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Subject Editor: Mikko Monkkonen Editor-in-Chief: Miguel Araújo Accepted 8 December 2016

41: 245–254, 2018doi: 10.1111/ecog.02779

doi: 10.1111/ecog.02779 41 245–254

Biological invasions are not only events with substantial environmental and socioeco-nomic impacts but are also interesting natural experiments, allowing the study of phe-nomena such as the cultural evolution of bird song following introduction. We took an excellent opportunity to compare the distribution of dialects of the yellowham-mer Emberiza citrinella, a small Eurasian passerine, in its native source region (Great Britain) and invaded range (New Zealand) more than hundred years after relocation. Recent field recordings (including those provided by volunteers within a citizen sci-ence project) were complemented by those from archives, each assigned to appropri-ate dialect by visual inspection of a sonogram, and the resulting spatial patterns of dialect distribution were interpreted using historical data on the yellowhammer inva-sion. The two countries differ markedly in the composition and distribution of dialects. New Zealand populations sing a greater number of different dialects, seven in total, five of which were not detected in the current British population, but have been reported by previous studies from the continental Europe. Two identified localities of capture (Brighton, Sussex, UK) and release (Dunedin, Otago, NZ) differ even more strik-ingly, having no dialects in common. The largely sedentary nature of yellowhammers allows for two mutually exclusive explanations for European dialects being detected in New Zealand but not in Great Britain: 1) the corresponding song types have emerged de novo in New Zealand, through convergent cultural evolution; 2) the dialects have disap-peared from Great Britain, while being preserved in New Zealand. Indirect evidence from the widespread occurrence of these dialects in continental Europe and the reported stabil-ity of yellowhammer song, supports the latter explanation. We suggest that the yellow-hammer dialect system is an avian equivalent of a phenomenon already noted in human languages, in which ancient words or structures are retained in expatriate communities.

Introduction

Biological invasions can be a potential threat to the environment (Pyšek et al. 2012, Kumschick et al. 2015, Martin-Albarracin et al. 2015) and economy (Pimentel et al. 2005, Kettunen et al. 2009), but they can also provide a golden opportunity to study evolutionary, behavioural or ecological phenomena on spatial and temporal scales

Research

Dialects of an invasive songbird are preserved in its invaded but not native source range

Pavel Pipek, Tereza Petrusková, Adam Petrusek, Lucie Diblíková, Mark A. Eaton and Petr Pyšek

P. Pipek (http://orcid.org/0000-0003-1116-1013) (pipek@natur.cuni.cz), T. Petrusková, A. Petrusek, L. Diblíková and P. Pyšek, Dept of Ecology, Faculty of Science, Charles Univ. Prague, Czech Republic. PP and PP also at: Inst. of Botany, Dept of Invasion Ecology, The Czech Academy of Sciences, Průhonice, Czech Republic. – M. A. Eaton, RSPB Centre for Conservation Science, Newcastle-upon-Tyne, UK.

––––––––––––––––––––––––––––––––––––––––© 2016 The Authors. Ecography © 2016 Nordic Society Oikos

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unachievable by traditional experimental approaches (Hale and Briskie 2007, Yoshida et al. 2007, Schrey et al. 2011). Animal populations often differ between native and intro-duced ranges, in morphological (Blackburn et al. 2013), life-history (Congdon and Briskie 2014) or behavioural traits (Pintor and Sih 2009), and biological invasions as ‘natural experiments’ can encompass vast areas, including whole continents (Norbury and Jones 2015), and several human generations (Lever 2005). Invasions may therefore provide insight into a range of natural phenomena.

Birds, particularly passerines (Passeriformes), are one of the most frequently studied animal groups in invasion ecology (Blackburn et  al. 2009). Passerines are charac-terised by their complex vocalisation, or birdsong, which plays important roles in their breeding ecology, and which is transmitted culturally in this taxon (Catchpole and Slater 2008). Bird introductions thus provide opportunities to study the cultural evolution of behavioural traits. Despite the relative ease with which song recordings can be obtained from free-living individuals (in contrast to, e.g., DNA samples or morphometric data) and the fact that detailed information about introduction events is often available, so far only a few studies have examined changes in bird vocalisa-tion after introduction (Jenkins and Baker 1984, Lang and Barlow 1997, Hamao 2015).

