Applied Animal Behaviour Science 75 (2001) 61±74
Vocalisation in chicks (Gallus gallus dom.)
during stepwise social isolation
G. Marx*, J. Leppelt, F. Ellendorff
Institute for Animal Science and Animal Behaviour Mariensee, Federal Agricultural Research Centre,
HoÈltystr. 10, D-31535 Neustadt, Germany
Accepted 20 July 2001
Abstract
Despite attempts to relate vocalisation of animals to their welfare status, very little is known in
farm animals and in precocious birds. We analysed vocal expressions of 50 chicks during step by
step social isolation. 91.20% of the recorded vocalisations (n ˆ 21 583 calls) consisted of four main
call types: distress calls (43.72%) were characterised by declining frequency with high energy;
short peeps (22.04%) by declining frequency, low energy and short duration; warblers (19.36%) by
repetitive bow-type elements with low energy; pleasure notes (6.08%) by increasing frequency and
low energy. 8.80% of calls were of other unde®ned nature. In all tests single isolated animals
showed highest total vocalisation. When two different hatches were compared, total vocalisation
differed signi®cantly (P < 0:001, F-test). Changes of vocalisation during stepwise reduction of
group size (beginning with four or more chicks) resulted in signi®cant differences (P < 0:01, Ftest) between steps. Low energy vocalisation (short peeps, warblers and pleasure notes) dominated
in group sizes of three and more chicks. As group size decreased, the number of distress calls
increased. Warblers and pleasure notes were absent when animals were isolated in the ®nal step of
the social isolation test indicating that a social target needs to be present for such call types to occur.
It is concluded that vocalisation is strongly dependent on social contacts in the chick and that
different degrees of social de®cits ®nd their expression in discrete changes in the pattern and
elements of vocalisation. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Chicken; Vocalisation; Group size; Call types; Social contacts
1. Introduction
Most animals communicate by a wide range of vocal signals. Vocal pattern changes with
certain mental states in the monkey (JuÈrgens, 1979; Hauser, 1993; Seyfarth et al., 1994; Au,
*
Corresponding author. Tel.: ‡49-5034-871186; fax: ‡49-5034-817143.
E-mail address: [email protected] (G. Marx).
0168-1591/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 1 5 9 1 ( 0 1 ) 0 0 1 8 0 - 0
62
G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
1997). Little is known in other species or farm animals (Weary et al., 1998; Zimmermann
et al., 2000a,b), but some studies are published on functional aspects of selected call types
(e.g. Evans, 1975; Algers and Jensen, 1985; Evans and Marler, 1992; Evans and Evans,
1999). The following analyses chicken vocal expressions in a social context (Marx and
Leppelt, 1997; Leppelt, 1997).
During the ®rst few days of life a pattern of intensive vocal communication develops
between mother hen and chick. Chicks can discriminate between their own mother and an
alien hen on the basis of the hen's cluck vocalisation (Kent, 1987), but the hen cannot
discriminate between own or alien chicks by vocalisation (Kent, 1992). Vocal patterns in
the chick were published previously (Wood-Gush, 1971; Collias and Joos, 1953): distress
vocalisation (``peep'' Ð Andrew, 1964; ``cri d'appel du poussin isoleÂ'' Ð Guyomarc'h,
1966; ``Verlassenheitslaut'' Ð Baeumer, 1962) usually occurs, when chicks are exposed to
con¯ict situations where assistance is called, for example, loss of group contact. Short
peeps are structurally very similar to distress calls but differ in duration and energy content
(Andrew, 1964). Pleasure notes (``cri simple ou rhythmique de plaisir'' Ð Guyomarc'h,
1966; ``StimmfuÈhlungslaute'' Ð Heinroth, 1924) and warblers (``cri rythmique d'assoupissement'' Ð Guyomarc'h, 1966; ``Ruf der Schlafbereitschaft'' Ð Baeumer, 1962) are
social contact calls, frequently observed with the chick's exploratory behaviour (Andrew,
1973). The warbler is used by chicks to express their readiness to sleep under the mother
hen. Fear trills (``cri d' inconfort psychophysiologique modeÁre'' Ð Guyomarc'h, 1966;
``Schrecktriller'' Ð Baeumer, 1962; ``Trill'' Ð Andrew, 1964) or fear notes (``cri d'
effroi'', ``cri aiger de peur'' Ð Guyomarc'h, 1966; ``Angstschrei'' Ð Baeumer, 1962) are
typically expressed when the chick is suddenly handled or in case of pending danger. To
date, the call types were not based on numerical evaluation of vocalisation. Type and
intensity of social stimuli have not been investigated. Further, functional description of
vocalisation is based only on few spectrographic presentations. Thus, characterisation of
the whole repertoire of vocalisation of chicks is one objective of the present study. To this
end chicks are placed into an experimental situation and vocalisation is evaluated by
spectral analysis.
