































33

THE SOCIAL CONTEXT OF CONTACT CALLS BY ROOKS

(CORVUS FRUGILEGUS)

Alexandru MUNTEANU*, DVM

Professor Ionel PAPUC*, PhD

Ira G. FEDERSPIEL**, PhD

Professor Nicola S. CLAYTON***, PhD

Nathan J. EMERY**, MD

Abstract

Communication is the link between individuals of one species and represents the essence of social life. Vocal communication is 
one of the most studied forms of information exchange, although it also comes with interspecific barriers that are still tricky to overcome.  
While we are able to understand the meaning of another human’s words, we fail to understand an animal’s utterances. Among these, 
bird song has become a field of particular interest. However, little is known yet about many species’ vocalizations, and even less about 
their significance or how different factors influence them. The presented study establishes the vocal repertoire of a group of rooks and 
further investigates the importance of contact calls between partners in an experiment. We found that test subjects and other group 
members produced more contact calls after than before partners had been separated from each other, indicating stress induced by 
physical isolation and/or the lack of visual contact as an important factor influencing the call frequency. Separating certain individuals 
seemed to affect the group differently, which indicates that the ‘importance’ of the animal to the group influences the group call rate. 
In this study, we have shown how social and environmental factors play a role in vocal communication in birds.

Key words: rooks, corvus frugilegus, contact calls, birds, communications

Introduction

Communication is all around us, we are all familiar with how to ‘do it’, and it is something that 

could not occur without social contact with others (McGregor 2005). But communication is not 

restricted to humans, all other animals also engage in various forms of communication, from vocal 

to tactile communication to using chemical cues. Over the last few decades the field of animal vocal 

communication has experienced a revolution in terms of methods and feasibility (Catchpole & Slater 

2008). Whereas before that, people had to draw spectrograms by hand, nowadays computers with 

* University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca, Faculty of Veterinary Medicine Cluj-Napoca, Discipline 
of Semiology, Ethology and Diagnostic Imaging

** Sub-Department of Animal Behaviour, Department of Zoology, University of Cambridge
*** Department of Experimental Psychology, University of Cambridge

Cluj Veterinary Journal, 15(1)/2009, pp. 33-43



34

intricate software do all the hard work, enabling researchers to focus on more important aspects 

of communication rather than trying to improve the methods for studying it. With the programs 

available to us nowadays, we are certainly getting closer and closer to disentangling the various small 

pieces that make up a bout of communication, i.e. ‘conversations’ between two or more beings, and 

in a few cases even understand ‘referential’ communication, e.g. alarm calls in a situation of arousal 

when a predator is close by or begging calls produced by an animal that sees another carrying food 

and wants to gain some of it. Even so, we may never really know for sure what goes on in the mind 

of another animal, how other animals perceive the outside world (Nagel 1974), or even what they 

mean when they produce vocalizations, because of the fundamental differences between our minds’ 

mechanisms (Quine 1973). 

One of the various smaller fields in animal communication that people have started working 

on extensively is bird song (e.g. black-capped chickadees, Poecile atricapillus: Avey 2008; tyrant 

flycatchers, Tyrannidae: Hughes 2008; Eurasian jays, Garrulus glandarius: Goodwin 1949), although 

the vocal repertoire has also been explored in mammals like degus, Octodon degus (Long 2007). 

Generally, bird vocalizations are divided into songs – long, complex and spontaneous vocalizations 

that are commonly produced by males mainly during the breeding season – and calls – much shorter 

vocalizations that are produced by both sexes, throughout the year (Catchpole & Slater 2008). The 

latter ones are given in certain contexts like danger, aggressive display or food desire and thus can 

be alarm calls, threat calls or begging calls. 

The smallest structural units of bird songs and calls, named elements or notes, are in fact sounds 

that, combined in a certain way, form syllables, and syllables in turn are combined to make up 

phrases (Catchpole & Slater 2008). A song may consist of a varying amount of the same or different 

phrases. A big advantage of vocal over non-vocal communication is that it uses sound as a means 

of communicating in situations where visibility is low or the distances are long (Catchpole & Slater 

2008). Sound is also a fast carrier of large amounts of information. Perhaps the energetic cost of 

sound production could be considered a disadvantage, but recent studies have shown that it is less 

expensive than other activities, such as flying (Oberweger & Goller 2001, Ward et al. 2004). Having 

advantages that outweigh the cost, vocal communication thus seems to be the most suitable way of 

communicating for birds.

