Grumpy gnomes and allies

By Lukas Pelikan and The Sound Approach

Little Bittern Ixobrychus minutus, Skala kalloni, Lesbos, Greece, 29 April 2002 (René Pop)

I vividly remember a night with some good friends on a cliff at Cape Emine, Bulgaria, in May 2014, when I almost certainly heard nocturnal flight calls of Little Bitterns Ixobrychus minutus for the first time. Every now and then the silence of the first few hours of this mild night was interrupted by calls of Water Rails Rallus aquaticus and numerous herons. At the time we could not readily identify all of them, partly because the ordinary bird sound CDs we had were misleading. Getting to bed I started to realise what a fascinating field of study I just became acquainted with. What I didn’t know at this point was that this experience would make such a deep impression on me.

One of the mystery sounds in particular stuck very clearly in my memory. It took me some time before I was certain which species gave the call. A recording from Schulze (2003) gave the impression that Squacco Herons Ardeola ralloides can have very similar calls. However, I later realised it must have been misidentified; the recording actually contains several Little Bitterns in flight, and is followed by one of a misidentified Purple Heron Ardea purpurea (cf Robb 2004).

I remember a second experience after Bulgaria, this time in Greece, where I was deeply asleep outside the house and suddenly woke up as I heard a series of penetrating ker sounds (the first probably still in my dream). What makes this story even more unbelievable is that I was sleeping on my audio recorder, so I even managed to start a recording in time. But it was only much later after several sightings of Little Bitterns calling ker and many of Squacco Herons flying silently, that it struck me: all of the ker calls must have been Little Bitterns.

Years later, in May 2016, I was thrilled to hear the typical ker in the silence of the night back at our house in inner city Potsdam, Germany. For your information: Little Bitterns breed scattered throughout Germany; Squacco Herons do not. Much to the disbelief of my fellow ornithologists I decided to record entire nights to avoid missing anything. Previously, when sitting on the balcony or rooftop, I only recorded what I had just heard, using the ‘pre-record’ function of my audio recorder. The results from this new approach were overwhelming! It turned out that Little Bitterns migrate more regularly over the city of Potsdam than anyone had ever imagined. In fact, these were the first known active migration records of Little Bitterns for the state of Brandenburg. Actually, I am not aware of any other nocturnal migration records in Germany prior to that year.

To get to know how inner-city nocturnal migration sounds if you are lucky enough to be able to put your devices on the rooftop, here is a recording from a night of strong and diverse migration over Potsdam. The advantage of recording in rather birdless city surroundings becomes obvious: besides the usual traffic and the occasionally drunk there are very few distractions.


Little Bittern Ixobrychus minutus Potsdam, Germany, 02:28, 28 August 2016 (Lukas Pelikan). Inner city flight calls (long-distance contact) at 0:04, 1:06, loudest at 2:20. The gaps between calls have been maintained to illustrate the natural spacing. Background: passing night bus, sirens.

Really intensive recording in Potsdam began in 2016, and very quickly Steve Klasan was also hooked. We live within earshot of one another, only 1 km apart. The two of us took turns recording nearly every night for three months during the late summer and early autumn of 2016. That year we managed to register 10 nocturnally migrating Little Bitterns, with a peak at the end of August. This even included a loose flock, possibly of five birds, on the night of the recording above. From observations by day I know that a single flying bird can be heard calling for as long as 1.5 or even 2 minutes from our house (but usually only 40 seconds). I keep this in mind when trying to estimate numbers from a series of calls.

This is what rooftop recording looks like in Potsdam, Germany, 13 September 2016 (Lukas Pelikan).

There are obvious advantages of such an inner city setup:

  • being able to put the microphone on the rooftop reduces traffic noise from nearby roads;
  • without lucrative habitat for most bird species in the surrounding area, migrating birds are more likely to be heard than just roosting or site-changing birds;
  • city lights can attract or disorientate flying birds, making them more likely to call (cf Watson et al 2016);
  • in our case the Havel river, which runs in a north-east/south-west direction through the city, may function as a sort of guideline, especially for waterbirds;
  • there are fewer barking dogs than in the countryside of, eg, Bulgaria or Greece.

But were those Little Bitterns really migrating? The ringing atlas for Germany (Bairlein et al 2014) indicates that even though most birds migrate to the south or south-south-east, some go south-west or west, so the migration routes seem to be diverse. No doubt there is also post-breeding dispersal in all directions. Taking into account that there is no breeding pair in the vicinity of Potsdam, however, it probably makes little difference whether targeted migrating or dispersal is concerned, as both can be difficult to tell apart anyway.

