Propagation of bird acoustic signals: comparative study of starling and blackbird distress calls

0 Acad6mie Neurosciences

des sciences / Elsevier. / Neurosciences

Paris

Propagation of bird acoustic signals: comparat.ive study of starling and blackbird distress calls Propagation des signaux acoustiques d’oiseaux : 6tude comparative des cris de detresse d’etourneau ~IICOLAS

MATHEVON”

’ 1.‘~ 1491 du CNRS,

2* , THIERRY

ALam, lnboratoire

des

‘Lnboratoire de biologic animale et appliq&,

AUBIN’,

JEAN-CLAUDE

et de met-/e

BI&MOND’

&canismes de communication, universitP Paris-XI, 91400 Orsay; fact&

des xczencef et techniques

universit~jean-IMonnet,

42100

Shnt-i%ienxe.

Franc,?

De precedents travaux ont montrt que l’information contenue dans un cri de detresse d’oiseau est codte a la fois par la repartition d’energie entre les harmoniques et I’evolution temporelle de la modulation de frequence. Dans la prtsente etude, en considerant ces paramttres, no ~1s avons compare la transmission de l’information a longue distance des cris de detresse d’etourneau Sturnus vulg~is et de merle Turdus merula B travers un environnement a vegetation dense. II apparait que I’atttnuation excessive des frequences elevCes (superieures B 4 kHz) apres une propagation a longue distance est responsable de modifications spectrales. L’energie des signaux propages tend a etre concentree dans une bande passante de 1,5-4 kHz quel que soit le spectre initial. Presentant un spectre large (0,8-T kHz), le cri de detresse de I’ttourneau est grandement mod&e. En revanche, ayant un spectre plus Ptroit (2-5,5 kHz). I e cri de detresse du merle n’est que peu rnodifte. Le cri de detresse du merle apparait comme bien adapt6 a une transmission h longue distance dans un environnement a forte vegetation. k l’oppose, le cri de derresse de I’ttourneau est beaucoup plus facilement degrade lors dune propagation. Neanmoins, la modulation de ftequence du cri de i’etourneau est conservte et ie decodage de i’information reste possibir ‘men clue sa fiabilite puisse diminuer. Ce resultat est examine dans une perspective ethoecologique, en prenant en consideration I’habitat et la structure sociale de ces deus oiseaux. Mats

cMs

: communication

acousfique,

propagaffon,

cris de dkfresse,

oisecrux

ABSTRACT I’revious works have demonstrated that the information rupport#Pd by a bird distress call is encoded both L’y the energy d’istribution among harmonics-and the temporal involution of thej&quency modzklntiox In the present study, using these parameters, we compared long-range information transfer in a dense tiegetatian environment between the starling Sturnus vulgaris and the blackbird Turdus merula ,distress calls. It appears that excessattenuation of hi
Note pr&snt& UOR remix

par Pierre Buser le 7 juillet 1997. accepr~e

‘Correspondence

ap+s revision Ic 1” ocrnhre 1937

and reprints

C. R. Acad. Sci. Paris. Sciences 1997.320.869-876

de la vie / Life Sciences

869

N. Mathevon

et al.

frequency mo d u I a t’zon of t h e starling call is preserved and message decoding remains message reliability may diminish. This result is examined5 om un et~o-ecologi,-i~lyuint into account both habitat and socialstructure of both birds. Key words:

acoustic

VERSION

communication,

propagation,

distres.3

calls,

ABREGEE

Les cris de detresse sont des signaux emis par de tres nombreuses especes d’oiseaux. 11s permettent d’avertir les congCn2res d’un danger. Quelle que soit l’espece d’oiseau consid&&e, les cris de detresse tendent vers la m&e structure de base (une frequence porteuse et ses harmoniques, le tout lentement modulte en frequence) et les lois de decodage semblent &tre intersp&ifiques. Vu leur structure complexe, les cris de dCtresse risquent d’Ctre fortement modi& aprks une propagation sur de longues distances en milieu B v@ation deme. Pourtant, pour assurer leur fonction biologique, de tels cl-is doivent pouvoir Ctre entendus et leur information decodte m6me g de longues distances de I’Pmetteur. On peut supposer que les oiseaux vivant dans des milieux B vegttation dense emettent des cris de dttresse plus adapt& aux propriMs acoustiques de cet habitat que ceux vivant en milieu ouvert. Nos recherches ont pour objet d’eprouver cette hypothtse en comparant deux espkces d’oiseaux : une caractkristique de milieu forestier, le merle Turdus merula, et une vivant le plus souvent en milieu ouvert, I’Ctourneau Sturnus vulgaris. Les cris de detresse de ces deux especes presentent la structure caract& ristique d&rite plus haut. 11sdiffirent du fait que le spectre du cri de I’Ctourneau est beaucoup plus large que celui du cri du merle. ConsidCrant que le processus de codage-decodage des cris de ditresse repose B la fois sur la repartition de l’energie entre les harmoniques et I’Cvolution temporelle frequentielle, nous avons pris en compte ces deux parametres pour norre etude, B l’exclusion de tout parametre d’amplitude.