To pinpoint the meaning of vocalizations, one usually starts by combining observations of the bird’s 

social relationship, interactions with its environment and recording calls and songs (Struhsaker 1967). 

This enables a better understanding of the context and thus of the meaning of a certain vocalization. 

However, grasping the whole range of utterances of one species is difficult to achieve, since there is 

a lot of variation, even between populations of the same species, stemming from different selective 

pressures and constraints working on their development, production, transmission and de tection 

(Ryan & Brenowitz 1985). A study with ravens (Corvus corax) suggests that vocalizations may very 

well vary even within populations, with new calls emerging and others being lost over time (Enggist-

Duebelin & Pfister 2002). Therefore, one can only attempt to categorize as many different song or 

call types as possible, either by collecting observational data and sound recordings or by conducting 

experiments tailored to look at a certain vocalization and how it changes in different contexts.

Rooks are members of the Corvus genus, the biggest and most widespread of the Corvidae family. 

Consisting of 48 species, this genus also includes ravens, crows, jays, magpies, and jackdaws 

(Grzimek B. 2002). Observations show that the typical crows of the Corvus genus are characterized 

by a spectacular black plumage with metallic iridescence, food caching abilities, complex social 



35

interactions and hoarse voices, although still little remains known about their vocalizations. Hardy 

(1979) in a study on Black and Blue Jays (Cissilopha) hypothesized that social species produce a wider 

range of vocalizations than solitary-living species, a theory launched after studying and establishing 

the vocal repertoire of four closely related species of the Cyanocorax genus, in relation to their social 

habits. We would therefore expect to find a rich vocal repertoire in rooks (Corvus frugilegus), since it 

is a very social species, breeding and foraging close together in colonies and gathering at communal 

roosting sites (Røskaft & Espmark 1982). 

Using the approach described above and combining observations with recordings, in the present 

study we tried to determine the different call types making up our rooks’ vocal repertoire. Further 

investigating the importance of one of the vocalizations, the contact call, we then conducted an 

experiment that looked at how the context can influence a vocalization.

Materials and Methods

Subjects and housing

In this study we used a group of eight rooks aged 6 years. The group consisted of five females and 

three males. For individual identification, they had been banded with coloured rings. All individuals 

were taken from the wild at a young age and hand-reared in captivity. The rooks were kept in a large 

outdoor aviary at the Sub-department of Animal Behaviour in Madingley, UK, measuring 10 x 8 x 4 

meters (length, width, height). The aviary consisted of two main compartments, similar in dimensions, 

separated from each other by a central small compartment.

Procedure

The present study consisted of two parts, each being designed to answer a different question: 

observations and an experiment. With the observations we wanted to establish the repertoire of rook 

vocalizations and determine the sex and individual differences in the calls; the experiment looked 

at the call frequency and call morphology differences between the test subject, its partner, and all 

other individuals. For both parts, a Canon MD 101 Digital Video CamRecorder and a Dell laptop with 

integrated SigmaTel stereo microphone array (SigmaTel High Definition Audio Codec) were used to 

make video and audio recordings.

Observations

We recorded vocalizations of all eight rooks over a period of 7 weeks (July-August 2008) for an 

average time of 1 to 2½ hours at different times of the day in order to avoid a possible bias caused by 

the time of day (e.g. influences of motivation or the nutritional status of the birds). The recordings’ 

sample rate was of 44100 samples per second, with a resolution of 16 bits per sample and a stereo 

channel format. The recorded frequency ranged from 0 to 22000 Hz. The audio files were stored in 

uncompressed wave format. After each vocalization notes regarding individual ID, time of vocalization, 

context of the vocalization, parallel behaviour accompanying the vocalization, distance to and visual 

contact with the partner and other members of the group and the subsequent vocalizations and parallel 

behaviour of the partner and other group members were taken. The vocalizations were classified 

into: neutral, agonistic (negative) and affiliative (positive).

Experiment

The experiment was conducted in July and August 2008. Each pair was only tested once a day, 

but in total received four test sessions: two sessions in two different conditions (see Fig. 1 and Fig. 

2). The order in which the pairs were tested and the order in which the partners were isolated from 

the group on the other side of the aviary was pseudo randomized.