Another major de®cit of previous studies (Gaioni et al., 1977; Jones and Harvey, 1988;
Jones and Merry, 1988; Jones and Williams, 1992) was that they were based on mere
counting of single vocal expressions of the distress call, while a characterisation of
vocalisation involving all call types is needed for a deeper understanding of vocal±
behavioural interaction. Thus, a second major objective of the study was to close the gap,
analysing in detail numerical changes in total vocalisation and to describe changes under
condition of stepwise social isolation.
2. Material and methods
2.1. Animals
A total of 50 chicks (White Leghorn, Institute strain) were obtained from two different
hatches approximately 12 h after hatch. Separated by origin of hatch, groups of 3, 5 or 10
chicks (®rst hatch) or 7, 10, 15 chicks (second hatch) were each placed into
G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
63
100 cm 60 cm 30 cm (length, width and height) temperature controlled commercial
rearing pens (Hemel BrutgeraÈte, Verl-Kaunitz, Germany) on ®ne wire mesh in a sound
proof room (±35 dB). The temperature inside the pens ranged from 26 to 328C at a relative
humidity between 30 and 40%. Animals were placed under 10 h dark/14 h light (lights on
06.00 h) using a 15 W incandescent electric bulb in each pen. A commercial chick diet and
fresh tap water were provided ad libitum. The weight of each chick was recorded daily.
2.2. The step isolation test (SIT)
SIT was carried out in a sound isolated test box (AC3, Industrial Acoustic Company,
NiederkruÈchten, Germany) 60 cm 60 cm 50 cm (length, width and height). The test
box reduced outside noise by additional 30 dB. The box was ventilated with room
temperature (20±228C) and illuminated with a 15 W incandescent electric bulb. Behaviour
was monitored by video (Panasonic WV-KS 512, Wiesbaden, Germany) and recorded on a
Panasonic AG 6024. Behaviour was only evaluated to detect gross disturbances.
The chicks were placed in groups of three to ®ve chicks into the test box for a period
of 15 min on day 2 and 5 of life to familiarise them to experimental handling and
environment.
The actual tests were carried out on days 6 and 7 of life between 09.00 and 12.00 h to
reduce the in¯uence of circadian rhythms. During this time it was possible to run two
repetitions for three groups per day; thus a total of four group repetitions of the test were
carried out for each treatment group. At least 50% of the chicks in a rearing box were
assigned to each SIT (Table 1).
The animals were carefully removed from rearing pens and placed into the test box,
which was then tightly closed. Subsequently, the chick's remained in the box undisturbed
for a period of four minutes while vocalisation was recorded. Thereafter, one animal was
removed and recording continued for 4 min, etc. till only a single chick was left in the test
box.
Care was taken to avoid that at test repeats the same chicks were in a given group or that
identical chicks were removed twice at the same step, during stepwise reduction of group
sizes. However, for the smallest group sizes tested (treatment group h1±r3±s2) this could
not be achieved.
Table 1
Assignment of number of chicks to starter groups of SIT
Treatment group*
h1±r3±s2
h1±r5±s3
h1±r10±s5
h2±r7±s4
h2±r10±s5
h2±r15±s7
*
Hatch
1
1
1
2
2
2
Number of animals
In rearing box
Initially in test box
3
5
10
7
10
15
2
3
5
4
5
7
h: hatch number; r: rearing group size; s: starter group size of SIT.
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G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
2.3. Voice recording and data evaluation
Throughout the test periods, vocalisations of all animals within the group were recorded
with a single capacitor microphone (B&K 4133, Naerum, Denmark) on tape recorder
(REVOX PR99, LoÈf®ngen, Germany).
In a pilot study a SIT was performed with ®ve animals in the starter group in four
repetitions. Then short lasting (<30 s) disturbances in vocalisation were observed immediately following removal of individual chicks. Another 30 s were allowed to pass before
data collection was started 1 min after closing the box and lasted for 2 min.
