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Frequency analysis of crane calls and their use for individual recognition

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Abstract and Figures

A sonographical analysis method is presented which was used and can be used for individual recognition of crane pairs and individuals
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3.2 Frequenzanalyse (Englisch)
Die Welt, w ie K ra n ich e s ie se h e n.
Th e w or ld , as seen by cr anes .
1. Kraniche in Bild und Ton
2. Kranicha rten
3. Individuelles Kenne nlernen
3.1 Einleitung
3.2 Frequenzanalyse (Englisch)
The method
Results (1): identification of
crane individuals / pairs
Results(2): life history facts, (a)
Results(2): life history facts, (b)
International projects
3.3 Freilandbeobachtungen
3.4 Links: „Intelligenz bei Tieren“
4. Graue Krani che
5. Mandschurenkraniche
6. Schreikraniche
7. Resumés
8. Anhang
Dr. Henne, Lei ter des Biosph ärenre servates
Schorfheid e-Cho rin, b ei der Beringung
eines Kran ichs
Dr. Bernhard Wessli ng m it
Sie sind hier:
3. Individuelles Kennenlernen3.2 Frequenzanalyse (Englis ch)
3.2 Frequenzanalyse von Kranichstimmen (in Englisch)
Individual recognition of cranes, monitoring and vocal communication analysis by s onography
A broad application of modern bioacoustic techniques
(Presentation at the IV. European Crane Workshop in Verdun, 11. - 14. Nov. 2000)
he purpose of this work is to provide a method for individual
recognition of crane pairs and crane individuals for long-term
monitoring and gathering of individual life history facts without t he need to
capture and band them. Results in various regions and with various crane
species will be described. In addition to the monitoring capability the
method opened the possibility of analysis cranes' vocal communication
and regionally s pecific "language" structures. The method is practically
applied also in projects for the protection of endangered crane species,
predominantly the Whooping crane, and here also in the raising and
training of captive reared cranes for release into the wild and ultraleight
airplane-led migration.
ystematical sonographical studies on c ranes had rarely been
conducted in previous years, first by Archibald [1]. His goal and
achievement was a taxonomical assess ment by similarities and
dissimilarities of t he unison call as displayed by the various species of
the cranes all over the world. No hints were given if any individual
characteristics were detectable.
A s econd study [2] on the sandhill c rane (grus canadiensis) was
performed in order to evaluate individual and regional similarities and
differences of their voice. Due to the use of the " guard calls" [3] and
rather insensitive sonographical techniques (electromechanical, as available at that time), not only the recording
method (a s ignificant disturbance) was inappropriate for our goals, also the manual evaluation technique of the
analogous sonograms is not effective. Moreover, the guard call was mostly expressed by the single crane in
absence of its mate, so that pairs can not be identified, and hence the results of the study questionable. The unison
call was not analysed by W eekly due to its complexity not manageable by this sonographical technique.
A third study [4] was devoted to sex determination of the Whooping Crane (grus americana) again by using the
guard c all. No hints were given concerning individual recognition possibilities, neither were the studies made on
pairs, nor over many years.
Generally, earlier sonographical st udies on unison c alls of c ranes have been sporadic and with low resolution, at
most focused on showing the qualitative frequency picture of the calls.
E. Henne in Germany, Brandenburg, had attempted in c reating a
collection of recordings on magnetic tapes for sonographical identification
of cranes and t heir life history, and has started to record unison calls in
certain c rane territories in 1988. However he did not s ucceed in
developing an analytic al method for analysing his unique records [5].
One single older study was found where sonography was used to
identify birds individually by acoustical methods: the Woodcock [6].
However in this case, it is the form of the song, not a characteristic
frequency, which distinguishes t he individuals.
The authors were recording birds whose territories were certainly too
distant from each other to have birds c hanging locations. They showed,
that an individual identification should in principle be possible, but did not
perform it in a given region with several neighbouring territories, nor over
several years.
In a more recent work (published after the development of the
technique described here), C. Walcott [7] et al. used a commercially
available program for analysing bird calls based on " Canary 1.2" and a
multivariate factor analysis running on Mac intosh and applied it on
banded common loons. They were mainly able t o recognise the loons
and to follow them when they changed their territory, however, the program also produced some uncertainties [8] due
to its concept (it automatically looks for empirically determined characteristic frequencies of the introduction, of
lengths of tacts , etc ., but would not be able to analyse unison calls where two birds are calling simultaneously).