Birdsong serves multiple purposes, including defending a territory, attracting sexual partners, and signalling a species’ identity. Although song thus acts as a species ‘fingerprint’, it can vary greatly within species in time and space. This geographic variation is apparent both on large scale, when the song differs among populations that do not meet and thus cannot potentially breed, but also on smaller scales, when there is an intense song sharing among neighbours (Catchpole and Slater 2008). A particularly interesting type of geographic variability in vocalisation is the occurrence of dialects with sharp borders, when song characteristics change abruptly rather than gradually (Podos and Warren 2007, Catchpole and Slater 2008).

Dialects are often regarded as a potential first step in speciation (Xing et  al. 2013, Potvin and Clegg 2015), although this is not the only reason to warrant research on geographic variation in song. Studies of dialects and other types of song characteristics can reveal important informa-tion about the ecology of the study species. For example, the intensity of song variant or syllable sharing among neighbours can indicate the degree of population fragmenta-tion (Laiolo and Tella 2005, Petrusková et al. 2010), a loss of song variability and disruption of the geographical pat-tern of dialects may reflect population decline (Holland et al. 1996), and dialects may also help to identify the source of a recolonizing population (Brunton et al. 2008).

Several hypotheses have been proposed for the origin of bird dialects (Podos and Warren 2007, Catchpole and Slater 2008) according to function. Dialect could be adaptive, and as such indicate that the singer is well adapted to local con-ditions (local adaptation hypothesis) or belongs to a certain social group (social adaptation hypothesis). They can also be

non-adaptive and arise as a by-product of selection pressures on other (e.g. morphological) traits. None of these hypoth-eses appears to be universal in application, and each can work in only some dialect systems (Podos and Warren 2007, Catchpole and Slater 2008).

Once established, dialect borders can apparently persist over decades (Harbison et al. 1999, Nelson et al. 2004, Wonke and Wallschläger 2009) and can be maintained even when the songs on both sides change (McGregor and Thompson 1988, Kopuchian et  al. 2004). Dialect borders can be sta-bilised either by strong assortative mating and low dispersal (Williams and Slater 1990, Ellers and Slabbekoorn 2003, Planqué et al. 2014), or, when dispersal is frequent, by post-dispersal learning (Ellers and Slabbekoorn 2003). For several species, playback experiments have also shown that reaction of birds towards familiar and unfamiliar dialects can differ, both in males (Petrinovich and Patterson 1981, Ratcliffe and Grant 1985, Searcy et  al. 1997) and females (Baker and Jenkins 1987, Searcy et al. 1997, Danner et al. 2011).

The understanding of the processes behind the evolution and maintenance of dialects can be enhanced by studying populations with the same background but which have been separated from each other, either naturally (Hill et al. 2013) or with human assistance (Parker et al. 2012).

Biological invasions give us the opportunity to study the effect of translocation over large geographic scales. Studies on chaffinches Fringilla coelebs introduced to New Zealand from England (Lynch et  al. 1989), Japanese bush-warblers Cettia diphone introduced to Hawai’i (Hamao 2015) and Eurasian tree sparrows Passer montanus (Lang and Barlow 1997) introduced to the USA have demonstrated that the structure of the song and the shape of individual syllables can change significantly after introduction. The last-mentioned study also showed that song diversity can be maintained despite an initial population bottleneck, due to subsequent rapid cultural evolution (Lang and Barlow 1997). In studies examining song evolution following natural colonization of new areas, i.e. a process similar to invasion (Hoffmann and Courchamp 2016; but see Wilson et  al. 2016), the results are equivocal. In some cases, new populations show lower song variability, in terms of number of different song types and syllables (Xing et al. 2013). In other situations, the song variation in exotic population can exceed variation in any of the original ones (Kroodsma et al. 1999, Baker et al. 2003), which might arise when individuals from multiple source populations combine (Pytte 1997).