 

Separating night flight calls of herons

Given that no one had noticed migrating Little Bitterns in our area, perhaps the explanation for this might be a lack of knowledge about their calls and how to separate them from other herons.

In my experience, herons most likely to be heard on nocturnal migration in central Europe are Grey Heron A cinerea, Eurasian Bittern Botaurus stellaris, Little Bittern, Black-crowned Night Heron Nycticorax nycticorax, and Purple Heron – roughly in this order, depending on the locality. I know from daytime observations that Western Great Egrets A alba regularly migrate over Potsdam but this species seems to be rather quiet in flight (unless flushed). The same seems to be true for Squacco Heron and Cattle Egret Bubulcus ibis. Little Egret Egretta garzetta is unfortunately rare at the locations where I record nocturnal migration. I never really stumbled upon a suspicious call. But members of The Sound Approach have recorded them on multiple occasions in Portugal and in Dorset, UK, so this is another heron species to listen for at night.

Compared to most passerine night flight calls (or ‘NFCs’), those of herons seem to show a greater variability. I believe that what we are hearing is not a wide range of different call types per species, but calls primarily altered by mood or degree of urgency. Some of the calls presented here give the impression that the migrant was mumbling to itself in a grumpy mood, especially when there are quieter calls that precede or follow a louder one.

Most of the species can be distinguished by ear instantly and easily but, if not, listening to a recording many times may help, or taking a look at a sonagram. What follows here are some tips for interpretation.

 

Little Bittern night flight calls

To separate NFCs of Little Bittern from those of the other small herons, frequency is the best clue. Their usual NFCs are so high-pitched that confusion with other heron species (at least in Europe) seems unlikely (mean frequency 2.1 kHz, n = 133). I determine mean frequency by measuring the lowest and highest possible part of the call and taking the mean of the two values. In poor recordings you will only see the power-carrying frequency alone and measuring is easy. Ironically, it becomes more difficult when the recording is good and the call shows particularly well in the sonagram. In this case I try to exclude harmonics when measuring. Usually, however, the mean values of either broader or narrower measurements do not differ considerably.

But looking at frequency alone is certainly not enough. The shape of the call can vary and there are two basic types of NFCs. The first type, which I associate with short-distance contact, is the one that makes them sound like grumpy gnomes mumbling to themselves. The calls are quieter and usually have a much denser distribution of frequency bands than the others. These may sometimes be associated with a flock of Little Bitterns calling to each other to maintain contact.

The second type, which I associate with long-distance contact, is both commoner and louder. It sounds more enthusiastic and has less densely distributed frequency bands, which only appear if the recording is good enough.
Both types of Little Bittern NFCs sound more or less like ker and both can have an additional croakiness, which makes them look more blurred in sonagrams.


Little Bittern Ixobrychus minutus, Potistika, Argalasti, Greece, 05:01, 3 September 2017 (Lukas Pelikan). Short-distance contact calls of a grumpy gnome mumbling to himself, calls at 0:01, 0:25, two at 0:27 and 0:39. Background: crickets and a party starting at 0:36

In general the NFCs (of both types) have an arched shape but with emphasis on the descending part, making the whole call sound slightly descending. Sometimes, with better sound quality, the call looks like a more complex construction, still with an arch visible but with additional parts above, which look like an incomplete bridge over the arch. I believe these are not harmonics related to the fundamental frequency but additional elements produced simultaneously. With increasing croakiness things become more and more blurred (ie, several elements merge together).


Little Bittern Ixobrychus minutus, Potistika, Argalasti, Greece, 00:48, 29 August 2017 (Lukas Pelikan). One single ker of a bird by night.