The m ethod
developed a lightweight sys tem consisting of a transportable minidisk
3.2 Frequenzanalyse (Englisch)
Ein Teil des no twendigen Equipm ents
Figure 1
Figure 2
Figure 3
recorder, fed by a long range microphone (Sennheiser 486) with highest
sensitivity and resolution plus an intermediate amplifier ("booster"),
digital filing on minidis k and analysis by two different computer based Fast
Fourier Transformation analyses.
In the area where I developed the method (Hamburg, nature preserve
"Duvenstedter Brook"), it is preferred to wait for the cranes to display their
early morning call (up to 1 œ hours before sunrise), but also calls
expressed in certain s ituation like territorial fights, copulation and others.
The hardware is completed by a megaphone (linked to a CD player) for
recording work outside of t his area when is limited time for recording. An
appropriate unison call played over t he megaphone, provokes the territorial
pair (and s ometimes also others) t o answer. The method allows to record
many calls at various places, from distance, without the need of
penetrating the territory or come even close t o the nesting area. Usual and practical dist ances are between 250 and
500 m, but recordings from a distance of 1 km or more have also been successfully made and analysed.
The recorded files are transferred into a PC after the field work. The files are recorded in Mono, and are transferred
into the computer with 20 k Hz sample rate.
The analysis is performed in two steps:
1. using the program "SoundForge 4.0", which generates a
qualitative sonogram (Figure 1) between 0 and 2000 Hz - in
contrast to earlier sonogram practic e to s how the full bands of
overtones up to 3 or 6 k Hz; the program is not made for voice or
speech analysis; therefore, the resulting sonogram is not a data
file, but only a s creen picture which can be printed, but not be
used for pattern analys is. It shows a qualitative pic ture of the
call, it helps to identify noise, other dist urbing bird calls (like
geese, ducks, cuckoo) and to cut the files into useful pieces
free from such complications.
2. based on "mathematica 4.0" (Wolfram Research), with a specially developed sc ript [10]
(protocol), a so-called "powerspectrum" is calculated (which is the plot of t he intensities at
certain frequencies, derived from the addition of all intensities at these frequencies over the
time of the recorded call); these results are data, or better: lists of data, which can be
analysed and compared. This analysis is preferably made between 600 and 1200 Hz at a
very high resolution, and c an be changed according t o needs (e.g., there are cranes which
call below 600 or above 1200 Hz). (Wit h this program also sonograms are created which are
now available as 3-dimensional lists of data - frequency, time, intensity - for further analysis
in the powerspectrum, but not y et in the sense of a pattern or image analysis.)
The "powerspectrum" was c onsidered by me to be a "vocal fingerprint" based on the fact, that the properties of the
sound generating organ (the trachea) will determine the frequency spectrum of the generated sounds of an adult
crane (after complete development of the voice).
Results (1): identification of crane indi vidual s / pairs
n identification of cranes and crane pairs would
be feasible, if (a) call characteristics are stable
enough within one season and from year to year (b)
the are different enough to differentiate them from
calls expressed by other cranes or crane pairs. In
1998, about 200 files from 7 pairs in their territories
and outside of it, one adult unpaired male, and a few
calls of unpaired youngsters had been recorded from
late March to May.
The main result is that the s onogram and the
"acoustic fingerprint" for a given pair are very stable
and can be safely recognised (Figure 2, Figure 3,
Figure 4).
Calls from the same pair only differ by less than 5% (as calc ulated by spectrum comparison) during a season or
from year to year.
3.2 Frequenzanalyse (Englisch)
Figure 4
Figure 5
Figure 6
All calls, all the fingerprint spect ra show enough different features from pair to pair so that they can be
differentiated, cf. Figure 5 and Figure 6 with 2 - 4. These differences are much greater than 5%.
As (especially in the beginning) no ring banded cranes were available for making sure that certain recordings are
definitely originating from a k nown pair, I t ested the method by analysing the loon yodels recorded by C. Walcott et
al [ 7]. The calls were safely identified, and even the confusion raised by changing the yodel c haracter after t erritory
changes could be resolved [11].
