Habitat-related birdsong divergence: a multi-level study on the influence of territory density and ambient noise in European blackbirds

Abstract

Song plays an important role in avian communication and acoustic variation is important at both the individual and population level. Habitat-related variation between populations in particular can reflect adaptations to the environment accumulated over generations, but this may not always be the case. In this study, we test whether variation between individuals matches local conditions with respect to noise level and territory density to examine whether short-term flexibility could contribute to song divergence at the population level. We conducted a case study on an urban and forest population of the European blackbird and show divergence at the population level (i.e. across habitats) in blackbird song, anthropogenic noise level and territory density. Unlike in several other species, we found a lack of any correlation at the individual level (i.e. across individuals) between song features and ambient noise. This suggests species-specific causal explanations for noise-dependent song differentiation which are likely associated with variation in song-copying behaviour or feedback constraints related to variable singing styles. On the other hand, we found that at the level of individual territories, temporal features, but not spectral ones, are correlated to territory density and seasonality. This suggests that short-term individual variation can indeed contribute to habitat-dependent divergence at the population level. As this may undermine the potential role for song as a population marker, we conclude that more investigations on individual song flexibility are required for a better understanding of the impact of population-level song divergence on hybridisation and speciation.

Fig. 1

Spectrogram showing the stereotypic singing style of blackbirds. Each song starts with a motif part followed by a twitter part and there is a short pause without sound in between two consecutive songs

Fig. 2

Illustration of the study areas. The schematic map at the top of the illustration shows an overview of the locations of the two study areas, Meijendel and Leiden. Barred areas indicate urban habitat, and the light-grey area is forest-dune habitat. The left and right maps at the bottom of the illustration are satellite images of parts of the study areas in Meijendel and Leiden, respectively. Circles show territories in which blackbirds were recorded. The colours of the circles represent the anthropogenic noise level in the territories measured during the dawn chorus in Meijendel and between 7:30 a.m. and 8:30 a.m in Leiden (see the “Materials and methods” section for details). The number of lines attached to the circles represents the number of neighbours around the territory that were heard singing during the dawn chorus (see the “Materials and methods” section for details)

Fig. 3

Overview of the song differences between forest males (n = 24) and urban males (n = 27) and the relationships of song characteristics with noise levels and territory density. The top row shows results for the peak frequency of the motif (a–c), the second row for the peak frequency of the twitter (d–f), the third row for the twitter proportion (g–i) and the bottom row for the pause duration (j–l). The first column consists of boxplots showing the median (line), interquartile range (box) and 95% range (whiskers) of the forest population (white bars) and urban population (grey bar) for each of the four song characteristics (a,d,g,j). The middle column has scatterplots showing the noise levels measured during rush hour plotted against the song characteristics of urban males (b,e,h,k). The right column consists of scatterplots showing the territory density plotted against the song characteristics for forest males (white dots, dotted lines) and urban males (grey dots, solid lines) (c,f,i,l). Lines are only shown for significant relationships in the combined analysis of urban and forest males