‘Bittern-chorus’, a hypothesis of a group-behaviour of Eurasian Bittern.

Introduction to the Eurasian Bittern

The Eurasian Bittern (Botaurus Stellaris) is a brown heron, a secretive bird found mostly in reed beds around Europe, Africa and Asia. In this assignment the focus is on the Mediterranean population, which is mostly sedentary thanks to the mild winters. (BirdLife International, 2020)

It is a solitary bird, where males are polygamous and mate with up to five females within the same large wetland. Studies show that nestlings are reared only by females (Puglisi et al., 2008), while the male keeps the territory safe and intruders-free for foraging. 

Its habitat requirements are quite specific, with preference for quiet lowland marshes near lakes and rivers with extensive dense young reed beds of Phragmites. (BirdLife International, 2020)

The focus of this study is on its ‘boom’ call, a unique call which consists of a series of resonant booms repeated at 1-2 seconds intervals. In preparation for the first boom of the sequence, the bird pumps air 5-6 times, to then delopy the actual booms. A series of pumps and booms is called ‘boom train’ (Puglisi et al., 2008). This is used by the bird during breeding and nestling seasons (european spring) for displaying and territorial defence. Although, Puglisi et all. (2008) declares that there is no objective proof of the latter. The call is mostly given at dusk and dawn, (Heron Conservation, 2020) but recordings found it calling throughout the entire night. 

The call has been rendered as “up, up, up, rumb”, “umb, uh, uh, uh, ub”, “up, up, up, rumb”, and “hu, hu, umb, ub”.
(Heron Conservation, 2020)











Case study and description of the behaviour

Case study Links

Google Drive, if Soundcloud has any issues.

  • Recording ‘Full-length’ is 2 hours and 12 minutes, starting at 4.20am.
  • ‘Bittern Chorus – Section 01’ is an extract from the Full-Length, happening 47 minutes in the recording, at 5.10am.
  • ‘Bittern Chorus – Section 02’ is an extract from the Full-Length, happening 55 minutes in the recording, at 5.14am.

This recording was captured on the 29th of April 2019 in Shapwick Heath National Nature Reserve in Glastonbury, Somerset, UK, at coordinated 51°09’18.3″N 2°48’07.4″W, starting at roughly 4.20am. 

The recording rig, equipped with an Ambisonics 360-degrees microphone (below) and a high-quality multichannel recorder, has been placed by the recordist and left unattended in between two reed bed areas, with the intention of not-disturbing the wildlife living in the reed beds while recording.


The main reason behind the choice of this recording for this assignment is its uniqueness of having multiple Bitterns calling at the same time, around 4 or 5, which in this assignment we recognise as ‘bittern chorus’, to distinguish it from other sections where only one or two bitterns call at the same time. This almost sounds like a fight between calls where it’s quite difficult to isolate each of the calls. It is not clear why the animals would behave this way, although previous research could lead towards various hypotheses.



Picture taken while setting up the recording gear.


Initial research and Tinbergen’s questions

Previous research by Puglisi et al. (2001 and 2008, Polak (2006), Alessandria et al. (2010) and Poulin, Lefebvre (2003), shows the reasons behind the booming calls are for territory defense and mating advertisement. Two hypotheses have been taken in consideration by Puglisi at el. (2001), the ‘Honest Advertisement Hypothesis’ and the ‘Locatability Hypothesis’. The former has the boom representing the individual’s fitness, to show to the mates their physical conditions, and the latter being the amount of enhancement made by one individual for its calls to be located by the receiver. 

By analysing the behaviour through Tinbergen’s questions, the function of this behaviour could be to show either the territory dominance during the nestling period or mates calling during breeding season.

In terms of causation, the pheromones possibly cause the bird to make these calls during the breeding period to showcase its size and strength, but also, in this case, the calls of other bitterns could be a stimuli that would make the bird call. It is important to remember that bitterns also call when they are on their own, not only when other bitterns are calling, but this could also be not entirely true, as we are not clear on the bittern capacity to receive the call signal, as it may be able to hear the calls much further then us humans.

In the previous studies, the development of the call is also taken in consideration with various hypotheses behind ‘poor booms’, which are booms that sound slightly ‘uncompleted’. Puglisi et al. (2001) explains that boom trains with or without poor booms have different costs, as poor boom could be the result of exhaustion, making boom trains without poor booms more costly for the animal, but sounding more ‘attractive’ and ‘powerful’. There is also the hypothesis that poor booms happen more often with young, inexperienced adults, as the control of full boom trains requires practice. (Puglisi et al., 2001).


Analysis of the behaviour and testable hypothesis 

The aim is to understand if there is a territory hierarchy. If one individual calls always from the same position, this could give more information regarding a possible ‘leader’ of the patch, and its booming strength, boom train length and presence of poor booms to evaluate the fitness of an individual. If the leader stays always in the same position, it may be that it’s holding the spot to make sure no one takes over its feeding ground, or showing its fitness to attract mates.

The proposed hypothesis aims to understand if this behaviour happens for mate attraction or territorial defense when multiple bitterns call at the same time, and to clarify if there is a ‘patch leader’, which would be holding the patch as its territory. 


Experimental design proposed

To lay-out an experimental design, there is the need to identify each bird by its booms characteristics, which of these birds stay in the same position over the entire time of the analysis, birds strength based on the poor boom amounts and how much do each of the birds call.

