Turkish Journal of Zoology
Turk J Zool
(2013) 37: 246-248
© TÜBİTAK
doi:10.3906/zoo-1207-4
http://journals.tubitak.gov.tr/zoology/
Short Communication
Hybridisation or vocal mimicry? A case of a mixed singing River Warbler
Locustella fluviatilis in eastern Poland
1
1
2,
3
Michał POLAKOWSKI , Monika BRONISZEWSKA , Łukasz JANKOWIAK *, Tomasz COFTA
1
Zachodnia 30A/8, PL-15-345 Białystok, Poland
2
Department of Behavioural Ecology, Faculty of Biology, Adam Mickiewicz University, Umultowska 89, PL-61-614 Poznań, Poland
3
Hoene 5A/5, PL-80-041 Gdańsk
Received: 04.07.2012
Accepted: 09.09.2012
Published Online: 25.02.2013
Printed: 25.03.2013
Abstract: The unusual mixed song of a River Warbler (Locustella fluviatilis) was recorded in eastern Poland. The identified male used
2 types of songs recalling the River Warbler and the Grasshopper Warbler (Locustella naevia) alternately, which was confirmed by
the interpretation of the sonograms. The most likely explanation of this unusual song is vocal mimicry, caused by the exposure of the
recorded individual to the song of the Grasshopper Warbler in its acoustic environment at an early stage of its life.
Key words: River Warbler, Grasshopper Warbler, vocal mimicry, hybridisation
The unusual song of a warbler from the genus
Locustella was recorded during the late afternoon of 6
June 2011, in Pałecznica village (Lubartów district, Lublin
province, 51°28′56.25″N, 22°40′20.63″E) in eastern
Poland (M. Polakowski, M. Broniszewska). A digital
recording of the song was made (136 s of the effective
singing in total; a link to the file can be found at http://
www.xeno-canto.org/103438). The bird was found in the
marginal habitat of any Locustella species (to compare,
see Cramp 1998; Kennerley and Pearson, 2010): small
bushes of the European Black Elderberry (Sambucus
nigra) in an agricultural landscape with buildings, as
well as other structures and a young established pine
stand. This individual was seen clearly enough to allow
an accurate visual identification as a River Warbler
(Locustella fluviatilis, RW) according to the diagnostic
features of this species.
The song of this bird was irregular, changing from one
speed to another and from one song type to the other,
rapidly, without pauses (Figure 1). It consisted of 2 types
of sounds that strongly recalled RW and Grasshopper
Warbler (Locustella naevia, GW) songs (Figure 2). The
passage from one type of song to the other was rapid,
without a transitional phase or pause. For more precise
analysis we used sonograms of the recorded song and
compared it with typical RW and GW sonograms (Figure
3).
*Correspondence: [email protected]
246
The GW sings very homogeneously. The differences
between individuals are clearer in the pitch than in the
speed. Irregularities in the song are slight and hard to
notice. The typical RW song is more irregular, the changes
in the pitch making the sound more or less swinging. The
mixed song was remarkably irregular (which distinguishes
it from the RW and the GW songs). It jumped from one
speed to another. The lack of any passages involving
acceleration or deceleration indicates that the bird used 2
different song patterns, corresponding to the songs of the
RW and GW, respectively.
The basic difference between the RW and GW songs
is the frequency of repetitions of successive notes. In the
case of the RW it is 9.5-16.5 notes/s, and in the case of the
GW, 22.5-25.0 notes/s (the variability range was measured
using different song recordings of these species: Kettle and
Ranft, 1992; Bruun et al., 1995; Génsbøl, 1998; Pałczyński,
2001; Xeno-Canto Europe, 2005–2012; Kruszewicz, 2006;
Schulze and Dingler, 2007; Celmins et al., 2012). In the
mixed song the speed ranges of the 2 song types did not
overlap. The lower range was 14.2–15.2 notes/s, which fits
the RW range, and the higher speed was 20.0–21.5 notes/s,
close to the GW range but somewhat slower.