In the meantime, unison calls and other calls have been recorded and analysed from 7 crane species:
Common Crane in Hamburg, Brandenburg, and in a bigger study area in Mecklenburg
(comprising around 120 breeding sites in 635 km² with 92 nesting pairs (in 2000), from which
in a first attempt 28 pairs have been characterised), altogether more than 50 different pairs
Red crowned Cranes (in captivity: Baraboo, ICF; Vogelpark Walsrode, Germany; wild on
Hokkaido, Japan, and in the Demilitarised Zone in Korea), in total over 60 different wild pairs,
5 of which are banded (recordings from banded cranes in winter 99/00, to be compared with
later recordings)
Whooper cranes in captivity (Baraboo, ICF, and Patuxent, Washington - almost 20 pairs), in
the wild in Aransas National Wildlife Refuge (USA) 27 pairs, and in the breeding area in
Wood Buffalo National Park (Canada), 10 pairs
sandhill cranes (captive ones in Baraboo, ICF, and some wild ones in ANWR)
White-naped Cranes (wild, DMZ in Korea), more than 30 pairs
Siberian Crane (in captivity: ICF, Baraboo, and Vogelpark Walsrode, 8 pairs)
Black-necked Cranes (captive in Hokkaido and Vogelpark Walsrode, 2 pairs)
Results(2): life history facts, (a) Ha mburg
ith these results, it was possible to c onclude (in difficult cases, the sonogram should always be consulted,
too), that in t he nature protection area in Hamburg / Stormarn in the season 1998.
7 pairs (including a new one) were present, 6 of them territorial (and breeding: M1F1, t hrough
M6F6), 1 non-territorial: M7F7
without any doubt, 3 pairs were t erritorial in an inacc essible hardly observable wetland and
breeding in clos e vicinity at corners of a triangle with sides 75, 150 and 200 m long (however,
this year with no suc cess), where doubts had been expressed the year before as visual
observations had indicated a third pair, which was not accepted as fact by many other
observers (M1F1, M4F4 and the third pair M6F6)
also, where doubts had been expressed the year before in an even less observable area, as
3.2 Frequenzanalyse (Englisch)
to whether the observation of a new pair in close vicinity of another older one was correct, the
presence of 2 distinct pairs could be confirmed, and they were breeding in close vicinity (less
than 100 m apart) (pair 3 and 5)
it was poss ible to prove that pair 5 was looking for a territory in the vicinity of its 1998 territory
by analysis of a video tape taken by another observer (where he was lucky to get a unison
call) in 1996
an unpaired (adult male) crane was trying to mate the female of the not territorial pair (pair 7)
and to separate this pair (however with no success until t he end of the breeding season).
out of a group comprising 4 young cranes, I was lucky to record a single unison call
expressed by 2 of these 4 young cranes (later pair 10).
In the next 2 years, several unique conclusions could be drawn, where visual observation failed; in 2000, nine pairs
were present, where visual observation only concluded 4 or 5:
pair 1
vocally known sinc e 1996 (identity proven for 96 also - video analysis - probably the oldest
pair, territory occupied since 81); no juv from 1996 - 1999; 2 juv in 2000 after selection of new
nesting site for 2nd breed (visually, a new pair would have be assumed);
pair 2
most suc cessful in last years; raised only in 2000 only 1 chick ; probably 2 chick each year
from 96 - 99 (assuming identity for 96/97), also here, this pair changed the territory;
pair 4
took a breeding pause in 2000: arrived late March, no copulation observed, it was very
inactive and seldom in the territory;
pair 6
first present in 1997; first breeding attempt in 98 (failed), 2 attempts in 99, one in a new
territory (both failed); in 2000, M6 arrives with F11, a new female; their breeding was
succes sful;
pair 5
took its territory first t ime in 1998 (1997 unoccupied, previously owned by pair 3, which took
neighbour t erritory after F of that territorial pair was killed in 95); in 99, not present - territory
taken by pa ir 7 (new in 98, no territory); back in 2000 (but now, pair 7 was missing)
However, it is proven from video analysis, that pair 5 was searching for territories in the
preserve in 96 (and observed in 97)
In 2000, 3 new pairs were looking for territory and probably established (each one undertaking no breeding
pair 8
first identified in a territorial fight with pair 2 in their second nesting place in 1999;
pair 9
also identified in 1999 calling from the area, where is now their territory;
pair 10
first identified and present in 1998 (!), when it appeared in a group of 4 y oungsters, landed in
a pond and unison called!
Figure 7, Figure 8 and Figure 9 (PDF) display a qualitative overview over the distribution of the crane territories and
the changes observed by sonography.