Little Bittern Ixobrychus minutus. Eight variations of ker calls, all except 3) representing long-distance contact. 1) Potsdam, Germany, 02:28, 28 August 2016 (Lukas Pelikan), 2) Hohne, Harz National Park, Germany, 01:25, 22 September 2017 (Lukas Pelikan), 3) Potistika, Argalasti, Greece, 05:01, 3 September 2017 (Lukas Pelikan), 4) Potistika, Argalasti, Greece, 03:02, 4 September 2017 (Lukas Pelikan), 5) and 6) Potistika, Argalasti, Greece, 22:23 and 22:59, 6 September 2017 (Lukas Pelikan), 7) Potistika, Argalasti, Greece, 00:48, 29 August 2017 (Lukas Pelikan), 8) Potsdam, Germany, 01:29, 9 August 2016 (Steve Klasan)

In more southerly places like Israel or Azerbaijan, Little Bittern flocks of around 40–50 or sometimes several 100 can be seen migrating by day (Shirihai 1996). At Besh Barmag, a well-studied site for nocturnal migration in Azerbaijan, Jonas Buddemeier recorded this flock of c 40 along with some Purple Herons flying by in the evening. You can hear the short- and long-distance contact calls and all sorts of croakiness. What a charming bunch of gnomes!


Little Bittern Ixobrychus minutus, Besh Barmag, Siazan, Azerbaijan, 19:00, 4 October 2017 (Jonas Buddemeier). Flock flying by on the windy Caspian Sea shore with Purple Heron Ardea purpurea flight calls from 0:40 onwards.

 

Eurasian Bittern night flight calls

Flight calls of Eurasian Bittern are more commonly heard in East Germany than those of Little Bittern, or at least there are more reports of them. Many know the typical graow (it rhymes with “cow”) from birds flying around in the breeding grounds or from nightly transits over rural areas. The NFC is markedly low-pitched, although not as extreme as Eurasian Bittern’s booming. It will usually linger or resonate around 1 kHz (mean frequency 1.1 kHz, n = 101). This means if the recording is not good enough you will often face the difficulty of separating it from a distant dog’s bark or, for this very reason, you may overlook it.


Eurasian Bittern Botaurus stellaris, Potsdam, Germany, 03:29, 8 August 2016 (Lukas Pelikan). Graow calls of a bird flying relatively close to the microphone, at 0:02, 0:37, 1:09 and 1:30. Background: funnily enough there is an Ortolan Bunting Emberiza hortulana migrating too, giving two plik calls from 1:16.

The shape of Eurasian Bittern’s call is almost exclusively a smooth arch. Just as in Little Bittern’s call there is usually an emphasis on the descending part. So you should hear something going down at the end of the call, indeed very similar to the word “cow” or the diphthong “aʊ”. Variation of the shape of the call seems to be rather limited. Since this call is at the lower end of the frequency spectrum, the sonagram must be stretched along the frequency axis (ie, vertically) to see minor differences in shape.


Eurasian Bittern Botaurus stellaris. Five variations of graow call. 1) and 2) Potsdam, Germany, 03:29, 8 August 2016 (Lukas Pelikan), 3) and 5) Khalkhgol, Dornod, Mongolia, 01:17, 23 August 2017 (Steve Klasan), 4) Trebelsee, Havelland, Germany, 19:30, 25 February 2015 (Bert Jahnke).

Another confusion risk for Eurasian Bittern’s graow is the flight calls of larger gulls, especially those of Pallas’s Gull Larus ichthyaetus and Great Black-backed Gull L marinus. These also rhyme with “cow” or “aʊ” and are low-pitched. They will usually be followed by different, more unmistakeably gull-like calls with much more variation, excluding Eurasian Bittern instantly. When looking at a single call, a very good starting point is to look for the harmonics: in flight calls of gulls the additional frequency bands of one call should each show similar power, whereas in Eurasian Bittern there should be an arch carrying the main power and several clearly less powerful frequency bands above or below. And finally, if you have additional croakiness blurring the call this should exclude a gull.

 

Evolution of a flight call

It may seem surprising to compare calls of Eurasian Bittern with calls of Little Bittern, but Eurasian’s graow call is actually not so different from the equivalent ker of Little. The difference is that it is pitched down to 50%. In this case the pitch change I am referring to is linked alteration of frequency and length (half the frequency means double the length). To understand this, just listen to this recording of three long-distance contact calls of Little Bitterns each pitched down by 50%.


Little Bittern Ixobrychus minutus. Three long-distance contact calls pitched down by 50% (−1200 cents). This does not work quite so well with short-distance contact calls of Little Bitterns. 1) Potsdam, Germany, 02:28, 28 August 2016 (Lukas Pelikan), 2) Hohne, Harz National Park, Germany, 01:25, 22 September 2017 (Lukas Pelikan), 3) Potistika, Argalasti, Greece, 00:48, 29 August 2017 (Lukas Pelikan).