However, all the studies mentioned above explored only post-introduction changes in song characteristics, not changes in composition and distribution of dialects. For that purpose, the invasion of yellowhammers Emberiza citrinella in New Zealand provides a rare opportunity. The yellowhammer is a widespread Palearctic songbird (del Hoyo et  al. 2011) with a very simple song and well defined dialects with sharp borders, the distribution of which is known from large areas (Petrusková et al. 2015). As with chaffinches, yellowhammers were introduced to New Zealand from Great Britain in the 19th century and the history of their capture and release is

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well documented (Pipek et  al. 2015). Moreover, for Otago and partly for Canterbury regions in New Zealand the source locality in England is known: Brighton, Sussex (Pipek et al. 2015). Unlike natural colonizers, New Zealand popula-tions of yellowhammers were isolated from Great Britain and, thus, if any change has occurred in New Zealand since the last introduction event, it has to be attributed clearly to evolution in situ. Moreover, British yellowhammers seem to be largely isolated from their conspecifics on the continent; there have been only five recaptures of birds crossing the sea between Great Britain and mainland Europe recorded since 1906 (Robinson et al. 2015), which constitutes only 0.2% of all yellowhammer recaptures in the UK.

Here, we compare the patterns of distribution of yellowhammer dialects in their European source area, Great Britain (Britain hereafter), and in the invaded range of New Zealand. Apart from our own data collection, we benefited from recordings available online and in sound archives, and from contributions of the general public to a citizen science project dedicated to this purpose. We com-bined the patterns obtained with historical data on introduc-tions (Pipek et  al. 2015), to test the following hypotheses: 1) the dialect diversity is greater in the native range, as only a small number of birds (ca 600–800 in total; Pipek et  al. 2015) were taken from the large British population to New Zealand, and this already small pool of dialects could have been further reduced by cultural drift, 2) new dialects evolved in the invaded range, as before the establishment and spread of yellowhammers in New Zealand there was a lack of song tutors for introduced birds, and errors in learning could have been more frequent and 3) dialect composition is similar in the localities of capture and release in the native and invaded range, respectively, due to birds’ common origin.

Materials and methods

Study species and its song

The yellowhammer is a small passerine, nowadays found typically in farmlands. Its native range includes a large por-tion of the Palearctic, from Spain in the west to central Asia in the east (del Hoyo et al. 2011). Between 1864 and 1875, more than 600 yellowhammers were brought from London as a biological control against crop pests and liberated in various localities across New Zealand, principally in Auck-land (North Island), Canterbury (South Island) and Otago (South Island) regions (Pipek et al. 2015). All the birds that were liberated in Dunedin (n 39) were collected around Brighton (Sussex); some (34 out of 222) of those released in Canterbury were also caught close to Brighton, the source of the remaining 188 birds is not known. In New Zealand, yellowhammers established quickly and spread to such an extent that they later became a target of organized culling (Pipek et al. 2015).

Yellowhammers are persistent singers, singing from early spring to late summer, morning until evening (Noble 1958).

Their characteristic song typically consists of two parts (Fig. 1). The initial quick repetition, usually one or two syl-lable types, is believed to be individually specific (Caro et al. 2009). The second part, a combination of two (rarely three) terminal syllables (labelled with letters B, C, D, E, X, of which B, C and X are shown in Fig. 1), is typically shared by males of a particular dialect area (reviewed by Petrus-ková et al. 2015). Such a distinction of individual dialects, suggested by Hansen (1985), has been supported by quan-titative analyses of syllable spectrotemporal characteristics (Wonke and Wallschläger 2009). Males with territories at the dialect border can sometimes alternate end phrases of both dialects, and are referred to as mixed singers (Glaubrecht 1989). Another apparent outcome in dialect contact zones is the emergence of intermediate three-syllable final phrases (Fig. 1, 2); these can sometimes dominate whole areas and thus be regarded as dialects themselves, such as XBC in northern Germany (Glaubrecht 1989) and Denmark (Hansen 1985). Song structure as well as dialect of a particular male are set in the first two years of its life and do not change later (Schön 1989).

Study areas and sampling strategy

Great Britain and New Zealand are isolated landmasses of comparable sizes, with New Zealand only about 10% larger and separated into two main islands. In New Zealand, yellowhammers reach population densities three times higher in comparable habitats (farmlands) than in Britain (MacLeod et al. 2005). We attempted to obtain recordings from throughout both countries representing the native and invaded range, but aimed for a higher density of recordings in New Zealand’s South Island and in southern England, reflecting our previous finding that most of the birds for release in the South Island were collected in southern England (Pipek et al. 2015).

Figure 1. Spectrograms of songs representing two most common yellowhammer dialects (XB, BC) and the intermediate dialect XBC.

