Tambourine Dove song and songs of other dove and pigeon species
The song of the Tambourine Dove we studied is equivalent to the perch coos described for other pigeons and doves in that it is a long-range signal aimed to attract potential mates and deter rivals (Baptista et al. 2018a). To accomplish these functions, a song has to convey information about the signalling species, as well as the quality, motivation and identity of the signaller (Bradbury and Vehrencamp 2011). As the song of the Tambourine Dove is relatively simple, one may ask how this information transfer can be achieved? When comparing the Tambourine Dove song with those of the well-studied Streptopelia doves, they have a considerably longer song duration with more notes within a song, and more variable number of notes within songs. In Streptopelia doves, songs are built from a few syllables (seven in the Laughing Dove (Streptopelia senegalensis) is likely the maximum) and last less than 4 s (de Kort and ten Cate 2004). Songs of the Tambourine Dove were, on average, three times longer and sometimes contained over 50 notes (Table 1). So, it seems that, at least in the time domain, there is a lot of ‘space’ for the coding of potentially important information.
Beside song duration, which was quite variable, songs of the studied species were highly stereotyped and characterized by a low overall frequency and narrow frequency band. Songs mostly consisted of unchanging, short and tonal-whistle notes (see Fig. 1). Such results may suggest that frequency and note characteristics could be cues for species recognition. However, real recognition is likely more intricate. The Tambourine Dove is one of the five species of the genus Turtur. All of them occur in Africa, inhabit habitats from forest to more or less wooded savanna where their ranges may overlap (Baptista et al. 2018b). All these Turtur species have very similar songs, typically consisting of 20‒40 short, unmodulated whistles that form long song strophes of 10‒18 s. The predominating song frequency band ranges between 300 and 700 Hz, meaning all of the species songs overlap in time and frequency (Baptista et al. 2018b). In addition, there is no obvious difference in the pattern of note organisation within a song for all species, with the notes of a song accelerating when closer to the end. Currently, there is no study focused on the between-species differences in Turtur species songs, and so it is hard to quantitatively represent how similar (or dissimilar) the songs are. With some confidence, one can state that the songs of these species are very similar, and if they are different, this difference is sophisticated, using multidimensional characters of the song.
The unmodulated-whistle of the notes and the low frequency of the song for Turtur doves, seem to be an adaptation for signalling in forested habitats. It is a trademark of this group, as their sister clade Chalcophaps indica (separated by ~ 13 million years), or the more distantly related, Gallicolumba luzonica and Geopelia striata (~ 23‒24 million years) have clear differences in their song, having longer whistles with less of a pure-tone (Khan and Arif 2013).
Pigeon and dove species have a relatively simple syrinx, reducing the ability to modify the frequency and tone of syllables produced (Suthers 2004; Elemans et al. 2004, 2008; Mindlin and Laje 2005). The low frequency and narrow bandwidth of sounds that build the Tambourine Dove song reflect the limitation the syrinx structure puts on vocal output. The initial note of the study species song was softer and the only one showing any difference in frequency (Fig. 1). Some syllables were produced very fast in quick succession, but again, they did not vary in frequency (Fig. 1). Unlike some Streptopelia doves, we also found no obvious amplitude modulation in notes (e.g. Slabbekoorn and ten Cate 1997).
All these characteristics seem to be common for all Turtur species and there is a need for a comparative study to find out which features of their song may allow for inter and intraspecific discrimination. It is likely that for each species, the differences are due to the note rhythm within the song and so pauses between notes may allow for better discrimination than other characteristics (Baptista et al. 2018a).
Species versus individual recognition
The second aim of this study was to find out if there are any song features that enable for the individual recognition of a Tambourine Dove. For such recognition, individuals must have a signal characteristic that is individual to them, where there is low variation within the individual but high variation between individuals (Budka and Osiejuk 2014). Similarly, for species recognition, signals must have low intraspecific variation but high interspecific variation (de Kort and ten Cate 2001). As previously mentioned there are few studies, all of which are on a single species, which suggest the occurrence of acoustic recognition in doves (Morris and Erickson 1971; Mairy 1977; Fusani 1994; Hutchison et al. 1997). In these species it has been found that both frequency and temporal variables differ between individuals, but the overall variability was greater in temporal variables.