So it is not only their vernacular name, bittern, but also their NFCs that bring the two species together. Botaurus and Ixobrychus have traditionally been grouped in the subfamily Botaurinae within the herons Ardeidae (del Hoyo et al 1992). DNA barcoding analysis supports the relationship between these species, with Botaurus and Ixobrychus forming a (monophyletic) clade Botaurinae (Huang et al 2016).

One could argue that there is an ancestral common flight call in Botaurinae, which was maintained or amended during the evolution of the different taxa. Depending on the body size, which correlates roughly with the size of the syrinx or vocal organ, the outcome (in this case the call) would therefore be subject to alteration in pitch. We can simulate this process by altering the pitch of the calls virtually on the computer. This suggests that flight calls can be used as a trait when assessing species relationships under an evolutionary view (cf McCracken & Sheldon 1997).

A call similar to graow or ker, or at any other pitch between them or even above or below, was possibly the flight call of the last common ancestor of both Eurasian Bittern and Little Bittern. In fact many (if not all) species of the same genera have similar NFCs. At present, I cannot securely separate those of, eg, Least Bittern I exilis from Little Bittern, or American Bittern B lentiginosus from Eurasian Bittern. Here is an example of American Bittern flight calls at dusk. As a probable first hint, this individual already sounds more rasping than the average Eurasian.


American Bittern Botaurus lentiginosus, Cape May Meadows, Cape May, New Jersey, USA, 19:00, 27 September 2004 (Killian Mullarney). Flight calls at dusk. To avoid confusion, flight calls of an American Black-crowned Night Heron Nycticorax nycticorax hoactli in the background have been edited out.

 

Black-crowned Night Heron night flight calls

Black-crowned Night Heron is one of the species that fascinated me from early childhood on. But many years passed before I heard its typical qwar in the night for the first time consciously, during that same Bulgaria visit in 2014.

Despite sometimes being compared to a frog’s call, the qwar call is actually quite pleasing to my ears. It is situated between Eurasian Bittern and Little Bittern NFCs in pitch (mean frequency 1.5 kHz, n = 106), which is roughly comparable to Grey Heron. It sounds, however, more similar to Eurasian Bittern’s graow. If you are not confident to distinguish between Black-crowed Night Heron’s 1.5 kHz and the 1.1 kHz of Eurasian Bittern, you should listen for a more nasal sounding qwar with a rather questioning open end. It rhymes with “what” or, as if surprise meets with disbelief, just “wha?” so it sounds rising in pitch.


Black-crowned Night Heron Nycticorax nycticorax, Potistika, Argalasti, Greece, Greece, 02:14, 3 September 2017 (Lukas Pelikan). A series of qwar calls of a bird coming closer, loudest at 1:26. Background: rustling leaves, bats, dogs, crickets and a party.

The shape of the night heron’s call is often an arch but appears every so often merely as an almost straight line. There is an example for this flat type among the seven variations below (6). Compared to Eurasian Bittern the emphasis is more on the rising part, the beginning, which is what makes it sound questioning. You could even say it sounds like a backwards, higher-pitched rendition of Eurasian’s graow. Additional croakiness can appear as well, which blurs it all together. Further loss of sound quality makes it look rather complex and unspecific concerning shape. However, one distinct feature which many flight calls show in sonagram is a rather abrupt step down (sometimes followed by a step up) in the arch or line.


Black-crowned Night Heron Nycticorax nycticorax, Besh Barmag, Siazan, Azerbaijan, 21:47, 4 October 2017 (Jonas Buddemeier). A long series of qwar calls.


Black-crowned Night Heron Nycticorax nycticorax. Seven variations of qwar calls. 1) and 2) Potistika, Argalasti, Greece, 02:14, 3 September 2017 (Lukas Pelikan), 3), 4), 5) and 6) Besh Barmag, Siazan, Azerbaijan, 21:47, 4 October 2017 (Jonas Buddemeier), 7) Besh Barmag, Siazan, Azerbaijan, 22:37, 17 October 2017 (Jonas Buddemeier).