Data were digitalised (Spectro 3000, MEDAV, Uttenreut, Germany) at a sampling
frequency of 51.2 kHz. The results were exposed to fast Fourier transformation (FFT, 512
points, hanning window, 51.2% window overlap). Each acoustic signal was represented by
a characteristic distribution of energy (dB) over frequency (kHz) and time (ms). Call
duration, shape of line of pitch frequency, energy content (represented by grey scale in the
spectrogram in Fig. 1) were used as criteria for assignment to call types. The method of
visual inspection is similar to manual labelling used as reference method in speech analysis
(Rabiner and Juang, 1993), thus speci®cation of call types were partially subjective. A ®rst
classi®cation of call types takes into account shape of line of either increasing (pleasure
note ``PN'') or decreasing (short peep ``SP'' and distress call ``DC'') or bow-type pitch
frequencies (warbler ``W''). The distress call is distinct from short peep by a duration of
more than 100 ms and peak energies beyond 92 dB. All calls that did not meet the criteria
mentioned were summarised as ``other calls''.
The inter-observer reliability based on three observers and 30 observations of 20 s
resulted in an intra-class coef®cient of correlation for vocal frequencies of DC ˆ 0:990,
SP ˆ 0:906, PN ˆ 0:899 and W ˆ 0:991; and for intra-observer reliability based on three
repetitions per observer of DC ˆ 0:990, SP ˆ 0:912, SN ˆ 0:903 and W ˆ 0:991.
The total vocalisation (TV) is de®ned as the sum of all calls per animal and minute.
Comparison of total vocalisation was carried out separately for each treatment group by
analysis of variance using the general linear model (GLM SAS 6.10). The ®xed effect was
the actual group size during the test. To compare the SIT of treatment group h1±r10±s5
(hatch 1, rearing group size of 10, starting with ®ve animals) to treatment group h2±r10±s5
a second analysis of variance was performed. Fixed effects of hatch and actual group size
during the SIT were considered.
The frequency of occurrence of speci®c calls is de®ned as the sum of calls per animal
and minute for each of the call types. Since the speci®c types of vocalisation are not
independent, multivariate analysis of variance was employed for comparison for each
treatment group with the group size during SIT as a ®xed effect (GLM SAS 6.10). To test
for differences in vocalisation the contrasts were used between two group sizes for each
step of reduction. Additionally, a two-way multivariate analysis of variance followed by
estimation of contrasts and Wilks'l-test (SAS 6.10) was carried out to compare the two
treatment groups with ®ve chicks at the start of SIT (h1±r10±s5 and h2±r10±s5) using the
group size during the SIT and the group as ®xed effects.
For description of vocalisation the least square means S:E:M: (GLM SAS 6.10) were
used. Differences of the speci®c vocalisations were tested between the two group sizes for
each step by a Tukey adjusted t-test (GLM SAS 6.10) and for the appearance of each call
G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
65
Fig. 1. Example spectrograms of call patterns for each of the four major call types in 6±7-day-old chicks (for
detailed description of calls, see text and Table 2). On the right hand the window in each spectrogram is enlarged
for a better presentation of the energy distribution within the calls by grey scale. (a) Distress calls with high
energy recorded from a single isolated chick; (b) short peeps from a group of three chicks; (c) warbler from a
group of three chicks; (d) pleasure notes from an undisturbed group of 10 chicks in the rearing crates. Calls (b),
(c), (d) are only expressed in groups of chicks, but not in isolated chicks. Calls in (a) and (d) show one or more
harmonics, i.e. multiples of the pitch frequency.
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G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
type by a t-test with H0: m ˆ 0. A value of P 0:05 indicated a signi®cant contribution of a
call type.
3. Results
3.1. Spectrogram pattern of calls
The majority of vocalisations could be assigned to the four major call types (distress call;
short peep; warbler and pleasure note) on the basis of distribution of energy over frequency
and time within calls (examples, see Fig. 1 and Table 2).
The main characteristic for differentiation was the difference in distribution of energy
over frequency and time (see spectrogram Fig. 1b and c). Overlapping in time of calls from
different animals were common in groups of more than three animals. They were clearly
distinguishable by spectrographic presentation. 91.2% of the 21 587 calls could be
assigned to one of the four categories described in Table 2. 8.80% followed different
and less consistent patterns. Usually the energy distribution over time and frequency was
very variable. Some of the calls showed pattern similarities to fear trills or fear notes
described by Wood-Gush (1971). These were too rare for statistical evaluation.