Our analysis of the potential for identity coding revealed that a single song or note feature does not provide enough information for identity recognition. Only the overall peak frequency of a song had a PIC value over 1, which indicates a relatively weak potential for identity coding (Charrier et al. 2001). Each of the remaining characters, when regarded as a single variable, was too variable to be a good identity cue.
A multidimensional approach (DFA) in which different song features were used together, revealed that individual recognition by song in the studied species might be potentially quite an efficient method. Using just a few parameters of the whole song allowed for the correct assignment of 77% of individual songs to particular males. In the final analysis we were able to correctly classify 96.7% of songs. Without a doubt the strongest effect was the between note duration of the initial syllables (allowing for 92.3% correct classification), which carried more individual information than note peak frequency alone (58% correct classifications). This result suggests that in the studied species, the temporal organisation within a song is more variable among individuals, thus providing a background for individual differences. Similar studies (Ballintijn and ten Cate 1997a; Slabbekoorn et al. 1999) have discussed how temporal parameters are the best at discriminating between different vocalisation types and different individuals and so this is likely to occur for all Columbiformes due to the limitations presented by the syrinx structure. These results suggest that features that differentiate individuals are those linked with the rhythm of note production and that they are restricted to the initial notes of the song. This makes sense as song duration varied within an individual depending on whether the final notes within the song were produced, but the beginning notes always remained highly stereotyped.
It is worth noting that the DFA was calculated for the whole set of recordings using 41 individuals at a time, while in natural situations birds are not faced with the problem of discriminating between such a large group of conspecifics at once. In most situations they have to recognize whether the song belongs to a neighbour or stranger (male perspective), or if the song belongs to their mate or an individual who has the potential for being a future mate (female perspective) (Tibbetts and Dale 2007). The high values of correct DFA classification we found for automatic recordings at the same points but separated by longer period also suggest that songs of males are very stable in structure. However, this result should be confirmed by the analysis of recordings of individually marked birds in the future. Hence, high values of correct DFA classifications should be interpreted as a very big potential for individual recognition in the studied species. On the other hand, our result should be treated with caution as finding even huge between-individual differences is not a guarantee that birds used them for recognition (e.g. Budka and Osiejuk 2014). For example, the Pink Pigeon (Nesoenas mayeri) from Mauritius was even found to respond similarly to playback of close conspecific calls from Madagascar (Wolfenden et al. 2015). Hence, there is a strong need for experimental studies on both between- and within-species vocal discrimination in doves.
There is no doubt that song in Columbiformes is inherited genetically (Lade and Thorpe 1964). A question arises as to how birds may achieve individual specificity without losing species specificity, if indeed the signal is genetically transmitted? Or, how individual differences appear during ontogenesis and why they are only linked with specific song features? Cues for answering such questions were provided by Lade and Thorpe (1964). Using hybrids of Streptopelia doves they found that there was no radical change in the tonal quality of the sound produced, with some hybrids producing songs with a broken or intermediate rhythmic pattern. Later, ten Cate and Ballintijn (1996) experimentally revealed that notes are elementary units of sound production in the Collared Dove (Streptopelia decaocto). Therefore, one may expect that doves are more flexible in changing the pauses between notes (hence changing rhythm or rate) than in changing note characteristics. This however does not mean that doves are not able to produce differentiated notes. In the Collared Dove, males produce modulated or unmodulated coos and it was shown that differences in frequency affect the strength of response (Slabbekoorn and ten Cate 1997). Additionally, a study on coo development supports the hypothesis that frequency modulations play an important role in intra-specific communication and may signal age and sex of an individual (Ballintijn and ten Cate 1997a, b). However, in the Tambourine Dove and other Turtur sp. they have virtually no frequency modulations, thus, it seems that in this clade this kind of song variation was lost during their evolution or never gained.