The sonogram (Figure 3) presents typical images
for both species. In the case of the RW it is a wider and
longer stripe with irregular horizontal wavy striations in
the volume maximum. For the GW, the sonogram shows
POLAKOWSKI et al. / Turk J Zool
8000
Hz
6000
4000
2000
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14 s
Figure 1. Sonogram of the River Warbler’s mixed song recorded in Poland on 6 June
2011. The song is irregular, consisting of 2 note types alternately repeated in short series.
Hz
8000
6000
4000
2000
RW
0
1
GW
2
3
4
s
Figure 2. Part of the sonogram of the River Warbler’s mixed song. Vertical
lines were added to illustrate differences in speed and note length. RW – River
Warbler type of song, lower speed, longer note, wider frequency spectrum;
GW – Grasshopper Warbler type of song, higher speed, shorter note, narrower
frequency spectrum.
River Warbler
mixed singer
RW
Grasshopper Warbler
GW
Figure 3. Sonogram image comparison. River Warbler typical song – single note
longer, main frequency spectrum wider, harmonics visible as horizontal bars or
waves, no double narrow sharp elements. Grasshopper Warbler typical song – single
note shorter, main frequency spectrum narrower, no visible harmonics bars, double
narrow sharp elements within each note. RW – River Warbler-type song of mixed
singer; GW – Grasshopper Warbler-type song of mixed singer (recordings of the
typical songs made by T. Ogrodowczyk, available in Kruszewicz, 2006).
2 vertical lines separated with very narrow pauses and the
volume maximum going diagonally downward.
We are convinced that the RW mixed singer was a
transient bird, because the place is a rather unsuitable
habitat for the nesting of this species (Kennerley and
Pearson, 2010) and no other RW was found there during
the 2011 breeding season (authors’ unpublished data).
It is possible that some RW males sing atypically at
the beginning of the spring migration (at faster tempi;
Mackowicz, 1989), but the recorded mixed song does
not correspond to any known RW song types. The
clearly defined parts of the RW and the GW songs in this
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POLAKOWSKI et al. / Turk J Zool
mixed song could suggest a case of hybridisation of the
2 species. However, the probability of that is rather low,
as there are no known cases of hybridisation between
the European Locustella species (e.g., Kennerley and
Pearson, 2010) and the individual’s species was precisely
identified. On the other hand, there are known to be a
very few cases of hybridisation in the genetically related
genus Acrocephalus, e.g. Reed Warbler (A. scirpaceus) ×
Marsh Warbler (A. palustris; Lemair, 1977), Reed Warbler
× Great Reed Warbler (A. arundinaceus; Hansson et
al., 2004), and Great Reed Warbler × Clamorous Reed
Warbler (A. stentoreus; Hansson et al., 2012). Therefore,
in our opinion the most probable explanation is the effect
of a wrongly learned sound as the result of a mistake
during the learning process (Deregnaucourt et al., 2004).
The song patterns in Passeriformes are shaped by learning
early in the birds’ life (Nottebohm, 2005). Some birds use
learned sounds in the wrong context as vocal mimicry
(Kelley et al., 2008). The discussed mixed song could be a
result of this adaptation process. In the literature there are
examples of the RW’s vocal mimicry, in showing a strong
similarity to the song of the Great Tit (Parus major;
Bergmann and Helb, 1982). The motifs of the RW songs
are rather constant; however, some variation between the
individuals occurs, probably due to rare local variants or
mimicry (Bergmann and Helb, 1982). The most probable
explanation is the interspecific mimicry coupled with the
possibility that the recorded bird, at an early stage of its
life, was exposed to the song of the GW in its acoustic
environment.
Acknowledgements
We are very grateful to Roch Mackowicz, Łukasz Ławicki,
and Przemysław Wylegała for their comments on the
manuscript, as well as to Agnieszka Piróg and Joanna
Przybylska for translation and language corrections.
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