Results(2): life history facts, (b) Brande nburg
ith these tools and experiences in hand we replayed and digitalized analogous recordings on magnetic tape,
taken by E. Henne over many years in Brandenburg, in an area of comparable s ize lik e the one in Hamburg
with 7 - 9 territories, several of which had been very close together, and analysed them with the same method.
Including 1998, the files are comprising 37 "crane-pair years".
The analysis (summarised in table 1) showed a remarkable st ability of the crane pairs in their territories over
years, although the places were rather c lose to each other. In all 6 territories, for 2 and more (up t o 6) subsequent
years it could be documented that the same pairs were present: 34 t imes, territories were occ upied the next year,
too, 29 times by t he previous pair.
We found that in the nesting place Bb not only in 1997, as was observed by Henne, 2 pairs were breeding, but
also - visually undetected - in 1994 and 1995, and these were the same ones as in the later y ears. The sonography
had detected t he presence of the second pair where t he visual observation had failed for 2 years. Whereas one pair
(No. 7) was mis sing there in 1997, it appeared again in this joint territory in 1998. Also it was shown, that - as in Ss
and Fb - the apparently continuous use of a territory / nesting site may indeed be a sudden replacement of one pair
by a new one, or a stepwise replacement with a phase of one to several years of tolerance at a clos e distance.
Pair No."3" bred in territory Zb from 1993 to 1996, it was joined in this territory by pair "4" for one year (1994).
In one territory (Ss) however, in contrast to the others, 3 different pairs of males and females were breeding
between 94 and 98. Pair 9DB (male D and female B) bred in 1994, pair 8AC in 1995, pair 8AB (i.e. with the same
female as in 1994 in DB!) in 1996 and 1997.
We conclude, that pair bonding and partner loyalty is quite strong in the Eurasian Crane, as is their loyalty with
their territory. It seems to be more common than concluded from visual observation, that cranes tolerate another pair
close to t heir nesting site, allow it to breed there for some y ears, whereby the latt er pair may become the only pair
after some time (disappearance of the first pair, like 1997 in Bb).
During the time and for the territories observed by Henne, three times a crane formerly bound with a mate has
changed his mate (1991/94 Fb; 1994/ 95 and 1995/96 Ss). This would be equivalent to a rate of about 10%, and (as
shown in Ss) not necessarily because of the death of the partner. Also, a territorial pair with longer tradition may be
displaced by a new one and find a different territory nearby.
3.2 Frequenzanalyse (Englisch)
In 4 cases (about 15%) a new pair appears in a territory originally owned by another pair (1991, 1995 and 1998 in
Fb, 1993 Zb, 1995 Ss).
Surprisingly, these results are in good accordance with . preliminary conclus ions drawn from a broad field study
running now over 7 years with sandhill cranes [12]. Here, an increasing number of adult cranes pairs was banded
(now 35 pairs), and some radio-tagged.
Of well over 100 crane-pair-years observed until now in only 12 cases mates had changed, in the other cases , the
same pair returned to the territory, i.e. again about 10% " divorce rate". It was found that most of these divorces had
not been due to one of the mates having died.
It is therefore not any more unlik ely that partner loyalty may be a matter of the degree of the partners "fitting
together" and a change of the mate a matter of choice by both sexes. This is supported by our observations in
Hamburg, where a single adult crane tried for a whole spring season to win a mate bound with another male, however
without s uccess . Male 6, obviously a young individual, came back with a new female after the first 2 unsuccess ful
breeding attempts .
In another even broader s tudy in Mecklenburg near Goldberg, we will build a database c omprising around 90 pairs
in 635 km² observation area for a long-term monitoring; 28 pairs have been acoustically characterised as a first st ep.
2 new pairs have been found in a densely populated small moor, 2 new occupied territories (visually unknown) in a
forest. The sonographical monitoring will be part of the continuous visual observation going on there since many
years, which is accompanied by banding and radio-tracking work.
Interna tional proje cts
n Japan (Hokkaido), we have begun to build the basis for a longer term monitoring with t he resident red-crowned
cranes. Recording of spontaneous unison calls at winter feeding stations showed the feasibility, as well as did a
later recording (using provocation by playing calls from CD via megaphone) in 16 breeding territories in South and
Northeast Hokkaido.
Even later, it was possible to record unison calls of pairs with at least one banded crane individual at winter
feeding station. These pairs can be followed over the y ears even in winter, provided that they will c all in the presence
of a recorder [13].
