In this study we have recorded singing male sunbirds in many locations along the Rift and analyzed the spectrogram of their trill component. We found that each location had a slightly different dialect, and built a network of locations according to their dialects. We used data of the distance between each two dialects as the basic matrix. Since we had a full matrix, we had to set some threshold in order to remove some of the ties and get a meaningful network. We set the threshold to be the largest distance that still allowed all locations to be connected as one component, in order to be able to relate each location to the others.
|The trills in different locations|
The network of locations by song distances revealed locations that are "connected", i.e. their songs are relatively similar, while other locations were "disconnected" - their songs were quite different from each other. This network shows that there are "communities" of locations with similar songs (we determined the communities using the Girvan-Newman algorithm). Some of these communities consist of locations from the same geographical region, while others have a mix of locations from different regions. The locations in the Arava valley, the natural habitat of the sunbirds before the expansion, all belong to the same community. From there began the habitat expansion to the north. Three of the most isolated locations in the network indeed represent villages in the extreme north, suggesting these sunbirds do not interact much with more southern populations. Also interesting are the locations along the dead sea: sunbirds were present in Ein Gedi before the settlements, and their songs closely resemble the Arava songs. However, the songs of nearby Kaliya and Mitzpe Shalem, which were settled in the 1970s', are found in a different community, together with northern settlements, suggesting that these two locations were inhabited by sunbirds coming back from the north, and not directly from Ein Gedi.
|The network of locations according to sunbird songs|
Another interesting finding is that network centrality, which depicts how central were locations of singers in the network, was negatively correlated with genetic variability. This fact implies that the most central locations in terms of song dialects, which are Ein Gedi, Bet Zera and Sede Eliezer, host established populations of sunbirds which resist intruders. These locations are probably a source of dispersal to other places.
Overall, our results support the historical processes hypothesis of dialect formation, which predict song dialects of nearby locations which were occupied at the same time to be similar. This work illustrates the power of network analysis in describing not only social relations between animals, but also other types of relations. I believe it could be useful for many other types of analyses.