36

Before each session, the birds were familiarized with the experimenter’s presence. In both 

conditions, each session consisted of three phases, each of 10 minutes length: a pre-separation phase 

((1) in Fig. 1 and 2), a separation phase ((2) in Fig. 1 and 2) and a post-separation phase, i.e. reunion 

of the partners in the group ((3) in Fig. 1 and 2).

During the pre-separation phase, both partners were in the group and could move around freely; at 

the beginning of the separation phase, experimenter 2 separated one or both partners from the group 

and at the end of the phase, the partners were reunited in the group for phase 3, a control phase. In 

condition 1, one of the individuals was physically, but not visually isolated from its partner and the 

rest of the group during phase 2 (Fig. 1).

Fig. 1. Condition 1. Arrows show the three phases of the experiment: (1) pre-separation, (2) separation of the 
test subject (partner stays with group), (3) post-separation. The recording device is indicated by a square, 
experimenters are represented by figures. Dashed lines indicate wire mesh.

Fig. 2. Condition 2. The set up is the same as in condition 1, with an exception during the separation phase 
(2), where both partners were separated from the group. Solid lines represent visually isolated compartments, 
dashed lines indicate wire mesh.

In condition 2, both partners were physically isolated from the group and each other. In addition, 

one was also visually isolated in the small compartment, while the other one could still see the group 

(Fig. 2).



37

The latter condition was conducted to control for the influence of the group on the partner that 

stayed with the group in condition 1. In both conditions, observations of the group (including one 

of the partners in condition 1) were taken by experimenter 1, whereas observations of the separated 

subject(s) during phase 2 were taken by experimenter 2 from outside the aviary. 

Data Analysis

The dominance hierarchy was calculated with Matman on the basis of observed displacements 

(i.e. a bird retreats after having been approached by another). We used 10,000 randomizations and 

calculated the Landau's linearity index (h) and the directional consistency index to determine linearity 

of the hierarchy and the consistency of the result of the calculation (both indices lie between 1 and 

0). An h of 1 indicates a linear hierarchy, whereas 0 point towards an ever-changing hierarchy.

For the analysis of the different call types, the vocalizations were extracted from the main recorded 

sound file, saved as wave files with the same format and added into a library, with the different call 

types categorized for each bird. Video data was then transferred onto the computer and analyzed 

together with data from the observation sheets. In order to save disk space and for easier storage the 

raw videos were then compressed using Microsoft Windows Movie Maker in windows media video 

file format with a 3 mb/s bit rate quality, preserving the original display size (720 x 480 pixels), aspect 

ratio (4:3), frames per second (30) and audio quality (16 bits per sample).

For recording, editing and analysis we used the following programs: Acoustica Premium (version 

4.1.0, Acon Digital Media GmbH, 1994-2008), Avisoft-SASLab Pro (version 4.40, Avisoft Bioacoustics, 

1990-2008), Windows Movie Maker (version 6.0, Microsoft Corporation); for statistical analysis we 

used SPSS 16.0 (SPSS Inc., 1989-2006). Using a spectrogram generated in Avisoft, we analyzed 

the following parameters: temporal parameters (duration of element, interval between elements, 

distance from start to maximum), sub-elements (number of elements), spectrum-based parameters 

(peak frequency, peak amplitude, fundamental frequency, minimum frequency, maximum frequency, 

frequencies of peaks, amplitude of peaks), locations of measurements (start of element, end of element, 

centre of element, maximum amplitude of element, mean spectrum of entire element, minimum 

parameter of entire element, maximum parameter of entire element). For classifying the vocalizations 

into different call types and looking at individual and sex differences, we used cluster analyses, 

Mann-Whitney U tests and Kruskal-Wallis tests. For analysing the number of contact calls given in 

the experiment, we calculated the average call number per individual across the two test sessions 

(separately for each condition) to rule out influences of the weather, the ‘mood’ of the birds, nutritional 

status etc. For analyses involving the ‘other’ six group members we did the same but also divided 

the number obtained by 6 (the total number of the birds excluding the test subject and the partner). 

We used Friedman ANOVAs and Wilcoxon matched pairs tests. All tests were nonparametric, two-

tailed, and alpha was set at 0.05.