The analysis of these calls was the basis for a comparison of Japanese red-crowned c rane unison calls with t hose
from mainland China. These calls have been recorded by me in winter 99/00 in the Demilitarised Zone between
North- and South-Korea, where a significant portion of Chinese red-crowned cranes (and also white naped cranes) is-
After a detailed analysis [14] it is evident, that the call structure is completely different between the Japanese and
the Chinese cranes of the same species. Call structure (degree of complexity), most prominent frequency of male
and female, call length, female calls per male tact and other features are basically different. The two populations are
separate since probably 300 years or more, which did not create the cranes to form a different species (recent gene
analysis has shown that the population st ill are genetically to be considered one species) [15].
We therefore have to conclude that t he c all st ructure is influenced not only by genetic, but also strongly by s ocial
Red-crowned cranes (especially at their wintering feeding sites) seem to provide an excellent study field for
beginning to understand the cranes' vocal communication codes ("language"). We have found several different vocal
expressions, more than have been observed before (and also s ome very interesting ones in response to CD calls by
megaphone). Observations are leading to the conclusion, that cranes inform each other about living objects on earth
(with no or low degree of danger, but some degree of interest), like a "deer", and with a different sound about a living
flying object representing a certain danger, like an "eagle".
We have recorded and analysed the respective sounds [16].
The certainly most important st udy is c onnected with the North-American Whooping Crane, the most endangered
crane species with only 188 living individuals in the last surviving wild and self-reproducing population. B anding of
young whooping cranes has been s topped about 12 y ears ago. In a first expedition t o t he wintering grounds in the
Aransas National W ildlife refuge in Texas in 1999, 27 pairs have been characterised with the "acoustic fingerprint",
22 of them by unison call, 17 of them by their guard call, and 12 with both guard and unison call. For this purpose,
more than 200 recordings have been analysed.
In another expedition to the breeding grounds in the W ood Buffalo National Park in North Canada (by Brian Johns,
using my equipment), it was possible to record 10 pairs in their breeding territories.
The analysis of the calls and t he comparison with the analyses of the winter calls showed, that 6 of the 10
recorded pairs were in the group of 27 characterised in the previous winter. So it was possible for the first time to
follow wild birds from the winter territory to t heir summer territory and find out which bird lives together with his mate
in which territory - without having them touched any time, or even disturbed in any way. These are the results:
spring 2000
territory in ANWR winter 99/00 pair number in WB NP
8 3
10 1
17 2
19 12
32 30
33 13
This can be the basis for a long-term monitoring of t hese endangered birds, providing the possibilit y of learning
more about their life, helping to establish even better protection means.
3.2 Frequenzanalyse (Englisch)
The growing k nowledge about the importance of vocal communication for cranes has become an important part
also of the project (carried out by the "Whooper Crane Recovery Team") to establish a third population (after the
second one which is being established in Florida as a residential non-migrating flock) by release of c aptive reared
cranes into the wild. The 3rd flock will be released in W isc onsin and it is planned to teach them t o migrate with t he
help of ultraleight airplanes ("Operation Migration").
As a last test run before doing so with captive reared W hooper cranes, the team is flying the same way with
captive reared sandhill cranes. For the first time in such raising and training for UL-led flights, calls recorded from
wild (sandhill) cranes were used to c ommunicate with the young cranes. 6 different calls have been provided by the
author to be played using wrist-held s mall loudspeakers and powerful megaphones attached to the airplanes. The
calls are interpreted to be used by the sandhills with the understanding of "attention", "danger", "continue flight",
"come here / c ontact", and the guard and the unison calls.
The first experiences are very promising: the vocal imprinting seems to be much more powerful than pure visual
imprinting (after hatching): whereby it was very complicate in earlier trial projects to bring the y oung cranes t o follow
the airplane in the first weeks (because they preferred to s tay the ground together with the - c ostumed - caretaker),
they now followed the subject or object mak ing the "contact" c all. It is easy to tell the cranes to follow the airplane or
the c aretaker, to go in or out of the fenced area. It was even possible to c all a group of young c ranes back to the
training centre when they had been "kidnapped" by a wild crane pair.
he new sonographical method for identifying cranes provides a relatively simple and easy to handle tool for
identifying and monitoring c ranes and their life (partner and territory fidelity) It also allows to count how many
different crane pairs are present in a certain area, much easier than by just visual observations.