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Recording collection and analysis

We set up a citizen-science project ‘Yellowhammer Dialects’, inspired by a similar Czech project (Procházka et al. 2013). Volunteers from Britain and New Zealand were encouraged to record songs of yellowhammers in the field and upload them to a dedicated website (< http://yellowhammers.net >), or to send them by email to the authors. Some recordings were already stored in digital audio or video formats online.

Additional recordings were obtained from sound archives (British Library Sounds, BBC Sound Effects library and McPherson Natural History Unit Sound Archive). Further-more, New Zealand Dept of Conservation shared with us recordings obtained through automatic recorders distrib-uted across New Zealand. Finally, a significant portion of New Zealand recordings and some of the British ones were collected by authors (PP, AP, TP) in the field. Most of the recordings were recorded in 2013–2015, since the launch

Figure 2. Distribution of yellowhammer dialects in Great Britain and New Zealand. Coloured symbols represent different dialects. These are depicted with schematic spectrograms. The figure is separated into two parts: A) comparison of both countries, B) zoomed regions of interest, source region in Great Britain (Sussex) and two target localities in New Zealand (Otago and Canterbury). Map sources: Esri, DigitalGlobe, Earthstar Geographics, CNES/Airbus DS, GeoEye, USDA FSA, USGS, Getmapping, Aerogrid, IGN, IGP, and the GIS User Community.

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of the citizen science project ‘Yellowhammer Dialects’, but we also obtained some recordings from the pre-digital era. The oldest British recording was from 1951 (Aberdeenshire, Scotland), the oldest recording from New Zealand was from 1955 (Fiordland National Park, South Island). The list of all recordings used in our analysis is given in Supplementary material Appendix 1, dataset A1.

All recordings were converted into WAV format and visu-alised in Avisoft SASLab Pro ver. 5 (Specht 2007). Record-ings of low quality and those not containing the complete terminal part of the songs (thus not allowing assignment of dialect) were excluded from further analyses. Each recording with a complete song was assigned a corresponding dialect (or two dialects, in case of recordings of mixed singers). We used the nomenclature introduced by Hansen (1985), based on the shape and frequencies of terminal syllables. How-ever, as the differentiation between dialects XsB and XlB, based just on the duration of the X syllable (Hansen 1985) is questionable (Wonke and Wallschläger 2009, Petrus-ková et al. 2015), we pooled these two into a single dialect category XB. Similarly, we simplified the nomenclature of intermediate three-syllable combinations containing the X syllable (XBC, XBB).

Rarefaction analysis

As was already mentioned, the distribution of recordings was not even; furthermore, the sampling effort differed also between the two studied countries (see Results). In order to account for this, we performed a rarefaction analysis, in which the sampled elements were not recordings but squares 10 10 km, which are typically used as a basic unit in atlases of breeding birds (Robertson et al. 2007, Balmer et al. 2013). Recordings occupying the same 10 10 km square were pooled together and all distinct dialects among them recorded (Supplementary material Appendix 1, dataset A1). On the squares that contained at least one recording with identified dialect we then performed a rarefaction analysis, developed in R (R Core Team) (Supplementary material Appendix 1, Script A1).

We created random subsamples of various size (from 1 square to the total number of squares with recordings within the given country) and counted the number of captured dialects within these subsamples. For each num-ber of squares, resampling was performed 1000 times. This allowed us to compare the saturation curves (median number of detected dialect types within 95% central range; i.e. 2.5 to 97.5% quantiles) for both countries.

Results

In total, we obtained 510 recordings of yellowhammer songs with spatial information (305 from Britain and 205 from New Zealand). After removing songs lacking the final, dia-lect-specific phrase, 365 recordings were suitable for analysis, 224 from Britain and 141 from New Zealand. We achieved similar coverage in both countries in terms of number of squares (10 10 km) in national grids (OSGB36, NZMG), from which we had at least one finished song (136 for Britain and 105 for New Zealand).

In total, nine distinct dialect types (i.e. distinct combi-nations of terminal syllables) were detected. Britain and New Zealand differed markedly in dialect composition (Fig. 2A, Table 1). While two dialects were shared (BC, XB), seven were found in only one country: two in Britain, and five in New Zealand. This difference was not caused by sam-pling effort as illustrated by the rarefaction analysis (Fig. 3); the 95% central range band for the two countries ceased overlapping even in subsamples four times smaller (28) than our actual sample size (105 for New Zealand).