 

Night flight calls of Grey Heron and Purple Heron

NFCs of Grey Heron and Purple Heron are in a different category from those of the three species mentioned above. They differ in having more pronounced croakiness, due to a wider frequency bandwidth. When you make sonagrams from calls recorded at close range, it will be difficult to identify the modal or power-carrying frequency of the call and to decipher harmonics and the fundamental frequency. However, with lower recording quality (eg, at a distance to the bird) the bandwidth shrinks and you begin to discern the core structure, especially in Grey Herons. The pitch of the call seems to be strongly influenced by general agitation. Because of that, it varies strongly in both species. NFCs of Grey Herons are pitched roughly between Black-crowned Night Heron and Little Bittern, and NFCs of Purple Heron are pitched roughly in the area of Little Bitterns. Surprisingly, as a result, Purple can actually be confused at certain times with the most croaky Little Bitterns, especially when viewing sonagrams. The diffuse croakiness of both Grey and Purple should nevertheless always be distinguishable from the accentuated clarity of the three species above, if not by sonagram then at least by ear.

I assume most people are familiar with the rrank flight call of Grey Heron. Although usually very different, there may occasionally be danger of confusion with Black-crowned Night Heron, as both can have generally rising pitch. When Grey Heron uses something similar to a short-distance contact call (eg, a lower pitched, less agitated and quieter call preceding or following a louder one), confusion becomes more likely. Any confusion risk increases with distance. In the following I describe the NFC of Purple Heron only, comparing it to that of Grey Heron.


Grey Heron Ardea cinerea, Hohne, Harz National Park, Germany, 00:10, 22 September 2017 (Lukas Pelikan). Series of rrank calls of a nocturnal migrant over vast spruce forests. Background: Red Deer Cervus elaphus galloping around.

 

Purple Heron night flight calls

Separating Grey Heron and Purple Heron NFCs is much more difficult than separating the bitterns and night heron. Bandwidth is equally wide and pitch in both Grey and Purple is similar, though Purple tends to be a little higher-pitched. In my experience, most variants of Purple can be readily separated from Grey by ear, and the difficulty centres on a small part of the variation. When the two species become higher-pitched and more strident (probably due to agitation), they tend to sound more similar. I remember a situation where an Ardea heron was approaching us in Bulgaria with strident calls and we first tried to identify it aurally. After we agreed on the identification as Grey we raised our binoculars and, frankly, had to admit it was in fact a Purple. In general, however, the sound of Purple’s krrrk is croakier, more rasping and drier. Lower-pitched calls may even resemble calls of male Garganey Anas querquedula in timbre.

Looking at sonagrams, you will usually see a wide block of a call in either species. In Purple Herons the different harmonics should be merged beyond recognition, whereas in Grey Herons at least part of the call will usually have clearly separated harmonics. As for the croakiness of Purple, sometimes this shows as prominent vertical lines, similar to the ones in a male Garganey’s call. This appears to come from a broader spacing of the particular elements that together make the krrrk call; logically this should sound more rasping.

All in all, the differences are much clearer as soon as you listen to these calls. In fact, some differences that I can hear are not yet visible for me in sonagrams. So it is always safest to listen to a recording thoroughly, likewise with the other species above.


Purple Heron Ardea purpurea, Besh Barmag, Siazan, Azerbaijan, 18:30, 10 October 2017 (Jonas Buddemeier). A rather quick succession of krrrk calls in night flight.

 

Little Egret night flight calls

As I have no experience in the field with Little Egret’s NFC, I can only summarise what I have learned so far from recordings. The NFC can be described as having an intermediate position among those described for the other species. There is a real confusion risk with Black-crowned Night Heron as they share mean pitch (mean frequency 1.5 kHz, n=103), yet Little Egret sounds different most of the time. The timbre is much the same as in calls of Little Egret flushed by day. As in those much longer and more stressed calls, the NFC sounds essentially gurgling and guttural and can be transcribed as argh (like the human exclamation but shorter) or airgh, so it rhymes with “air”. It is as if the bird has a mouth full of water and is yelping for “air”.


Little Egret Egretta garzetta, Sagres, Vila do Bispo, Portugal, 22:58, 15 October 2015 (Magnus Robb). Flight calls of a flock of nocturnal migrants flying over a fortress and out over the Atlantic.

The intermediate character becomes obvious when looking at the shape of the call. Although it appears to vary, most of the time it looks like a diffuse arch, or an arch within a cloud. As the description argh suggests it goes down slightly at the end and sometimes even rhymes with “aʊ”. But confusion with Eurasian Bittern is unlikely owing to its high pitch. On other occasions you will see dense frequency bands (to an extent that I have not seen in Black-crowned Night Heron yet) as in short-distance contact calls of Little Bittern, but once again frequency excludes the latter quickly. If the downward inflected “aʊ” is not apparent, confusion with Black-crowned Night Heron becomes more likely. In this case listen for the gurgling timbre and additional croakiness, which seems to be more common in this species. Looking at several calls I see a varying but distinct zigzag line instead of a smooth arch; something I have not seen in other heron’s NFCs. It may prove to be diagnostic when present.