Introduction

warn other prey of danger, 31. They are well-known

by numerous species a predator. They serve

and to confuse predators to evoke an interspecific

of ‘to IIres-

ponse [4-71. This interspecificity results from the structural convergence between signals of different species. Distress calls tend toward the same basic form: a carrier frequency with numerous harmonics, slowly modulated in frequency [8]. The structure of distress calls differs among bird species with regards energy distribution between spectrum harmonics, values of fundamentals and number of harmonics. Different species of bird use the

same 1 -A

two simple

basic slope

laws

of decoding

(a slowly

for

increasing

call

recognition: or decreasing

frequency modulation) applied to a carrier frequency that follows the acoustic shape of a natural distress call is sufficient to confer a distress meaning to the signal [8]. The

870

of view,

even

if

taking

birds

11 en r&Ite que le spectre du cri de dPtresse de l’ttourneau est grandement mod&e apt& une propagation B longue distance en milieu g veg&ation dense. En revanche, le spe’ctre du cri du merle est bien conservP. Ces r&ultats, obtenus it partir de signaux naturels, sont en accords avec ceux obtenus pr&Pdemment avec des signaux acoustiques artiflciels. A l’emission, I‘Cnergie sonore du cri de l’etourneau est repartie sur un large spectre (X00-7 000 Hz). Les frtquences supirieures g 5,5 kHz ne resistent pas P la propagation et jont absentes du signal propage. En revanche, l’energie sonore du cri du merle est concentrCe sur les premiPres harmoniques. Cette repartition est consen+e aprks une propagation g longue distance. Du point de vue spectral, le cri du merle est done bien adapt6 B une propagation dans un milieu g vegetation dense tandis que celui de I’Ptourneau ne l’est pas. Cependsnt, une caract& ristique importante est prtservCe dans les deux cas: la modulation lente de frPquence. 11apparait done que l’information port&e par le cri de dktresse de l’ttourneau sera rPduite apres une transmission B iongue distance B travers une v6gCtation dense, au contraire de celle portee par le cri du merle. D’apri:s cette Ctude, il est possible de mettre en evidence des diffirences de structure acoustique entre de:; signaux provenant d’oiseaux vivant dans des biotopes diffiirents et de correler ces diffkrences aux contraintes acoustiques particul2res h ces biotopes.

frequency the distress

Distress calls are signals given bird when they are seized by

possible

modulation is therefore call for triggering the

an important feature responses of birds.

of

2 - Another feature, the harmonic structure, plays a role in the coding/decoding process of distress calls. An ethological message is encoded in the harmonic structure by the energy distribution between the spectrum harmonics 191. Spectra with a high intensity upper part produce stronger response than other types of spectra. Wide-band frequency spectra elicit a stronger response than narrowband

spectra

Birds ulation Distress

a

191.

do not use parameters for decoding [lo]. calls

are

uttered

relative

to amplitude

by numerous

species

modliving

in

different habitats [5]. Species living in an environment with dense vegetation must cope with important acoustic degradations [I l-l 31. Owing to their complex strl;cture, distress calls cannot be transmitted without being altered. As the message must be heard by the receiver.

serves to warn other individuals, at long range and trigger a quick We can suppose that birds living

C. R. Acad.

Sci. Paris, Sciences

the calls response in dense

de la Me / Life Sciences 1997. 320.669-876

Propagation

K.hz

starling

t

cf distress

blackbird 6 --------------------------------

calls

KtlZA

-^-----~.~

4 ..____ ---_

4

+

240 ms Figure h’o:e

1. Spectrograms the harmonic

vegetation properties

of distress structure,

calls.

widespread

over

a broad

use distress calls more adapted of this habitat than birds living

frequency

bar !d.

to the acoustic in open field.