We recorded four dialect types in Britain, although one of these was sung by a single male only. British recordings were dominated by the XB dialect, followed by the BC dialect and an intermediate, three-syllable dialect XBC (Table 1). The southwest of England was dominated by a single dialect (XB)

Table 1. Proportion of dialect types recorded in the two countries.

Country Total XB BC XBC XBB BhBl BlBh BD BBe BDB Mixed singers

Great Britain birds 224 150 42 28 1 – – – – – 3squares 136 97 38 9 1 – – – – – 3

New Zealand birds 141 66 3 – – 20 18 16 8 4 6squares 105 50 3 – – 18 13 15 6 4 4

Figure 3. Rarefaction curves constructed in order to remove the bias potentially caused by uneven sampling effort and different sample sizes in the two countries. We used national grids with squares 10 10 km (OSGB36, NZGM). We created random subsamples from all squares containing at least one song with identified dialect. For each size of the subsample (ranging from 1 to the total number of squares with recordings per region) we made 1000 permutations and calculated the median value of number of dialects (represented by solid line) and 2.5 to 97.5% quantile range (shaded areas). NZ – New Zealand, GB – Great Britain.

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while in other parts of the country dialects alternated across relatively short distances creating a mosaic-like distribution (Fig. 2A). Three recordings from the adjacent island of Ire-land were all of XB dialect.

We found substantially more yellowhammer dialects in New Zealand (seven) than in Britain (Table 1). The most frequent in New Zealand was XB, while three others, BhBl, BlBh and BD, were also common and were recorded with similar frequency. The remaining three dialects (BBe, BDB and BC) were less frequent. There were also mixed singers, capable of singing XB songs in combination with character-istics typical of another dialect: BhBl (4 recordings from 2 squares), BD (n 1) and BlBh (n 1).

As in Britain, the XB dialect prevailed in New Zealand, being found across large areas, particularly coastal regions of the South Island. In other areas, birds singing different dialects were more intermixed. For example, three dialects (BhBl, BlBh and BBe) were found within the 10 km radius around Dunedin (Otago, South Island). Five dialects found in New Zealand (BhBl, BlBh, BD, BBe and BDB) were not found in Britain. Interestingly, dialect BC, the second most common dialect in Britain, was recorded only three times in New Zealand, in a limited area of North Island (Bay of Plenty).

Direct comparison of the Brighton area (Sussex) and two regions in South Island (Canterbury and Otago), corre-sponding to the localities of capture and release, respectively, showed even more striking differences between the native and invaded areas (Fig. 2B). While the coastal area of Canter-bury region was, similarly to southern England, dominated by the XB dialect, birds in Otago sang mostly dialects not found in the UK.

Even though we had substantially fewer recordings with complete songs from North Island (n 19) than from South Island (n 121), the sample was large enough to cover all but one dialect (BDB) recorded in New Zealand, including the dialect BC which we did not find on South Island.

Discussion

History preserved in a song: a greater diversity of dialects in the invaded range

Our study is the first to explore post-invasion changes in the song dialects of an invasive bird species. We found a striking difference in the total number and spatial distribution of yel-lowhammer dialects recorded in the original range in Britain and introduced range in New Zealand. Although we expected a reduction of the number of dialects in the introduced range due to the bottleneck effect, the opposite was true (Fig. 2A). While Britain was dominated by only three dialects, seven different terminal syllable combinations were recorded in New Zealand. These were often scattered over relatively small areas in a mosaic-like fashion, but in some areas, their spatial

distribution was more uniform. For instance, a large part of Canterbury is dominated by the XB dialect and the borders between different dialect zones might be indicated by mixed singers, as is the case in Europe (Glaubrecht 1989). Finally, the overall richness of dialects was high in both North and South Island, with six dialects each.

Two corresponding areas of origin and introduction identified from historical records (Pipek et  al. 2015), southern England and the Dunedin area on the South Island of New Zealand, do not match in dialect composition (Fig. 2B). In fact, the broad areas around Brighton and Dunedin have no dialects in common (the closest bird singing the XB syllable combination was recorded 65 km from the centre of Dunedin). Unfortunately, the source areas of birds liberated in other New Zealand regions are unknown (Pipek et  al. 2015) and thus do not allow such direct comparison. Interestingly, the dialects found in New Zealand but not Britain are known from continental Europe (Petrusková et  al. 2015). This holds even for birds singing a rare combination of terminal syllables BDB (Supplemen-tary material Appendix 1, Fig. A1), which had been previ-ously recorded in Denmark (Hansen 1985) and the Pyrenees (Matheu 1997).

