© A. R. Dean
Plate 1. A 'song switcher' between Willow Warbler and Chiffchaff songs. West Midlands. April 2009.
Individuals of one species copying the song of another can be distinctly confusing. There are two basic (but not exclusive) scenarios. In the first, one species switches between sensibly complete song-phrases of its own species and those of a separate species. The two song-types may be contiguous (i.e. delivered as a single phrase) or there may be a pause between the two. In such 'song switching', where complete song-phrases of two species are juxtaposed, the identity of the two species' songs usually remains evident to the human ear. Sometimes, however, the notes from the 'copied' species' song are delivered more rapidly or at a different pitch than is usual in that species, and then its character can be radically altered. The second and more complex scenario involves an intimate mixture of individual notes from the songs of two species in a single song-phrase. In such truly 'mixed song', the result can be especially disconcerting and may resemble the true song of neither species. Indeed, it may on occasion recall the song of a quite different species. In the case of true 'mixed singing' between Willow Warbler Phylloscopus trochilus and Common Chiffchaff P. collybita, the result can suggest the song of Siberian Chiffchaff P. (c.) tristis or even Greenish Warbler P. trochiloides. Other 'combination singers' can include phrases which strongly resemble the song of Iberian Chiffchaff P. ibericus (see Example 4 below), though the notes in such cases may not all originate from the normal songs of Willow Warbler and Common Chiffchaff.
Apparently, examples of 'song switching' and 'mixed singing' in the Willow Warbler are not significantly rare - see Cramp (ed), 1992 (BWP v6, page 657) and this probably applies to a range of species. For a review of 'mixed singing' in European song-birds see Helb et al. (1985). This paper is also available on-line by subscription; see Abstract.
It seems likely that consistent 'song switching' most-frequently involves simple song copying rather than indicating a hybrid between the two species. One species may imitate the song of the other when a close congener is seen as a potential territorial competitor. Conversely, consistent 'mixed song', involving a much more complex mix of individual notes from two species, seems to me less easy to explain from 'imitation' and may perhaps be a stronger indication of hybrid origin. Occasionally, though, both 'song switching' and 'mixed singing' are encountered from a single individual, so there is no clear-cut divide.
In the overlap zone between Iberian Chiffchaff and nominate Common Chiffchaff in NE Spain and SW France, most individuals combining elements of the two species songs are true 'mixed singers'. Using genetic AFLP analysis, most 'mixed singers' have been shown to be hybrids (Bensch et al 2002). In the overlap zone between Siberian Chiffchaff and Common Chiffchaff of the form abietinus, extending NW from the southern Urals, there is also a significant level of 'mixed song' (Lindholm 2008; Marova et al 2009). Here hybridization between the two forms has also been verified, from a combination of morphological, genetic and bio-acoustic characters (Marova et al 2009, Shipilina et al 2017).
However, Lawn (2018) has suggested that 'character convergence' (in this case involving song convergence) between Willow Warbler and Chiffchaff may be involved, as a result of population and habitat changes, and that this could have adaptive benefits.
Between April 2009 and June 2017 I encountered ten examples of such 'combination' singers in the West Midlands. All ten involved apparent Willow Warblers incorporating Chiffchaff notes into their repertoire and all ten at times gave normal Willow Warbler song-phrases (though these were very much a minority in the seventh example). None of these individuals employed the short, churring 'trrt' note with which Chiffchaffs preface many of their song phrases.
Details of these ten individuals follow.
© A. R. Dean
Plates 2 & 3. A 'song switcher' between Willow Warbler and Chiffchaff songs. West Midlands. April 2009.
The Willow Warbler Phylloscopus trochilus in plates 1, 2 & 3, photographed near Meriden, West Midlands, during April 2009, was switching between the songs of Willow Warbler and Common Chiffchaff Phylloscopus collybita. It re-appeared at the same location in April 2010. In all morphological respects it appears to be a perfectly standard Willow Warbler. Note especially the relatively long wing-point and the pale legs and feet. Several complete phrases of Willow Warbler song were followed by a pause, then several complete phrases of Chiffchaff song, and so on. The song phrases of Willow Warbler were perfect while the song phrases of Chiffchaff were a little hurried compared with a typical Chiffchaff.
Thus, this Willow Warbler was imitating complete phrases of another taxon and was hence a 'song switcher' or 'song imitator' as opposed to a true 'mixed singer'.
© A. R. Dean
Plate 4. Willow Warbler incorporating Chiffchaff notes into its song. Warks. May 2010.
On May 21st 2010, a Willow Warbler was observed in North Warwickshire (Plate 4) in which each song-phrase began with faltering and rather hurried Chiffchaff-like notes before concluding with normal Willow Warbler song .
During April 2011, a Willow Warbler in the Blythe Valley, West Midlands, was delivering a song incorporating notes from Common Chiffchaff song but with different timbre and more rapid delivery than usual in that species. As a result, these notes had a suggestion of Coal Tit as well as Common Chiffchaff.
In such juxtaposed 'song switching', it is quite often the case that the Common Chiffchaff-like notes are delivered at a faster rate than is normal in that species (see Cramp (ed), 2006, BWP v6, page 657).