The present study investigates this hypothesis paring two bird species: the blackbird Turdus the starling Sturnus vulgaris. We have chosen bird because, although this bird can be found

Playback

by commerula and the blackin various

an-:hropized environments, the original blackbird habitat is forest and dense bushes [I 41. Moreover, previous investigations have shown that the blackbird has behavioural adaptations to improve efficiency of song communication in dense vegetation [I 5, 161. In contrast, we have chosen the starling because this species spends most time foraging in open fields such as pastures

were days

!Distress

calls

of both

structure: modulated

of its activity or cultivated

the following broad, whereas

those

species

havethe

came

char-

a carrier frequency with harmonics, in frequency. However, they differ

aspects. The starling that of the blackbird

in

call spectrum is very is narrower (figure 1).

‘n the present study, we compare distress calls to transfer information environment with dense vegetation.

the ability at long Since

of these two range in an the coding/

decoding process of calls is based upon energy distributim between harmonics and temporal evolution of the frequency bandwidth, our work focuses on these two parameters. account.

Amplitude

modulation

was

not

taken

and methods

Playback

and

recording

P,+ciple

and

experimental

into

individual to another single representative blackbird.

conditions

C. R. Acad. Sci. Pas, l’G’97 320 I 869-876

were

is stereotyped stable from

and one

experiments, we chose a for the starling and the

Recording

February and microphone

March, saturation

calm and to

fluctuations. Both the loudwere at a distance of I.3 m ground effect [171. Playback (measured with a Bruel&Kjaer Bruel&Kjaer microphone type in front of the loudspeaker.

recorder at a tape speed of 19 cm/s, an Audax loudspeaker were used. the loudspeaker met tlie frequency signals.

A SeinheicJer M02 11 N omnidirectional connected to a NAGRA type IllB recorder 19 cm/s

on

material

was

microphone at a speed

of

used.

analysis

dktribution

between

transforms

(FFTs)

to focus on process and tem-

harmonics

Signals were examined in the frequency software developed in the laboratory distribution of energy accurately, we 256.point window size. The sampling Fast Fourier

Scief~,, -0~ de u !c vie / Life Sciences

performed

material

A U her 4000 report IC a IO-W
Energy

distress call are relatively

181. In our call each

calls

As mentioned in the Introduction, we chose the two parameters used in the coding/deco’ding of calls: energy distribution between harmonics poral evolution of frequency bandwidth.

procedure

Within a bird species, the its physical characteristics

of distress

performed between without wind to avoid

Playback

Sound

Materials

recording

limit random amplitude speaker and the microphone from the ground to limit intensity was 90 dB linear type 2235 connected to a 4176) at a distance of 1 m

lands. acteristic slowly

and

over long range (40 m) through dense bus11 (hawthorn Crataegus oxyacantha, Prunus spinosa, pri\/l-t Ligustrum vulgare). We then compared these propagated signals to undegraded control signals obtained after a 1 O-m propagation in an open field (under this condition, rangeand habitat-induced degradations are negligible). Experiments

were

domain using a [18]. To reveal the used a FFT with a rate was 16 kHz.

calculated

at the

tem-

$7 !-

N. Mathevon

et al.

poral middle of the signals. Ten propagated signals were used. Then, centages

we calculated per octave

control

signals

and

signal energy distribution using the Bessel-Perceval

ten

moves energy.

as perequation

than O-l

3/4 -kHz

ior the propagated and especially

signal 4-8-kHz

frequency ranges are attenuated strongly. ‘The latter represents l/3 of the control signal energy and less than 10% the propagated signal energy.

of

1191. Temporal

with

evolution

mdximum

of frequency

On the modified

bandwidths

amplitude

and 240

every ms).

20

For these dynamic commonly

ms

for

the

calculations, ranging used

blackbird we chose

from in bird

call

(call

to take

into

Statistical

control tion.

calculated only the

average frequencies

accouni.

a

(figure

and ten propagated ones were usecl Because we had rather small sample Mann-Whitney non-parametric test

between

harmonics

of data is modified

show that energy between control

Table

corresponding in comparison less than 2/3

I. Energy

distribution

frequency

range

to l-2and 2-4-kHz ranges with others. The octave2-4 oi the control signal energy,

among

octaves

24 4-8

32.1%

O-8

1 OOY” difference

from

control;

energy

distribu-

bandwidths

modifications in energy dornain may on the temporal evolution oifrequency. of signal temporal evolution between

the control frequency

significant

3.6 down

Discussion Signal Our and

are kliz but

have The the

signal is characterised modulation from the

by a slowly beginning to

to 2.9

kHz).

distress

call

and- conchs~on

degradation work shows the bllackbird distress

in a closed

blackbird

spectra

(comparison

undergone a long-range

between

control

and

Rlackbird

1 .O%

Control

O.cWo

signals).

