The songs of male pied flycatchers: exploring the legacy of the fathers

Translocated wild birds are predisposed to learn songs of their ancestral population

Population differences in socially learned mating signals like oscine birdsong are particularly vulnerable to breakdown through dispersal.1 Despite this challenge, geographic variation in learned signals is ubiquitous.2 A proposed explanation for this pattern is that birds express predispositions to selectively learn and produce population-typical songs.3,4,5 While experimental studies on lab-reared birds have shown the existence of within-species learning predispositions,6,7,8,9,10 it remains unclear whether and how learning predispositions influence song acquisition in the wild. Here, we investigated innate song learning predispositions in wild pied flycatchers (Ficedula hypoleuca) by measuring the songs of individuals translocated as eggs from a Dutch population to a breeding population in Sweden. We compared the songs of the adult males hatched from these translocated eggs with those from the ancestral and receiving populations. Songs of translocated males closely resemble the local Swedish songs to which they were exposed during development, supporting the importance of social learning. However, translocated males selectively learned those local Swedish song elements that sound the most "Dutch-like." As a result, their songs are significantly shifted toward those of the ancestral Dutch population. This suggests that innate learning predispositions track ongoing song evolution in wild populations of pied flycatchers. We propose that as songs continue to diverge over time, this coevolutionary relationship between song and learning predispositions may contribute to the emergence of incipient pre-mating barriers.

Species-specific song responses emerge as a by-product of tuning to the local dialect

Oscine birds preferentially respond to certain sounds over others from an early age, which focuses subsequent learning onto sexually relevant songs.^1,2,3 Songs vary both across species and, due to cultural evolution, among populations of the same species. As a result, early song responses are expected to be shaped by selection both to avoid the fitness costs of cross-species learning⁴ and to promote learning of population-typical songs.⁵ These sources of selection are not mutually exclusive but can result in distinct geographic patterns of song responses in juvenile birds: if the risks of interspecific mating are the main driver of early song discrimination, then discrimination should be strongest where closely related species co-occur.⁴ In contrast, if early discrimination primarily facilitates learning local songs, then it should be tuned to songs typical of the local dialect.^5,6,7 Here, we experimentally assess the drivers of song discrimination in nestling pied flycatchers (Ficedula hypoleuca). We first demonstrate that early discrimination against the songs of the closely related collared flycatcher (F. albicollis) is not strongly affected by co-occurrence. Second, across six European populations, we show that nestlings' early song responses are tuned to their local song dialect and that responses to the songs of collared flycatchers are similarly weak as to those of other conspecific dialects. Taken together, these findings provide clear experimental support for the hypothesis that cultural evolution, in conjunction with associated learning predispositions, drives the emergence of pre-mating reproductive barriers.

Estimating heritability of song considering within-individual variance in a wild songbird: The collared flycatcher

Heritable genetic variation is a prerequisite for adaptive evolution; however, our knowledge about the heritability of plastic traits, such as behaviors, is scarce, especially in wild populations. In this study, we investigated the heritability of song traits in the collared flycatcher (Ficedula albicollis), a small oscine passerine with complex songs involved in sexual selection. We recorded the songs of 81 males in a natural population and obtained various measures describing the frequency, temporal organization, and complexity of each song. As we had multiple songs from each individual, we were able to statistically account for the first time for the effect of within-individual variance on the heritability of song. Heritability was calculated from the variance estimates of animal models relying on a genetic similarity matrix based on Single Nucleotide Polymorphism screening. Overall, we found small additive genetic variance and heritability values in all song traits, highlighting the role of environmental factors in shaping bird song.

A Call to Expand Avian Vocal Development Research

Birds are our best models to understand vocal learning – a vocal production ability guided by auditory feedback, which includes human language. Among all vocal learners, songbirds have the most diverse life histories, and some aspects of their vocal learning ability are well-known, such as the neural substrates and vocal control centers, through vocal development studies. Currently, species are classified as either vocal learners or non-learners, and a key difference between the two is the development period, extended in learners, but short in non-learners. But this clear dichotomy has been challenged by the vocal learning continuum hypothesis. One way to address this challenge is to examine both learners and canonical non-learners and determine whether their vocal development is dichotomous or falls along a continuum. However, when we examined the existing empirical data we found that surprisingly few species have their vocal development periods documented. Furthermore, we identified multiple biases within previous vocal development studies in birds, including an extremely narrow focus on (1) a few model species, (2) oscines, (3) males, and (4) songs. Consequently, these biases may have led to an incomplete and possibly erroneous conclusions regarding the nature of the relationships between vocal development patterns and vocal learning ability. Diversifying vocal development studies to include a broader range of taxa is urgently needed to advance the field of vocal learning and examine how vocal development patterns might inform our understanding of vocal learning.

Like Father Like Son: Cultural and Genetic Contributions to Song Inheritance in an Estrildid Finch

Social learning of vocalizations is integral to song inheritance in oscine passerines. However, other factors, such as genetic inheritance and the developmental environment, can also influence song phenotype. The relative contributions of these factors can have a strong influence on song evolution and may affect important evolutionary processes such as speciation. However, relative contributions are well-described only for a few species and are likely to vary with taxonomy. Using archived song data, we examined patterns of song inheritance in a domestic population of Java sparrows (Lonchura oryzivora), some of which had been cross-fostered. Six-hundred and seventy-six songs from 73 birds were segmented and classified into notes and note subtypes (N = 22,972), for which a range of acoustic features were measured. Overall, we found strong evidence for cultural inheritance of song structure and of the acoustic characteristics of notes; sons’ song syntax and note composition were similar to that of their social fathers and were not influenced by genetic relatedness. For vocal consistency of note subtypes, a measure of vocal performance, there was no apparent evidence of social or genetic inheritance, but both age and developmental environment influenced consistency. These findings suggest that high learning fidelity of song material, i.e., song structure and note characteristics, could allow novel variants to be preserved and accumulate over generations, with implications for evolution and conservation. However, differences in vocal performance do not show strong links to cultural inheritance, instead potentially serving as condition dependent signals.