A recording of the song of the West Midlands individual at its natural speed is here ►.
A recording of the song slowed to 80% of its natural speed is here ►. In this slowed recording the 'identity' of the Common Chiffchaff components of the song is much more evident. The timbre still differs slightly from a 'normal' Common Chiffchaff song but the basic Chiffchaff structure is clear.
A sonogram of the song (at its natural speed) is below:
The Common Chiffchaff components are initially disconcerting but, in fact, they are not particularly abnormal except in the speed of their delivery. Thus, although the song phrases including these Chiffchaff components are initially perplexing, this proves still to be a case of 'songs switching' (or juxtaposing) rather than a truly 'mixed song' as defined above.
Below are sonograms of (a) the Common Chiffchaff-like notes from the Willow Warbler's song and (b) the standard song of Common Chiffchaff.
There are slight differences in average pitch and timbre but note the basic similarity in the structure of the notes. However, in the Chiffchaff-like song-phrase in the song of the Willow Warbler (a), there are nine notes in two seconds. In the normal Chiffchaff song (b), there are six notes in two seconds.
© A. R. Dean
Plates 5 & 6. Willow Warbler including Common Chiffchaff and Iberian Chiffchaff-like phrases into its song. Warks. April 2011.
The fourth individual was encountered in central Warwickshire, during late-April 2011 (plates 5 & 6). This Willow Warbler also included rapidly-delivered Chiffchaff-like notes juxtaposed with normal Willow Warbler song. However, it also included occasional song-phrases which incorporated several unusual and 'hesitant' introductory notes followed by rapid repetition of a single note, producing a Wren-like rattle though with different pitch and timbre ( c.f. the Wren singing in the background in second recording below). The note-sequence and timbre of this song-phrase strongly resembled the advertising song of Iberian Chiffchaff.
Recordings of the song of the Warwickshire individual are here ► and here ►.
Note how the final song-phrase in the first recording and several in the second recording are very like the advertising song of Iberian Chiffchaff.
A sonogram of the ibericus-like phrase from the Warwickshire bird's song is below. The structure of the first dozen notes corresponds quite closely with sonograms of Iberian Chiffchaff songs recorded in Europe.
For comments on Iberian Chiffchaff song and 'mixed singers', see Bensch et al (2002), and for the occurrence of Iberian Chiffchaff-like song in problematic individuals, see Collinson and Melling (2008).
© A. R. Dean
Plate 7. Willow Warbler incorporating Chiffchaff notes into its song. West Midlands. May 2014.
In early May 2014 an apparent Willow Warbler was discovered near Meriden, in the West Midlands, which was a truly 'mixed singer' (plate 7). Individual song-phrases incorporated a variable number of notes from the songs of Willow Warbler and Chiffchaff. Some phrases were thoroughly mixed and others dominated by Chiffchaff-style notes but most were dominated by Willow Warbler notes. The great majority of phrases were 'mixed' at some level and phrases which perfectly matched the song of Willow Warbler were in a minority. Phrases resembling Chiffchaff were quite evident but were slightly less even and slightly more sibilant than in true Chiffchaff song. Sonograms revealed some notes with an abnormal structure for either species, though the timbre and rhythm of notes was clearly attributable to one or the other species.
An extended sequence of song as delivered is here ►. A concatenated series of extracted song-phrases illustrating variable degrees of 'mixing' is here ►. A song-sequence here ► begins with Willow Warbler song; listen carefully and a Chiffchaff begins to sing in the background; the Willow Warbler appears to change its song in response.
Two short extracts of mixed song are here ►, with the corresponding sonograms below. Chiffchaff-style notes predominate in the first phrase while Willow Warbler notes predominate in the second phrase.
Finally, to 'compare and contrast', a clean Willow Warbler phrase, a 'mixed' phrase and a Chiffchaff-style phrase have been extracted and juxtaposed here ►, with the corresponding sonogram below.
Plate 8. Willow Warbler incorporating Chiffchaff-like notes into its song. Warks. Summer 2015.
In mid-April 2015 another 'mixed singer' was discovered near Packwood in Warwickshire. It remained in the area throughout the summer and consistently used mixed song. This individual delivered perfect Willow Warbler song at times but would frequently introduce mixed phrases and song-switching. At times it would utter phrases composed entirely of Chiffchaff-like notes. Some Chiffchaff notes were reasonable imitations whereas others had imperfect timbre and rhythm for that species. In the concatenated extract here ►, recorded in April 2015, there are onstructs with several consecutive Chiffchaff-like phrases and some efficient imitations. Another example of Chiffchaff-like notes but with evidently different timbre and rhythm is here ►, recorded in June 2015. These phrases have a cadence indicative of more genuinely 'mixed' song.
In June 2015 the song delivered by a 'mixed singer' near Meriden was dominated by phrases which suggested Chiffchaff song. Normal Willow Warbler phrases were very much in a minority. Although the rhythm of the dominant phrases recalls Chiffchaff song, the rapid delivery of the notes and their more fluted (less dry) timbre probably places these phrases into the 'mixed song' category. An example of this individual's song is here ►.