Results

Dominance hierarchy

The dominance hierarchy in the group was as follows: Plato (white ring) > Linnaeus (purple ring) > 

Mackintosh (red ring) > Einstein (yellow ring) > Darwin (green ring) > Aristotle (red and blue rings) 

> Da Vinci (blue ring) > Huxley (orange ring) (h=0.65, directional consistency index=0.98).



38

Vocal repertoire

We have established that the rook vocal repertoire 

consisted of nine structurally different calls, and 

several more variants. We have determined the 

function of three calls and have named them 

accordingly: contact and alarm calls, begging call, 

and submission calls. The remaining 6 calls and 

variants were named according to their acoustic 

similarity with other sounds: ‘rattle’, ‘purr’, ‘click’, 

‘hick-up’, ‘scream’, and ‘monkey’ calls. Contact and 

alarm calls were the most common call vocalized by 

the group and were in fact the same call but used in 

different situations. Examples of some of the calls 

can be seen in figures 3, 4, 5, 6, 7, 8. 

Do the test subjects, their partners or the other 

birds vocalize more before, during or after the 

separation?

In both conditions, we found a significant 

difference between the average number of calls test 

subjects gave before, during and after the separation 

(Friedman ANOVA: condition 1: n=6, χ2=6.09, 

p=0.048; condition 2: n=6, χ2=8.34, p=0.016; 

Fig.9). To determine the origin of the significant 

result, we conducted post-hoc Wilcoxon matched 

pairs tests. These revealed more calls of the test 

subject after than before the separation in both 

conditions (condition 1: n=6, Z=2.20, p=0.028; 

condition 2: n=6, Z=2.02, p=0.043) and more 

calls after than during the separation in condition 

2 (n=6, Z=2.02, p=0.043). 

Furthermore, a significant difference was found 

for the other group members in both conditions 

(Friedman ANOVA: condition 1: n=6, χ2=7.60, 

p=0.022; condition 2: n=6, χ2=6.34, p=0.042; 

Fig.10). In both conditions, they produced more 

calls during the separation of an individual than 

before (Wilcoxon matched pairs tests: condition 1: 

n=6, Z=2.20, p=0.028; condition 2: n=6, Z=1.99, 

p=0.046) and also more after than before the 

5

10

15

20
kHz

0.2 s

5

10

15

20
kHz

0.2 0.4 0.6 s

Fig. 3. Female contact calls

5

10

15

20
kHz

0.2 0.4 s

5

10

15

20
kHz

0.2 s

Fig. 4. Male contact calls

5

5

10

10

15

15

20

20

kHz

0.2 0.4 0.6

0.8 1.0 1.2 1.4 s

5

10

15

20
kHz

0.2 0.4 s

Fig. 5. Female begging call Fig. 6. Female ‘rattle’ call

Fig. 7. Female ‘hick-up’ call

5

10

15

20
kHz

s

5

10

15

20
kHz

0.2 0.4 0.6s

Fig. 8. Male ‘click’ call

Fig. 9. Box and whisker plots for the calls given by the test subjects 
in the three phases in condition 1. Boxes show the interquartile 
range; the box for the observers is divided by the median value. 
Whiskers indicate the largest and smallest value.



39

separation (condition 1: n=6, Z=1.99, p=0.046; 

condition 2: n=6, Z=2.20, p=0.028). There was 

no difference in the average number of calls the 

partner of the separated bird produced in any of the 

three phases in either of the two conditions.

Do the birds vocalize more before, during or 

after the separation of a certain individual?

Depending on which animal was separated, the 

birds called more or less before, during or after the 

separation in condition 1. We found a significant 

difference between the average number of calls 

the birds produced before, during and after the 

separation when Da Vinci (Friedman ANOVA: 

n=8, χ2=8.96, p=0.011), Darwin (n=8, χ2=11.08, 

p=0.004), Mackintosh (n=8, χ2=9.34, p=0.009) and 

Plato (n=8, χ2=6.34, p=0.042) were separated (e.g. 

Da Vinci, Fig.11). Results for the post-hoc Wilcoxon 

matched pairs tests can be found in Tab.1. 

Similarly, we found significant differences 

in the number of calls in condition 2 when the 

following birds were separated: Da Vinci (Friedman 

ANOVA: n=8, χ2=9.34, p=0.009), Linnaeus (n=8, 

χ2=9.34, p=0.009), Mackintosh (n=8, χ2=9.36, 

p=0.009), Aristotle (n=8, χ2=7.36, p=0.025), Plato 

(n=8, χ2=9.36, p=0.009) (e.g. Linnaeus, Fig. 12; for 

Wilcoxon matched pairs tests see Tab. 1).