NS

4.2 %, 73.0%

6.3% 79.0%

NS NS

22.8%

14.7%

*

1 00%

100%

propagated

Propagated

0.0%

NS

x 0.05,

by starling propagation

-__

~~

13.0% * 76.5:/;, 1 9.5’Y 0 IX

difference:

modifications calls after

environment.

Propagated

2.4% 7.5% 58.0%

no significant

and

Control

o-1 l-2

872

in starling

St3 rlfng

(kt-izj

regarding

distribuand pro-

pagated signals in both species calls it&/c!;!. however, these modifications do not affect signal structure in the same proportions. Indeed, as iar as the starling, call is concerned, the energy distribution is strongly modified: frequencies reiniorced represents

signals

afiects (I -2. and between

the end of the signal [from near 2 kHz up to more than 3 kHzi; this is also found in the propagated signal. Regarding the blackbird distress call, the temporal structure made by a first slowly increasing FM (,from 3.2 up to 3.6 kHz) and a second slowly decreasing FM is also preserved (from

Statistical comparisons tion among octaves

propagated

strongly blackbird

2).

tress call, increasing

Results distribution

not

control and the propagated conditions shows that one oi the acoustic parameters is preserved in both calls: the shape of slow frequency modulation. This phenomenon is obvious considering the mean of ireqliency values of maxirnum amplitude (figure 3). Regarding the starling dis-

Pll.

Energy

and

The above repercussions comparison

by men-

analysis

Ten control signals for each comparison. sizes, we used the

distribution is and propagated

Temporal evolution of frequency with maximum amplitude

results from is very smJll. becomes

frequencies within the

energy control

Therefore, after a 40-m propagation in a closed environment: the starling call bandwidth is very reduced, as a result of the attenuation of high frequencies. On the contrary, the blackbird call bandwidth remains the same

=

0 to -30 dB. The -30.dB limit is studies j201. The choice of-30 dB,

as a limit for bandwidth boundaries measures the fact that at -30 dB the signal/noise ratio Below this limit, transmission of information problematical. We taking into account tioned bandwidth.

duration

contrary, between

signals. Indeed, the only statistical difference the 4--8-kHz octave. The main part of the asignal 2-4-ktiz octaves) is not statistically different

Ten control signals and ten propagated signals were conpared. In order to follow the temporal evolufion of the frequency, the average frequency was calculated, for each experimental signal, at ten equally spaced times: every 50 ms for the starling call (call duration =: 500 ms),

NS:

up to more Accordingly,

100%

** 0.02.

C. R. Acad.

Sci. Paris,

Sciences

de

la vie / Life Sciences 1997. 320. 869-876

Propagation

Starling

dB 0

1

2

of distress

calls

Blackbird

3

4

5

6

kHz

7

12

3

4

I

I

5

kHz

61

-30

I 1

Figure &)th

2. Control are

cd/h

I

I

2

3

andpropagated affected

I

distress

by a propagation

but

4

5

call

spectra these

-

-

I

I 6

k.Hz

7

of both

I

studied do not

modihcat~on.~

1

2

affec-t

spectra

composition

to be concentrated initial spectrum

strongly attenuated. relatively preserved itable I and figure

The two proportion

previous uation

the blackbird spectrum is the considered frequencies results are consistent with

works dealing with frequency-dependent [I l-l 31. We show that frequencies

under

atten4 kHz

4

I

5

I

6

kHz

7

species.