This is not the first time that the written history of an invasion has come into apparent conflict with the interpreta-tion derived from an alternative methodological approach. For example, molecular data indicate that the spread of Euro-pean hedgehogs Erinaceus europaeus through New Zealand was in the opposite direction to that implied from histori-cal documents (Bolfíková et al. 2013). However, the absence from Britain of some of the yellowhammer dialects found in New Zealand can be explained without discarding the his-torical record. It is often assumed that when populations in native and invaded ranges differ, it is the exotic population which must have changed (Lockwood et al. 2005, Congdon and Briskie 2014). However, it can be the other way around because factors such as changes in land use in the native range can have a similar or even stronger influence than the novel environment. For example, morphological changes during the last ca 150 yr are in fact more apparent in British birds than in those introduced to New Zealand, with contempo-rary British yellowhammers being smaller than those in the mid-19th century (Blackburn et al. 2013). We argue that, in a similar vein, the most likely explanation for the observed differences in song dialects is that New Zealand yellowham-mers have retained song structures that were present in both countries originally, but were lost from the source popula-tion in the UK subsequently. Such a phenomenon has been demonstrated in the much more complex system of human languages, for example in colonial English dialects (Hickey 2005) or in isolated Czech villages in Romanian Banat (Costachie et al. 2011).

Yellowhammers have experienced a significant population decline in Britain in recent decades, in particular between the 1980s and 1990s (Robinson et al. 2016) due to the influence of modern agricultural practices (Chamberlain et  al. 2000).

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This apparently resulted in a loss of genetic variation (Lee et al. 2001) and could also have been associated with a loss of song variability. Corn buntings Emberiza calandra, which have declined even more sharply in response to the same pressures, lost some dialect variation as a consequence of land-use change (Holland et al. 1996). In Poland (Osiejuk and Ratyńska 2003) and in the UK (McGregor 1981), they showed clear dialect borders only in areas with high population densities.

Alternative scenarios: no evidence for sampling bias and winter immigration

There are, however, several alternative scenarios, which might have resulted in the current contrasting patterns between Britain and New Zealand, although we do not find them as plausible as the above scenario. First, since we have sam-pled only a small portion of birds (300) out of the whole population of yellowhammers in Britain (approx. 700 000 pairs) (Musgrove et  al. 2013), some of the dialects could have ‘missed the net’. Sampling bias, the simplest expla-nation for observed differences, seems unlikely however (Fig. 3). In fact, in only 19 complete songs from North Island of New Zealand, there were more distinct syllable patterns than in the whole of Britain from which more than ten times as many recordings were available.

An alternative but also unlikely explanation of the mis-match between the British and New Zealand dialect com-position (particularly in the well-studied southern regions of both areas) would be a different origin of the birds intro-duced to New Zealand than assumed. Most of the birds for Otago departed from the UK in winter (Pipek et al. 2015), and thus some of the birds captured in southern England for this purpose might have been winter immigrants from elsewhere, possibly including continental Europe (Knox and Parkin 2009). However, yellowhammers are generally sedentary, both in Britain (Wernham et al. 2002) and else-where in Europe (Cepák et al. 2008). They rarely cross the English channel: between 1906 and 2015, only two yel-lowhammers out of 2500 recaptured in Britain were ringed outside of the British Isles (Robinson et al. 2015), and only three yellowhammers ringed in Britain were recovered else-where in that period (Robinson et al. 2015). Furthermore, the mainland European localities connected with Britain through known ringing recoveries seem to be dominated by the XB dialect (Petrusková et al. 2015) (see Supplementary material Appendix 1, Fig. A2 and dataset A2). Therefore, oversea migration is also a highly unlikely explanation of the observed discrepancy between British and New Zealand dialect composition.

In the 19th century it was not uncommon for birds caught in continental Europe, e.g. game birds or rare species such as ortolan buntings Emberiza hortulana, to be sold on London markets (Shrubb 2013); yellowhammers obtained that way might have sung with different dialects. However, it was probably not economically viable to import this species to Britain, where it was amongst the most common

bird species (Holloway 2002). Furthermore, this scenario cannot explain the patterns around Dunedin, as it is well documented that all yellowhammers released in that area were caught in the vicinity of Brighton (Pipek et al. 2015).