Who vocalizes more: the subject, the partner, 

or the others?

We compared the average number of calls the 

test subjects, their partners and the other birds 

produced overall for each of the three phases 

(Friedman ANOVAS) and found no significant 

differences.

Is there a difference between the two conditions?

We looked at the three different phases separately 

and compared the average number of calls the test 

subjects, their partners and the other birds gave in 

condition 1 and 2. Except for the number of calls 

Fig. 11. Box and whisker plots for the calls given during the 
separation of one bird in condition 1. Boxes show the interquartile 
range; the box for the observers is divided by the median value. 
Whiskers indicate the largest and smallest value.

Fig. 10. Box and whisker plots for the calls given by the test 
subjects in the three phases in condition 2. Boxes show the 
interquartile range; the box for the observers is divided by 
the median value. Whiskers indicate the largest and smallest 
value.

Fig. 12. Box and whisker plots for the calls given during the 
separation of one bird in condition 2. Boxes show the interquartile 
range; the box for the observers is divided by the median value. 
Whiskers indicate the largest and smallest value.



40

the subjects produced during the separation we 

found no significant differences. When separated, 

the subjects called more in condition 2 (in the small 

compartment) than in condition 1 (at the other side 

of the aviary) (Wilcoxon matched pairs test: n=6, 

Z=2.02, p=0.043; Fig. 13).

Tab. 1. Results for the comparison of the average number of calls all birds produced before,
during and after the separation of certain individuals for (a) condition 1 and (b) condition 2.

(a) phases compared Z P

Da Vinci
during-after 2.02 0.04 more after
after-before 2.37 0.02 more after

Darwin
before-during 2.20 0.03 more during
during-after 2.20 0.03 more during

Mackintosh
before-during 2.20 0.03 more during
after-before 2.20 0.03 more after

Plato
before-during 1.99 0.05 more during
after-before 2.20 0.03 more after

(b) phases compared Z P

Da Vinci before-during 2.20 0.03 more during

after-before 2.20 0.03 more after

Linnaeus before-during 2.20 0.03 more during

after-before 2.20 0.03 more after

Mackintosh before-during 2.02 0.04 more before

during-after 2.20 0.03 more after

Aristotle after-before 2.20 0.03 more after

Plato
during-after 2.02 0.04 more after

after-before 2.20 0.03 more after

Discussion

The present work revealed a large vocal repertoire in the studied group of rooks similar to the one 

Røskaft & Espmark (1982) found in their group. 

We found that the test subjects produced more contact calls after the separation from their partners 

than before it, irrespective of the condition, i.e. whether the separated partner was in the small 

compartment or in the other side of the aviary. When an individual was isolated from its partner in 

the small compartment, it gave more contact calls after than during the separation. Similar to post-

conflict affiliation, when rooks affiliate with each other after a fight in the group (Seed et al. 2007), 

rooks seem to call more after a stressful situation than before it. This points in the direction of stress as 

an important influence of calling for the subjects. Thus, due spending energy on increasing their heart 

Fig. 13. Box and whisker plots for the calls given by the test 
subjects in the two conditions. Boxes show the interquartile 
range; the box for the observers is divided by the median value. 
Whiskers indicate the largest and smallest value.



41

and breathing rate, they might lack the energy for calling during separation. Also, the restricted dark 

space may be an unnatural situation they want to get out of by any means, and for them, the best way 

for doing that may seem to be through the cracks, where they can see light coming in. Continuously 

trying every possible way of escaping, calling becomes less of a priority, and as stated above, they 

could at the same time save energy for the flight. The separated bird produced fewer calls in the small 

compartment in condition 2, in which he could not see the partner, than afterwards, when he was 

reunited with his partner. Nevertheless, compared to condition 1, the subjects produced more contact 

calls during separation in condition 2, indicating an overall higher level of arousal in condition 2, 

which may be due to the additional stress of lack of visual contact with the separated bird. This has 

also been a noted result in a study on marmosets, where the subjects produced a “separation phee” 

call during short periods of absence from the group (Norcross et al. 1999). A study that is currently 

being conducted with chimpanzees looks at the influence of visibility in the forest on call rate and 

may reveal similar results (Slocombe et al. in prep.).