3.2. Total vocalisation (TV)
Mean total vocalisation (sum of all calls per animal/min) relative to group size is shown
in Fig. 2. When only a single chick was left in the test box (Fig. 2a±f) highest total
vocalisation occurred in chicks of both hatches and independent of initial group size when
compared to other group sizes (P < 0:05, t-test for differences of least square means). For
small starter groups of two to three chicks (Fig. 2a and b) an increase in total vocalisation
followed each reduction in group size (P < 0:01, Wilcoxen sign test). Starter groups of ®ve
or seven animals (Fig. 2c, e and f) resulted in a somewhat different total vocalisation. A
decrease was noticeable after the removal of the ®rst chick (P < 0:05, t-test). No signi®cant
changes in total vocalisation occurred during the next steps down to two chicks.
Table 2
Call type characteristics
Call type
Durationa Maximum Pitch
Main
range
of energya frequencya frequency
range
range (kHz) modulation
(ms)
Harmonicsb Other characteristics
Distress call 100±250 0.02±2
2±6.5
Short peep
40±100 0.004±0.01 3±5
Descending
Descending
Warbler
Ascending and 0±1
descending
Ascending
0±1
100±900 0.004±0.02 3±5.5
Pleasure note 23±90
a
b
0.001±0.02 2.5±5.5
Measured on 200 calls of each call type.
The harmonics are multiples of the pitch frequency.
1±4
0
Bouts of three and more calls
Usually in bouts of two to
three calls
Consisted of 2±14 elements
±
G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
67
Fig. 2. Mean total vocalisation (TV, calls per animal/min) from different group size during the step isolation test
for each of the six treatment groups. Group description implies hatch number (h), rearing group size (r) and
starter group size (s) of SIT. The error bars show the S.E.M. Asterisks between the two bars indicate significant
differences of TV induced by the reduction of group size (P < 0:05, t-test) for differences of least square means.
When treatment group h1±r10±s5 was compared to treatment group h2±r10±s5 during
SIT a smaller total vocalisation was observed in chicks of hatch 2 (P < 0:001, F-test).
Then, on day 6 of life, body weight was lower in the chicks of hatch 2 when compared to
hatch 1 (55:2 0:63 versus 63:6 0:94 g, P < 0:001, Mann±Whitney U-test).
3.3. Specific vocalisation
Fig. 3 displays the frequency of occurrence of speci®c vocalisation during reduction of
group size. With only a single animal in the test box the most obvious change was the
consistent increase in the frequency of occurrence of distress calls in all six treatment
groups (P < 0:05, t-test, see Fig. 3a±d and Fig. 4a and b). The other obvious result was, that
the frequency of occurrence of the non-distress calls was zero when the group size was
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G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
Fig. 3. Frequency of occurrence of special calls (calls per animal/min) with each of the five call types for four treatment groups (least square means and S.E.M.
(indicated by error bars); n ˆ 4 group repetitions; white bars: least square means greater than zero (P < 0:05, t-test); black bars: least square means not greater than zero
(P > 0:05, t-test); asterisks between two bars indicate significant differences in frequency of occurrence of special calls induced by the reduction in group size
(P < 0:05, t-test)).
G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
69
Fig. 4. Frequency of occurrence of special calls (calls per animal/min) with each of the five call types for
treatment groups starting with five chicks from both hatches (least square means and S.E.M. (indicated by error
bars); n ˆ 4 group repetitions; white bars: least square means greater than zero (P < 0:05, t-test); black bars:
least square means not greater than zero (P > 0:05, t-test); asterisks between two bars indicate significant
differences in frequency of occurrence of special calls induced by the reduction in group size (P < 0:05, t-test)).
reduced to one in 16 out of 24 observations. Other speci®c vocalisations were less
consistent upon removal of chicks.
However, the Wilcoxen sign test showed that the frequency of occurrence of distress
calls is higher after reduction to two chicks when compared to the presence of three
chicks (Fig. 3b±d and Fig. 4a and b) and that frequencies of occurrence of other call types
are then oppositely directed, i.e. lowered (Fig. 3f±h, j±l, n±p and r±t and Fig. 4c±j). For
single isolated animals and for most groups with only two chicks the frequency of
occurrence of distress calls also showed means greater than zero (P < 0:05, t-test,
Fig. 3a±d and Fig. 4a and b). At the begin of the test only two second hatch treatment
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G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
groups (h2±r7±s4 and h2±r15±s7, Fig. 3c and d) showed a frequency of occurrence of
distress calls greater than zero.