Most of the previous research audio recording with equipment for studio analysis is not used, but on-site human notes recording, increasing the overall statistical error. With the technology used here, the recording includes various elements that can be analysed:


  • Direction from which the call is coming from.
    By using an advanced audio workflow and analysis tools, it is possible to see where each call comes from, as shown on this video.

Above, the rectangle shows a 360-degree frequency representation of what is happening around the microphone, ‘unwrapped’ from a sphere, where the center is the front, and the sides are the back. The red dot is the bittern calling.


  • Spectrogram analysis of each call.
    This will help understand how many booms are happening for each train, and when they are poor. Here is a video preview.


A screenshot of a spectrogram of a section of the recording, with a boom train included in the green rectangle.


  • Loudness of each call, and possible approximation of distance from the microphone.
    The bittern call is based on a dominant frequency, which is quite low in frequency (between 100 and 300 Hz generally). Calculating the time it takes for the sound generated by the bittern to reach the microphone is quite simple, and this would help determine its position in the patch. This technique needs to be analysed further. (OpenStaxCollege)


On the biological side, the following aspects can be analysed:

  1. How many bitterns are calling, this would lend to choose which sections can be catalogued as ‘bittern chorus’, and when this is happening.
  2. Where the bitterns are calling from, which would help to understand which individuals are stationary and which ones are moving, to possibly understand which one is the ‘patch leader’.

If the observation is made by an observer while listening to the recording in the studio, it would be ideal to use the behaviour sampling rule with zero-one sampling happening every 5 minutes for 10 seconds. In this way the observer can note down the direction where the call is coming from, the type of boom train for individualisation. 5 minutes is proposed by Poulin & Lefebvre (2003) as an optimal recording interval, and 10 seconds zero-one sampling as it is the average length of a boom train. Zero-one has been chosen over instantaneous because of the length of the boom trains, and the possibility to miss a boom as it being overall short. Ideally, as the sampling could start half-way through a boom train, there should be a rule where also the booms before the start of the interval would be counted, if part of the same boom train.

As a long-term audio recording approach, it would be ideal to record every day at dawn (Poulin & Lefebvre, 2003) from just before the breeding season, being early March, until the end of the nestling period, around mid-May.

There will be the need to initially catalogue the behaviours in an ethogram:

  • Call Direction (where it comes from) = CD
  • Call fundamental Frequency (to identify the bird) = CF
  • Full Booms (per interval/boom train) = FB
  • Poor booms (per interval/boom train) = PB


This will then need to be processed initially in a matrix, to start cataloguing all the data.

CD (degrees) CF (Hz) FB (number) PB (number)

And then, a second phase of observation

  • CD’s frequency
  • CF’s frequency

The CF can be used to identify each animal, which will be identified with a number. Then, for each individual, it will be counted how many times it is called from the same direction, how many FB and how many PB.


ID #1



ID #2



ID #x



With this information, a kinetic diagram can be compiled representing each individual, its fitness and also the busiest areas where the calls come from, to better understand the overall behaviour of each individual within the patch, and possibly finding a pattern to see who is ‘dominating’ it.



In this experimental design, the aim is to find a way to identify each bird, to see their movement within a certain area and evaluate their fitness, to figure out if there is one individual, or more, that holds the territory as a patch leader. This, combined with other field work such as nesting positions, could lead to a better understanding of the reason behind the Eurasian Bittern’s call and its reproductive behaviour.



Alessandria, G. & Carpegna, F. & Toffola, M.. (2003). Vocalizations and courtship displays of the Bittern Botaurus stellaris: We report on the courtship behaviour of the Eurasian Bittern from direct observation in rice fields. Bird Study. 50. 182-184. 10.1080/00063650309461311. Link

BirdLife International (2015). Botaurus stellaris. The IUCN Red List of Threatened Species 2015: e.T22697346A60160978. Downloaded on 22 March 2020. Link

BirdLife International (2020) Species factsheet: Botaurus stellaris. Downloaded from http://www.birdlife.org on 26/03/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 22/03/2020. Link

Brown, Andy & Gilbert, Gillian & Wotton, Simon. (2012). Bitterns and Bittern Conservation in the UK. British Birds. 105. 58-87. Link

Heron Conservation. (2020) Eurasian Bittern, Botaurus stellaris (Linnaeus). Visited on 20 March 2020. Link

OpenStaxCollege. (2020) Speed of Sound, Frequency and Wavelength. Link 

Polak, M.. (2006). Booming activity of male Bitterns Botaurus stellaris in relation to reproductive cycle and harem size. Ornis Fennica. 83. 27-33. Link

Poulin, B. & Lefebvre, G.. (2003). Optimal sampling of booming Bitterns Botaurus stellaris. Ornis Fennica. 80. 11-20. Link

Puglisi L., Pagni M., Bulgaelli C. & Baldaccini E. (2001) The possible functions of calls organization in the bittern (Botaurusstellaris), Italian Journal

of Zoology, 68:4, 315-321, DOI: 10.1080/11250000109356425. Link

Puglisi, L. & Cima, O. & Baldaccini, N.. (2008). A study of the seasonal booming activity of the Bittern Botaurus stellaris; what is the biological significance of the booms?. Ibis. 139. 638 – 645. 10.1111/j.1474-919X.1997.tb04686.x. Link

RSPB, The application of science: a case study – the bittern story. Downloaded from the RSPB website. Link