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.

Captive Rearing Experiments Confirm Song Development without Learning in a Tracheophone Suboscine Bird

“…Similarly, one of the only known hybrid tracheophone suboscines was an antpitta ( Grallaria ) that produced a song structurally intermediate between the songs of its putative parent species [43]. This contrasts with the situation in hybrid oscines in which the structure of songs is typically unchanged from one or other of the parental song types, either because hybrid offspring copy songs from the parent male, or produce repertoires containing songs from both parental types (mixed singing) [44]–[46]. It has also been noted that there is little evidence of individual song variation, mimicry, repertoires or dialects in tracheophones, all suggesting an absence of vocal learning [30], [47], [48].…”

Early song discrimination by nestling sparrows in the wild

“…When closely related species come into contact, courtship signals could diverge as a consequence of reproductive character displacement (Grant and Grant, 2010;Kirschel et al, 2009;Seddon, 2005), or converge when there is no selection against heterospecific copying (Haavie et al, 2004;Laiolo, 2012;Secondi et al, 2003;Sorjonen, 1986;Tobias and Seddon, 2009;Vokurková et al, 2013). We currently lack a mechanistic explanation for these contrasting patterns of song divergence versus convergence; the presence or absence of early song discrimination could be one such mechanism that determines whether cultural traits diverge or converge.…”

The use of network analysis to study complex animal communication systems: a study on nightingale song

“…Songs were assigned to song types using a semi-automated method to identify nightingale song types using a combination of spectrogram cross correlation and visual comparison developed by M.W. (for details, see [35]). …”