Also, the other group members gave more contact calls during the separation of a bird than before, 

and more after than during the separation, further supporting the idea of stress as a factor. 

Interestingly, the partner of the isolated bird did not change the rate of contact calls in any of the 

phases in either of the conditions. In both conditions the partner was not physically constricted, it 

could see the group members and could freely communicate with them. Only in the second phase of 

the second condition the partner was physically isolated but in a familiar enclosure comparable in size 

and visibility with the one it was separated from. So, one explanation can be that the partner could 

better assess the situation than the test subject and thus would communicate at a regular frequency. 

Another explanation could be that the distance and physical isolation from the group make the bird 

instinctively try to conceal its location to an eventual predator, and so minimize the chances of being 

in imminent danger. Instead of using compromising vocalizations it spends its energy on finding a 

way to get to its partner or the group members. This is because for a social species, rooks are safer 

when gathered in large groups, while one bird is more vulnerable on its own. 

Separating certain individuals seemed to affect the other group members differently, indicating an 

influence of the ‘importance’ of the animal to the group on the call rate, potentially arising from the 

dominance status or the sex of the given bird. Social dominance hierarchy has been shown to play 

an important role in determining the long-distance call frequency and structure in male chimpanzees 

(Pan troglodytes), in the way that high-ranking males produce more calls than low-ranking ones 

(Mitani and Nishida 1993).

Other studies have shown that not necessarily the rate, but for example the structure of vocalizations 

changes when animals are separated from their group or partner. The oldest example in this direction 

is the ‘sympathetic song’ of two Australian magpies (Gymnorhina tibicen). The two birds had succeeded 

in learning a flute song that both had been exposed to and managed to reproduce it as a duet, in 

which each one uttered a half while the other one would finish the song (Waite 1903). Gwinner and 

Kneutgen have shown that because both the common raven (Corvus corax) and the common shama 

(Copsychus malabaricus) bond for life and have individually different vocalizations they have a 

much stronger vocal relationship. When an individual had not been in the presence of its mate for 

some time, the latter one would produce the missing partner’s vocalizations. Upon hearing its own 

vocalizations, the mate would then return to the caller (Gwinner and Kneutgen 1962). The same 

phenomenon has also been reported for the eastern bluebird (Sialia sialis) (Morton et al. 1978) and 

the black-headed grosbeak (Pheucticus melanocephalus) (Ritchison 1983).



42

Similar findings have been reported in nonhuman primates that show fine differences in their call 

structure as a result of social changes. When paired with a new mate, pigmy marmosets (Cebuella 

pygmaea) modify their trill vocalizations structure (Snowdown and Elowson 1999). Periods of social 

instability determine the emergence of novel variants and transformations in frequency structure of 

the ‘combined harmonic’ calls in female Campbell’s monkeys (Cercopithecus campbelli) (LeMasson 

and Hausberger 2004). An acoustic variability has been demonstrated to occur in the male chimpanzee 

(Pan troglodytes) typical long-distance calls, the pant hoot, in relation to pre and post-traveling 

situations of temporary associated parties (Mitani and Brandt 1994).

Future studies could take into account the stress of the act of separating a bird on the group and 

include control experiments to rule out the actual separating as an influence on the call rate, e.g. 

by catching a bird, letting it go again and measuring the contact call rate immediately afterwards. 

Furthermore, to increase the sample size of just three rook pairs, more data should be taken from 

different rook groups. In addition, field observations could prove helpful in determining the actual 

relevance of our study, by noting the different stressful factors that can naturally occur (e.g. predator 

chasing one of the partners, or partner dies). Recording quality could be improved by installing 

microphones within the aviary or the rook territory to reduce the stress factor created by the presence 

of an experimenter. This would generate more accurate data, as the recordings could continue over a 

longer period of time. Miniature microphones could also be installed directly on the individuals’ bodies 

to reduce the possibility of confounding certain birds’ vocalizations. In order to better determine the 

context in which one vocalization is being produced, CCTV cameras could be installed in different 

angles within the aviary, thus reducing the necessity of human presence and in this way eliminating 

the additional stress created otherwise.

Overall, we have shown that vocal communication in rooks, as a highly social species, is a very 

flexible process influenced by different social and environmental factors.

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