The starling spectrum is greatly modified after propagation; in particular, the frequencies above 4 kHz are In contrast, whatever 2). These

3

I

in the same

proportion

in a I-4-kHz energy distribution

calls studied regarding

here their

bandwidth (table

whatever

the

/).

are not modified spectrum after

in the same long-range

propagation. This is obvious if distress call spectra are compared (figure 21: as far as the blackbird is concerned, control and propagated spectra are quite similar; on the

propagate with less degradation than others. High frequency attenuation results from sound scattering and absorption by vegetation 112, 131. Studies with artificial signals have shown that the lower the frequency is the better the sound is carried, particularly in closed environments (bushes, dense forests). However, close to the

contrary, the starling ones are very different. As a consequence of these modifications in the frequency domain, the starling call bandwidth becomes narrower after longrange propagation in dense vegetation. On the contrary, in these propagation conditions, the blackbird call bandwidth remains the same.

ground, sounds below 2 kHz are excessively attenuated [22l. We cannot, however, provide evidence of this last phenomenon because the distress calls studied contain

We notice that the spectrum of the control blackbird distress (call is very similar to the spectrum of the propagated starling distress call. Thus, it appears that the spectrum structure of the blackbird distress call is well-

no or few frequencies cies below 500 Hz susceptible has been

below 1 kHz. and above 3-4

to degradation. called ‘sound

The window’

In any case, frequenkHz are particularly

interval 500 by Morton

At the emission, starling call energy a broad spectrum (800-7000 Hz). 4 kHz are not resistant to long-range

Hz to 4 kHz 1231.

is widespread Frequencies propagation

over above and are

adapted dispatched long-range

to long-range propagation since signal over frequencies relatively preserved propagation in a dense environment.

trast, the spectrum appears to be badly conditions.

structure adapted

of the starling to propagation

lacking in propagated signals. On the contrary, blackbird call energy is concentrated on the first harmonics (frecluencies lower than 4 kHz, belonging to Morton’s window); therefore this distribution is relatively preserved

However, an important feature of evolution is preserved in both calls: frequency modulation. Regarding slowly increasing FM is preserved.

cluring ment.

bird call, (figure 31.

long-range propagation, even After a long-range propagation,

C. R. Acad.

SCI. PCm,

1097. 320. 869-876

Sciences

de

in a closed environsignal energy tends

ia vie i iik

S&XeS

the

temporal

structure

energy is during In con-

distress under

call these

distress call temporal the shape of the slow the starling call, the Observing the blackis

also

preserved

873

N. Mathevon

et al.

Starling

Blackbird

Control

Control 5000

01

I 50

150

250 Tune

350

40

450

60

120 Time

(ms)

Propagated

160

200

(ms)

Propagated 5000,

‘““:1----.A 150

50

Figure In each

3. Temporal case,

evolution

the shape

250 Tl,“C

of average

of the frequency

350

450

(“IS)

frequency. modulation

i> preserved.

As a consequence of these results, we would say that the blackbird distress call structure is more preserved than the starling one after long-range propagation in dense vegetation. Both the blackbird and the temporal evolution preserved. On the contrary, tains its temporal evolution tion; its spectrum composition Message Among always

call spectrum composition of frequency bandwidth are the starling call only mainregarding frequency modulais harshly modified.

transmission birds, the basic acoustic shape of distress the same: a complex carrier frequency with

calls is nume-

rous harmonics and a slow frequency modulation applied to the carrier from the beginning to the end of the signal 181. Studies on starling have shown that a synthesised distress call with a wide-band frequency spectrum elicits a stronger response from birds than one with a narrow-band spectrum. Moreover, spectra with a high intensity upper part (ie, high energy in high frequency harmonics) produce a stronger response than other types of spectra [9]. Therefore, it appears that information supported by the starling distress call will long-range propagation energy

874

distribution

between

be reduced and modified in dense vegetatiorr since harmonics

is harshly

after the

altered.

These

modifications

message Regarding between

are

so important

that

the ethological

tie,

presence of danger) could be deteriorated. the blackbird call, the energy distribution harmonics is different from that of starling. The

call sipectrum has a low intensity upper part (ie, low energy in high frequency harmonics). The energy distribution between harmonics is then preserved after longrange propagation in dense vegetation. The physical characteristics of the call are little modified by propagation. The ethological racteristics can cant cleterioration. All birds

message supported by these phvsical chabe conveyed at long range without signifi-

use the slow

frequency

modulation

as a decod-

ing parameter. If this FM is lacking, the call loses its distress meaning 124, 251. The shape of thus slow FV can vary among bird species. For instance, the black-headed gull L8arus ridibundus recognises a distress message from an increasing or a decreasing FM. The black-headed gull distress call begins with a slowly increasing part and then continues with a slowly decreasing part urltil the end. The starling identifies the distress message c)nly if the FM is slowly increasing. No experiment has been carried out concerning blackbird call decoding; the call temporal structure

is however C. R. Acad.