The collection of recordings made by systematic visits to the areas in question especially early in the morning,
but also statistically distributed over the days gives a picture of the minimum number of pairs (and individuals)
present in the area under supervision.
Wherever it is not possible or not advisable to band (which is the majority of areas) this method is the only method
which can be practically applied by almost every ornithologist or observer.
A long term monitoring of cranes leading to individual life history facts is possible.
All these advantages without even touching the c ranes, from distances between their flight distance up to several
Acknowledgeme nts
would like to thank my son Bengt and Inga Roedenbek for helping me to make the first recordings in 1998.
Thanks go also to Frank Golchert for helping me to as semble the hardware, and to Holger Hoffmann for his
contributions during the development of the mathematica sc ript.
I thank George Archibald for his consults and his generous help in providing me the possibility to record at the ICF
in Baraboo and for introducing me to the W hooper Crane Recovery Team, to the Japanese and Korean crane experts
who helped me in my field work there (Yulia Momose, Prof. Masatomi, Kimiya K oga, Mr. Matsuo, Mr. Pae). My
thanks go also to the Whooping Crane Recovery Team and especially George Gee who arranged the possibility for
my recording in Patuxent, Tom S tehn who supported my recording in Aransas, and Brian Johns, who recorded in
WBNP. Thanks also go to Operation Migration (Bill Lishman, Joe Duff) for their support for applying the vocal
communication tec hnique during the UL-led migration.
Refere nces
1. Archibald "Crane Taxonomy as revealed by the Unison c all"; roceedings of the International
Crane Works hop Barathpur Baraboo (ICF) 1987, p. 225 - 251
2. F. Weekly Mast er Thesis work Univ. Wisconsin 1985
3. acall displayed by a single crane when disturbed, here after intrusion by the researcher and a
dog into the nesting s ite, inducing a flight of the sitting crane followed by the guard call, the
contary of our goal to identify cranes from the distance
4. G. Carlson, C. Trost The Condor 94 532 - 536 (1992)
5. E. Henne Proceedings of the 3rd European Crane Workshop 1999 (h. Prange, g. Nowald, W .
Mewes, eds) Halle 1999, p. 66 - 72
6. Y. Ferrand, Gibier Faune Sauvage, 4 214 - 254 (1987)
7. C. W alcott et al., Bioacoust ics 10 101 - 114 (1999)
8. in 2 years, 2 different male loons occupied a territory which the factor analysis was not able
to distinguish; in another year, a known loon invaded the territory of another k nown loon and
displaced the former owner; the dis placed loon established nearby but its "yodel" call
changed so that it was not grouped any more in t he same factor s pace as before, so would
have been considered to belong t o a different bird according to the program used; also t he
invador's yodel had changed, but less dramatically; several other examples confused the
authors s o that they concluded that changing the territory also changed the loon yodel; cf.
footnote 11
9. thanks go to Frank Golchert, who consult ed in purchasing of hardware and modified it
according to my concept
10. thanks go to H. Hoffmann, a former "mathematica" support technician, who helped me to
develop this special protocol
11. using my analysis, als apparently changed calls showed the same fingerprint as before
territory c hange, and t he c onfusion raised by "Canary 1.2" became obvious: the birds were
partially "overblowing", so that the program did not find the correct frequency of the
3.2 Frequenzanalyse (Englisch)
introducting tone (which was to be found at double frequency as overtone only), and some
other slight changes in the call structure not at all significant had nevertheless put the
respective calls outside of the factor space
12. performed by the International Crane Foundation, B araboo, Wisconsin, in Briggsville, WI; Jeb
Barzen personal communication
13. K. Koga, B. Wessling, t o be published
14. B. Wessling, to be published
15. K. Koga, personal communication referring to a Japanese publication
16. K. Koga, B. Wessling, t o be published
ResearchGate has not been able to resolve any citations for this publication.
Crane Taxonomy as revealed by the Unison call"; roceedings of the International Crane Workshop Barathpur Baraboo (ICF)
  • Archibald
Archibald "Crane Taxonomy as revealed by the Unison call"; roceedings of the International Crane Workshop Barathpur Baraboo (ICF) 1987, p. 225 -251
  • Y Ferrand
Y. Ferrand, Gibier Faune Sauvage, 4 214 -254 (1987)
  • C Walcott
C. Walcott et al., Bioacoustics 10 101 -114 (1999)