in the supercilium
well marked dark eyestripe, particularly in front of the eye
mottled ear-coverts
no real paleness or greyish cast to the upperpart tone
dark centred greater coverts
dark bar at the base of the secondaries contrasting with strong, white greater covert bar
dark tertials with neat, narrow white fringes
The legs and feet especially were pretty pale

Along with the appearance of the bird, the calls heard at lunchtime confirmed (to me at least!) that this was a YBW. The sonogram below shows this birds calls alongside a YBW recorded on Sanday (Orkney) in September 2015. Despite the Sanday bird having a slightly wider frequency range, the calls are clearly very similar, 'V' shapes with the bottom halves 'pinched in'.

So what about the calls that fooled us so much first thing the same morning?

Well, I guess the first lesson is that a sonogram can tell you much more about a call than your ears can! Will and I were both convinced that the call we'd heard repeatedly that morning was monosyllabic, but the sonogram tells a different story, showing a very short, quick call with a similar 'V' shape to a YBW

In comparison with other calls though, these morning calls are consistently different. They only cover roughly half the frequency range of the standard YBW calls (the higher half), lack the 'pinched in' look, and they are shorter in their duration, lasting for approximately 0.2 seconds rather than the 0.3 seconds of the standard call.



The two bird theory has been suggested several times in this instance, and while it would make sense, I'm convinced there was only one stripy phyllosc present. So - was this a Hume's and we dropped the ball, or was it a weird variant YBW call?

More ways to remove unwanted noise - Noise reduction and Pass filters

Much of the noise that you’ll want to remove from your recordings will occur across a limited frequency range, so can’t simply be cut out as demonstrated in this post. In this recording of a grey wagtail in flight, you’ll notice that the bottom half of the sonogram (the lower frequencies) is quite dark compared to the upper half (the higher frequencies). I made this recording not far from a busy road, so much of the low frequency noise will be that of cars passing by in the background.

There are three ways to reduce or eliminate this sort of noise from a recording, and all use features in the Effect menu. The tools to use are: Noise reduction, Pass filters, and the Equalisation tool. 

Noise reduction

Highlight a part of your recording that contains the noise you want to remove, but does not contain any of the important stuff you want to retain. Then select Noise reduction from the Effect menu. In the window that appears, click on the ‘get noise profile’ button.

When this is done, highlight the whole of your recording (click and drag) and then return to the Effect menu, selecting Noise reduction again. This time, just press OK and Audacity will reduce the sounds that you ‘profiled’ and reduce them across the whole recording. 

You will see in the image below that the darker area at the bottom of the sonogram has been greatly reduced.

Pass Filter

To reduce unwanted low frequencies, as with this example, choose the High pass filter from the Effect menu. 

A window will appear, and in the box labelled ‘Frequency’, type the frequency below which you want to reduce the levels. In this example I’ve typed 5000, as this is roughly where the background noise disappears, and crucially, is lower than the lowest frequency shown by the calls you’re interested in. Press OK and the High pass filter will reduce much of your unwanted noise.

You’ll be able to hear the difference in your recordings of course, as well as seeing it on the sonograms. Beware however that over editing of this sort can produce rather unnatural sounding recordings - with too much editing of this nature making recordings sound down right unpleasant. It's an effective way of cleaning up sonograms though, so if you're submitting a sonogram and a recording for a description, for instance, consider 'cleaning' the sonograms but sending a different, less edited r