One could also argue that our sampling is biased towards southern England and that some birds from less sampled British areas, such as Scotland, could migrate southwards in winter (Balmer et al. 2013, BirdLife International 2016). However, even within Britain the birds do not move far; 95% of recaptured yellowhammers were recovered within 25 km from the locality they were ringed in, with the median dis-tance being less than 1 km (Wernham 2002). The longest known distance between ringing and capture within Britain is only 153 km (Robinson et al. 2015). There is no indication that yellowhammers migrate over long distances in New Zea-land either (Angus 2013), although it must be admitted that in this country they are banded much less frequently than in Britain (Cossee 1998, Jamieson et al. 2016).

Alternative scenarios: convergent evolution is possible but less likely than preservation of dialects in the invaded range

Finally, the patterns observed could result from conver-gent evolution of the yellowhammer songs in New Zealand leading to similar patterns as in continental Europe. As this species’ song has a relatively simple structure, and the dia-lects differ in frequency and modulation of only two or three terminal syllables, there is not infinite space for innovation. During the initial phase of invasion, there was probably a lack of tutors (birds from which the males learn their song in the sensitive period of their life) as well as lack of positive or negative feedback from conspecifics. Under such conditions, there is a room for errors in learning and development of song elements. For example, New Zealand native saddlebacks Philesturnus rufusater evolved completely new song types in just a few decades after translocation (Parker et  al. 2012). Similarly, alien Eurasian tree sparrows have replenished lost song variability with brand new syllables (Lang and Barlow 1997). It seems likely that initial phrases of yellowhammer song also undergo fast evolution, as they can show extreme variability even on a very small scale (Caro et al. 2009). How-ever, in our study we focused not on overall song-element variability but only on the final part of the song, which is, unlike initial phrases, clearly connected to geography. Fur-thermore, in contrast to saddlebacks, yellowhammers learn and fix their song in the first two years (Schön 1989), and only extremely rarely acquire some song characteristics from other bird species (Swart and Zavadil 2015). Therefore, it is reasonable to assume that they would more often lose syllables than develop new ones.

Unfortunately, there are only very few old recordings available, which prevents the evaluation of possible temporal trends in dialect composition. While there is no record of ‘missing’ dialects since the 1950s in Britain, one of the ‘new dialects’, BDB, was recorded in New Zealand almost 60 years ago, confirming its long-term presence in the country.

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Most dialects as found in New Zealand have been recorded in several well separated places around Europe, such as Roma-nia, Czech Republic, Spain or Finland (Petrusková et  al. 2015). Whether this pattern has arisen through independent evolution or through rare long-distance migration remains unanswered. If new syllable combinations developed easily in low-density yellowhammer populations, however, we would expect greater dialect variability in Britain, especially follow-ing the species’ recent decline (Robinson et al. 2016).

To conclude, we can dismiss with confidence the sam-pling bias and winter immigration scenarios as a cause of the patterns observed in yellowhammer dialects. Although we cannot do so for the convergent evolution of song structures, we believe that the more parsimonious explanation is the loss of dialects in the native range due to population declines, rather than the independent emergence of numerous syl-lable combinations in New Zealand and Europe. Thus, we propose that yellowhammer songs in New Zealand are in a sense a living archive of those sung in 19th century Britain.

Acknowledgements – We thank all authors of recordings; G. Helbling, C. Tipp, L. McPherson and J. Mortimer for providing access to recordings of BBC Sound Effects Library, British Library, McPherson Natural History Unit Sound Archive, and NZ Dept of Conservation, respectively. British Ornithological Union, British Guides, Ornithological Society of New Zealand, Royal Forest and Bird Protection Society of New Zealand, Wildlife Recording Society, 10 000 Birds and Veronika Meduna from Radio New Zea-land helped us spreading the information about our citizen-science project. We also thank T. Blackburn for editing the manuscript and P. Scofield for enriching discussions. Finally, we thank the late V. Jarošík for being always open to new ideas. The project was financially supported by long-term research development project RVO 67985939, Praemium Academiae award from the Czech Academy of Sciences (to P. Pyšek) and by the Grant Agency of Charles Univ. in Prague (project no. 312213).

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Supplementary material (Appendix ECOG-02779 at < www.ecography.org/appendix/ecog-02779 >). Appendix 1.