Little Egret Egretta garzetta. Five variations of argh call, from a minimum of three different individuals on the same night. 1) 23:22, 2) and 3) 22:20, 4) and 5) 22:58. Sagres, Vila do Bispo, Portugal, 15 October 2015 (Magnus Robb).

 

Summary

In order to summarise the most important anchor points when distinguishing these heron NFCs, here is a simple table.

 Main frequency (mean)ShapeEmphasisSound descriptionRhymes withBandwidth of frequencyFeatures (if present)
Little Bittern2.1 kHzArchDescendingkerNarrowIncomplete bridge in overtone
Eurasian Bittern1.1 kHzArchDescendinggraow"cow"NarrowLow pitch
Black-crowned Night Heron1.5 kHzArch or lineRisingqwar"what" or "wha?"NarrowAbrupt step down (and step up) along arch
Purple Heron~2.1 kHzDiffuse blockRaspingkrrrkWideBroad bandwidth, vertical lines
Grey Heron~1.8 kHzDiffuse blockStridentrrankWideBroad bandwidth
Little Egret1.5 kHzDiffuse archGurglingargh"air"MediumZigzag line

 
With these hints, identification of most heron NFCs in Europe should become easier. Bear in mind the variation due to changing levels of agitation and possibly how far they intend their calls to carry (calling grumpily to themselves, or long-distance calls), which can alter both pitch and timbre. They can add varying degrees of croak, for whatever reason or function. With decreasing sound quality in your recording (eg, due to distance to the bird) the number of visible harmonics is reduced, as well as the surrounding croakiness, which otherwise appears as a blurry cloud around the usual arches.

Fig. 1 Little Bittern Ixobrychus minutus, Eurasian Bittern Botaurus stellaris, Black-crowned Night Heron Nycticorax nycticorax and Little Egret Egretta garzetta: mean position of NFCs (deep red) along frequency axis. The central deep red begins to fade at mean of highest frequency and mean of lowest frequency measured within a call. The outer ends of the fading colours represent the maximum of the highest or minimum of the lowest measured frequency within a call, respectively.

I hope that more people will now be able to experience the excitement of one of these species flying over and calling in an utterly unexpected place, and I hope that my excitement may prove to be contagious! Some of these species occur in very low densities, and most are tremendously difficult to spot in daytime due to their secretive lifestyle. Hearing a Eurasian Bittern calling graow over our house in Potsdam is always a cause to rejoice, even though it breeds within Potsdam’s boundaries. In fact we quickly managed to work out the best time of the night to hear them, so we could actually sit outside in a chair (which we regularly do for nocturnal migration) and wait for them to pass by – and it worked quite well.

Might you be able to detect cryptic migration of some of these species over your area? Black-crowned Night Herons are famous for calling as they fly over at night, but perhaps Little Bitterns should be too. Carpe noctem!

 

References

Bairlein, F, Dierschke, J, Dierschke, V, Salewski, V, Geiter, O, Hüppop, K, Köppen, U & Fiedler, W 2014. Atlas des Vogelzugs: Ringfunde deutscher Brut- und Gastvögel. Wiebelsheim.
del Hoyo, J, Elliot, A & Sargatal, J 1992. Handbook of the bird of the world 1. Barcelona.
Huang, Z H, Li, M F, Qin, J W 2016. DNA barcoding and phylogenetic relationships of Ardeidae (Aves: Ciconiiformes). Genetics and Molecular Research 15 (3): gmr.15038270.
McCracken, K G & Sheldon, F H 1997. Avian vocalizations and phylogenetic signal. Proceedings of the National Academy of Sciences of the USA 94: 3833–3836.
Robb, M 2004. Review of Andreas Schulze 2003. Die Vogelstimmen Europas, Nordafrikas und Vorderasiens. Dutch Birding 26: 319–324.
Shirihai, H 1996. The birds of Israel. London.
Watson M J, Wilson, D R, Mennill, D J 2016. Anthropogenic light and nocturnal flight calls of migrants. The Condor 118: 338–344.

 

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