When compared to all other call types the short peep was the most frequent with a
frequency of occurrence greater than zero (P < 0:05, t-test, Fig. 3e±h and Fig. 4c and d).
Short peeps were observed in all groups with three or more chicks (see Fig. 3e±h and Fig. 4c
and d). When the starter group consisted of four or more animals only low rates of short
peeps (ranging from 0 to 4.3 calls per animal) were recorded from the ®nal group, i.e. single
chick (Fig. 3g and h and Fig. 4c and d) In treatment groups h1±r10±s5 and h2±r15±s7 the
®rst reduction of group size resulted in a signi®cant decrease in frequency of occurrence of
short peeps (P < 0:05, t-test, Fig. 3h and Fig. 4c).
The frequency of occurrence of warbler, signi®cant from zero, was always observed
in a group size of three or more animals (P < 0:05, t-test, Fig. 3j±l and Fig. 4e and f).
Three groups with two chicks displayed warbler calls as well. However, the warbler was
never observed in isolated animals. Reduction of group size from three to two or from
two to one decreased mean frequency of occurrence of the warbler signi®cantly (P < 0:05,
t-test, Fig. 3i±l and Fig. 4e and f) in all treatment groups except in the treatment group
h2±r15±s7.
Pleasure notes reached a mean frequency of occurrence greater than zero (P < 0:05, ttest, Fig. 3m±p and Fig. 4g and h) in ®ve instances. High frequency of occurrence of the
pleasure note occurred in the two animals of treatment group h1±r3±s2 and in the three
animals of treatment group h2±r7±s4 (Fig. 3m and o). In all other groups the means of
frequency of occurrence were lower (0±3.8 calls per animal/min).
Frequency of occurrence of other calls was low in most groups but in some cases means
were elevated signi®cantly above zero (P < 0:05, t-test, see Fig. 3q±t and Fig. 4i and j).
The changes in frequency of occurrence of other calls were not signi®cant.
Since frequencies of occurrence of various call types are mutually dependent, a
multivariate analysis of variance was used to test for differences in vocalisation between
group size. Signi®cant changes in vocalisation were established for stepwise reduction of
group size in ®ve treatment groups (see Table 3). Only treatment group h1±r5±s3 with an
initial group size of three animals did not yield signi®cant differences (P ˆ 0:1946, F-test).
Table 3
Calculated F-value of Wilks'l for the results of multivariate comparison of frequency of occurrence of special
calls during the SIT in each treatment group
Treatment group
F-value (d.f.)a
h1±r3±s2
h1±r5±s3
h1±r10±s5
h2±r7±s4
h2±r10±s5
h2±r15±s7
38.73 (5;
1.75 (10;
6.54 (20;
4.54 (15;
2.68 (20;
6.19 (30;
a
Degrees of freedom (denominator; numerator) for F-test.
P < 0:05.
**
P < 0:01.
***
P < 0:001.
*
2)*
10)
37)***
22)***
37)**
70)***
G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
71
Table 4
F-value and significance table for changes in the frequency of occurrence of specific calls tested by multivariate
contrasts (Wilks'l) between the group size before and after each step of reduction for the treatment groups
Treatment group
6
h1±r3±s2
h1±r5±s3
h1±r10±s5
h2±r7±s4
h2±r10±s5
h2±r15±s7
d.f.a
Group size after reduction
3.39*
5
0.75 (NS)
4
5.01*
2.57#
0.29 (NS)
3
6.71**
1.21 (NS)
0.37 (NS)
0.56 (NS)
2
1
1.52 (NS)
5.04*
3.01#
0.1 (NS)
4.15*
38.74*
1.26 (NS)
12.01***
21.07***
6.03**
45.16***
5;
5;
5;
5;
5;
5;
2
5
8
11
11
17
a
Degrees of freedom (denominator; numerator) for F-test.
P < 0:05.
**
P < 0:01.
***
P < 0:001.
#
P < 0:1.
*
The changes in vocalisation triggered by each reduction of group size were calculated by
contrasts between each pair of consecutive groups. In all groups with signi®cant changes in
vocalisation reduction from two to a single chick alters distribution of call types (see
Table 4). The remaining pattern of response was less consistent: after the ®rst reduction of
group size, treatment groups h1±r10±s5, h2±r7±s4 and h2±r15±s7 showed signi®cant
changes in vocalisation. Changes of vocalisation in treatment group h2±r10±s5 did not
reach a level of signi®cance.