based Sci. Paris,

upon Sciences

the same de

shape

la vie

1997

as that

/ Life Sciences

320.869-876

of

Propagation

the black-headed decreasing FM that the blackbird the distress call

gull: a slowly increasing followed by (figures 1 and 3). We can then suppose uses this shape of slow FM to recognise meaning [8]. Regarding our experimental

results, the temporal shape of FM is preserved for the two studied calls. Therefore, this important decoding parameter can be used by both birds, even after a 40-m propagation in dense vegetation. Neither the blackbird nor the starling parameter,

are

penalised since there

An etho-ecological

by an eventual is no alteration.

point

alteration

of this

of view

size, and vocalisation depend mostly on size

possibilities [271.

Ac:knowledgements:

thank

among

M. Rehai’lia

improvement

SD.. Tuckfield In birds. Am.

studies on vulgaris).

the use Science

of a 119,

R.C. 1976. Distress Nat. 96,418-430

screams

as

3 Hill G.E. 1986. The function of distress m ce (Parus b~color): an experimental 34, 590-598

calls given approach.

by tufted titAnim Behav.

4 Brbmond J.C.. Gramet P.. Brough T., Wright E.N. 1968. ri!;on of some broadcasting equrpments and recorded calls for scaring birds. J. Appl. Ecol. 5,521.529 5. Busnel acousfique c’rseaux. 6 Morgan jackdaws 48 l-49 1

R.G Giban J. (eds). des cultures ef a&es lnra Presse, Paris P.O., (Corvus

7. Morgan P.O.. vus munebula) Pehav. 22,688.694

Howse P.E. monedula)

.4 compadistress

1960. Coiloque suf /a protection moyens d’effarouchemenf 1973. Avoidance to distress calls.

Howse P. E. 1974. to normat and

Conditioning modified

@. Aubin T. 1991, Why do distress calls ses? An experimental study applied Elehav. Process. 23, 103-l 11

conditioning An/m. Rehav. ofjackdaws distress calis.

evoke interspecific to some species

111. Brbmond J.C., Aubin T. 1992. The role of amplitude il-1 distress-call recognition by the black-headed Lundus). Ethoi. Ecol. Evol. 4. 187-191 Paris,

Sciences

de

la vie

/ Life

gull

Sciences

des of 2 1,

(CarAnim. responof birds.

9 Aubin T., Brbmond J.C. 1992. Perception of distress call monic structure by the starling (Sfurnus vuigaris). Behaviouf 15-163

(1:. R. Acad. Sci. 1997. 320,869.876

birds that call uttered

modulatiorl (Laws

clense flocks. to relay disrange. There-

have heard and by one of them,

The density

blackbird social of individuals

factors may be invoived to expiain between the starling and blackbird to know which of them is preponderant

English.

11. Cosens tion and habitats.

H , Jumber J. 1954. Preliminary sound to repeal starlings (Sturnus S.. Fretwell of kinship

take offs of some decoded a distress

may be different

could be a releasing stimulus for the other flock members [visual signal, may be acoustical - wing fluttering). The selective pressure on the starling call structure is probably

of the

REFERENCES

2. Rohwer a neasure

Starlings live in quite signals are susceptible information at long

calls

strucbeing

acoustics/ calls. It is but all perrnadistress for the

blackbird than for the starling: birds cannot see each other, only acoustical communication is possible but is seriously under pressure by a scattering environment. In response to this strong seiectltie pressure, the biZickbird.cailspect.rum is narrower than that of the starling. Energy is concentrated on few harmonics and the signal is very resistant over long-ran;ge propagation in a closed environment.

passerines

for the

elements is very

operate in the same way: individual distance and nent unfavourable acoustical environment make call information conveyance much more difficult

However, this is not sufficient to explain the inadequacy of the starling distress call. Moreover, we can presume that the blackbird and starling have comparable vocalisation capacities. Indeed, both species are of comparable

1. Frings specific 318-319

fore, the correctly

Severai differences impossible

diffeforest open

environment. As a consequence, the blackbird distress call is wellin a very sound scattering habitat.

We

among these species. In such a group, other tress calls and convey

some other social structure

weaker than for the blackbird. ture is very different, the much slighter.

First, the habitats of the two birds studied are qurte rent. The original habitat of the blackbird is dense [I41 while the starling lives in various habitats from field to a very closed one can expect that adapted to propagation

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har120.

ridi-

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