The causes and evolutionary consequences of mixed singing in two hybridizing songbird species (Luscinia spp.)

Abstract

Bird song plays an important role in the establishment and maintenance of prezygotic reproductive barriers. When two closely related species come into secondary contact, song convergence caused by acquisition of heterospecific songs into the birds' repertoires is often observed. The proximate mechanisms responsible for such mixed singing, and its effect on the speciation process, are poorly understood. We used a combination of genetic and bioacoustic analyses to test whether mixed singing observed in the secondary contact zone of two passerine birds, the Thrush Nightingale (Luscinia luscinia) and the Common Nightingale (L. megarhynchos), is caused by introgressive hybridization. We analysed song recordings of both species from allopatric and sympatric populations together with genotype data from one mitochondrial and seven nuclear loci. Semi-automated comparisons of our recordings with an extensive catalogue of Common Nightingale song types confirmed that most of the analysed sympatric Thrush Nightingale males were 'mixed singers' that use heterospecific song types in their repertoires. None of these 'mixed singers' possessed any alleles introgressed from the Common Nightingale, suggesting that they were not backcross hybrids. We also analysed songs of five individuals with intermediate phenotype, which were identified as F1 hybrids between the Thrush Nightingale female and the Common Nightingale male by genetic analysis. Songs of three of these hybrids corresponded to the paternal species (Common Nightingale) but the remaining two sung a mixed song. Our results suggest that although hybridization might increase the tendency for learning songs from both parental species, interspecific cultural transmission is the major proximate mechanism explaining the occurrence of mixed singers among the sympatric Thrush Nightingales. We also provide evidence that mixed singing does not substantially increase the rate of interspecific hybridization and discuss the possible adaptive value of this phenomenon in nightingales.

Figure 1. Spectrograms of representative recordings of studied species.

Spectrograms of representative recordings of Common Nightingale, Thrush Nightingale “mixed singer”, and allopatric Thrush Nightingale (A), with songs characteristic for Common Nightingale in the mixed singer’s repertoire indicated by horizontal bars. The first Common Nightingale song from A is enlarged (B) to demonstrate the typical song organization of this species, consisting of alpha, beta, gamma and omega parts (after [52]).

Figure 2. Song analysis workflow: from a recording to category assignment.

Visualization of the semi-automated song assignment process comparing each song to be analysed with the catalogue of Common Nightingale song types, and assigning it to one of five song categories (bottom). Visual comparisons were conducted by two people independently of each other. Terms in italics indicate observer decisions (see Methods for details).

Figure 3. Examples of song categories, taken from repertoires of sympatric Thrush Nightingales.

The Common Nightingale song type included in the catalogue (A023), to which the examples for categories “LM catalogue” and “LM partial catalogue” were assigned, is shown on top. Different elements in the catalogue example and their counterparts in Thrush Nightingale songs are numbered; the asterisk indicates similar but distinct variants. Note that a particular song type is characterized by the order of different elements; the number of their repetitions is variable and does not influence the assignment to particular categories. See Methods and Fig. 2 for definitions of song categories and the workflow of category assignment.

Figure 4. Proportions of song categories in Nightingale recordings from sympatry and allopatry.

Proportions of song categories in song samples of allopatric and sympatric Common and Thrush Nightingales. Results averaged for different taxa (pure species and hybrids) and origin (allopatry vs. sympatry) are shown together with a map showing areas of sympatry and allopatry in the studied region (after [47]) and geographic origin of studied males (A). Numbers of analysed males are given in each pie chart; data for allopatric Common Nightingales are shown separately for the German population (the source for the song type catalogue), and for the Czech population. Repertoire compositions of individual analysed birds, ordered according to source region, taxon, and proportions of Common Nightingale song types, are shown in stack bars (B).