The comparison of treatment group h1±r10±s5 and h2±r10±r5 (two-way multivariate
analysis of variance) shows the differences of vocalisation between the hatches
(P < 0:001, F-test) and a signi®cant interaction between hatch and reduction of group
size (P < 0:05, F-test). Signi®cant differences in Wilks'l of multivariate contrasts were
observed between the two hatches in the groups with ®ve (F5;26 ˆ 5:30; P < 0:01), two
(F5;26 ˆ 4:76; P < 0:01) and one animal (F5;26 ˆ 4:94; P < 0:01).
4. Discussion
Based on the observation that the numerical distribution of the chicks pattern of
vocalisation changes under successive increase in social isolation, it is concluded, that
recording and analysis of the vocal pattern may be a new and reliable source of information
to detect acute stressful situations aversive to the chick.
While the pattern of vocalisation in the chick undergoes considerable changes as
stepwise isolation progresses, the most consistent changes occur when the chick is isolated.
Even in the presence of one pen mate distress calls increase dramatically over all other call
rates, while all other call types diminish or disappear. This provides strong evidence, that
social isolation is vocally expressed in the young chick by distress calls. The distress call is
not only dominant in situations of social isolation (Montevecchi et al., 1973; Jones and
Merry, 1988; Vallortigara and Zanforlin, 1988), it is also a common pattern in the regular
vocalisation of chicks (Schmidt and Marx, 1998). The decline in call rates of the remaining
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G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
call types had not yet been described. Jones and Harvey (1988) simply counted distress
calls during behavioural observation of the remaining chicks. The present study bases its
evidence on computer aided visual inspection of energy distribution over frequency and
time. The absence of information on non-distress calls during social isolation may be due to
the lower level of loudness of these calls and to dif®culties by the human ear to perceive call
differences. Decline in activity of non-distress calls is not a sudden event occurring when a
chick is left alone; it rather develops progressively as groups are reduced in size.
The other three call types are less consistent in their response when stepwise reduction
occurred in groups larger than three animals, but were more consistent when smaller
groups containing three or two individuals were reduced. Short peeps are frequent in chicks
of all groups with the exception of the single chick group. Observation of single animals
showed that short peeps always occurred at the begin of vocalisation or after a longer break
(>3 s) in vocalisation which had also been noticed in the ®rst description of short peeps by
Andrew (1964). Since the warbler is characteristic for groups of three or more chicks it may
serve as a tool for group communication. Warbler-like modulated vocalisation in chicks
were also described when the chicks are crouching under the hen (Baeumer, 1962).
Pleasure notes occurred rather infrequent relative to other call types but were more frequent
in groups of three and more chicks when compared to smaller group size. This supports the
general observation that the chick needs a larger group size for adequate social behaviour.
Based on our own results and those of others (Guyomarc'h, 1962; Andrew, 1964), there
is no doubt, that not only a discrete vocal pattern exists in the new-born chick but also, that
expression is related to social environment. It can be assumed that social isolation is
stressful to the chick. The increase of the so called distress calls with a concomitant
decrease in short peeps and so called pleasure notes underlines the assumption. On the
other hand distress calls and short peeps are increased while pleasure notes are decreased in
the ®rst step of SIT having four and more chicks in the starter group. This may result from
changes of environment at the start of SIT and will need further attention.
Comparison of the two hatches at identical group size displayed differences between
hatches in vocalisation upon successive reduction of group size. Both groups are different
in body weight. It cannot be excluded that post natal development differences expresses by
body size may modify vocal response to social isolation.
Even though only 8.8% of all vocalisations were not a attributable to one of the four call
types, they may represent call types in transition as described previously (Andrew, 1964)
and are probably of lesser importance in the context of present study. For classi®cation of
call types rather obvious criteria were drawn upon such increasing, decreasing and bow
type pitch frequencies. Overlapping in time could occur but could clearly be recognised so
that unambiguous classi®cation was assured. However, it cannot be excluded that ``other
calls'' contained elements of heretofore not described rare call types.
5. Conclusion
Stepwise social isolation is followed by a gradient in vocalisation indicative of a stressful
situation. Furthermore, our data show, that analysis of vocalisation in the chick may be
useful to assess the chicks well being.
G. Marx et al. / Applied Animal Behaviour Science 75 (2001) 61±74
73
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