Plate 9. Willow Warbler incorporating Chiffchaff-like notes into its song. Warks. April 2016.
In April 2016 a 'mixed singer' was present in Kingsbury Water Park in Warwickshire. Its song was dominated by Willow Warbler phrases but Chiffchaff phrases were incorporated regularly. Several phrases began with two or three Willow Warbler notes before changing seamlessly into Chiffchaff-like notes. A sequence incorporating several genuinely mixed phrases can be heard here ►. It comprises ten song phrases, of which phrases 1, 5, 8 & 9 are 'mixed'. A sonogram of the eighth phrase is below:
In May 2016 a 'mixed singer' near Berkswell also delivered predominantly Willow Warbler song but incorporated phrases which began with Willow Warbler notes before changing seamlessly into Chiffchaff-like notes. In the recording ►, there are ten phrases of which numbers 1, 5, 8 and 9 are of this 'mixed' type while the remainder are normal Willow Warbler phrases.
In June 2017, again near Berkswell, a Willow Warbler (on appearance and inclusion of perfect Willow Warbler song at times) appended Chiffchaff-like notes to some phrases. These were delivered with great speed and vehemence and were of a more even pitch than true Chiffchaff song.
A phrase commencing with typical WW song and then changing without a pause into the CC 'burst mode' is <here>.
An isolated burst of CC-like notes is <here>. There is a hint of both WW and CC in the timbre of these 'burst modes'. A sonogram of this sequence is below:
For some further examples and discussion of 'combination' songs involving Willow Warbler and Common Chiffchaff, see this item and this item from the Gwent OS website.
Additionally, an apparent Common Chiffchaff with a 'combination' song was recorded during April 2011 in Yorkshire - see Martin Garner's website.
The website of Annika Forsten and Antero Lindholm contains details and recordings of a 'song switching' Common Chiffchaff in Finland in June 2008. Note, especially, the unusual song-variation in 'recording 4'.
A warbler in Sweden in 2010 was identified as Iberian Chiffchaff but doubts crept in when it began to incorporate into its song Chiffchaff-like notes and Willow Warbler song-phrases (Lars Svensson in lit.). A video and recordings of its song can be found here.
For more information on 'mixed singing' and 'song switching' see Cramp (ed), 1992, (page 657) and 'The Sound Approach to Birding', 2006, (pages 129-130).
For data on organisation and repertoire in the song of Willow Warbler see Järvi, T., Radesäter, T. and Jakobsson, S. (1980) and Gil, D. and Slater, P.J.B. (2000).
BENSCH, S., HELBIG, A.J., SALOMON, M., & SEIBOLD, I. 2002. Amplified fragment length polymorphism analysis identifies hybrids between two subspecies of warblers. Molecular Biology 11: 473-481.
COLLINSON, J.M. and MELLING, T. 2008. Identification of vagrant Iberian Chiffchaffs – pointers, pitfalls and problem birds. British Birds 101: 174–188.
CONSTANTINE, M., & THE SOUND APPROACH. 2006. The Sound Approach to Birding: a guide to understanding bird sound. The Sound Approach, Poole.
CRAMP, S. (ed.) 1992. The Birds of the Western Palearctic. Volume 6. OUP, Oxford.
GIL, D. and SLATER, P.J.B. 2000. Song Organisation and Singing Patterns of the Willow Warbler, Phylloscopus trochilus. Behaviour 137: 759-782.
HELB, H.-W., DOWSETT-LEMAIRE, F., BERGMANN, H.-H. and CONRADS, K. 1985. Mixed Singing in European Songbirds — a Review. J. Comp. Ecol. 69: 27–41.
JӒRVI, T., RADESӒTER, T. and JAKOBSSON, S. 1980. The Song of the Willow Warbler Phylloscopus trochilus with Special Reference to Singing Behaviour in Agonistic Situations. Ornis Scandinavica 11: 236-242.
LAWN, M. R. 2018. Interspecific territoriality by mixed-singing and song-switching Willow Warblers: a possible case of character convergence. British Birds 111: 334 - 348.
LINDHOLM, A. 2008. Mixed Song of Chiffchaffs in Northern Russia. Alula 14: 108-115.
MAROVA, I.M., FEDEROV V.V., SHIPILINA, D.A., & ALEKSEEV, V.N. 2009. Genetic and vocal differentiation in hybrid zones of birds: Siberian and European Chiffchaffs (Phylloscopus [collybita] tristis - Ph. c. abietinus) in the Southern Urals. Doklady Biological Sciences 427: 384-386.
SHIPILINA, D., SERBYN, M., IVANITSKII, V., MAROVA, I. & BACKSTRÖM, N. 2017. Patterns of genetic, phenotypic, and acoustic variation across a chiffchaff (Phylloscopus collybita abietinus/tristis) hybrid zone. Ecology and Evolution 2017; 1–12.
Dean, A. R. 2017 et seq. 'Switched' and 'mixed' songs in Willow Warbler Phylloscopus trochilus and Common Chiffchaff P. collybita. deanar.org.uk/general/articles/wwmixedsong.htm.