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Oscine songbirds learn vocalizations that function in mate attraction and territory defense; sexual selection pressures on these learned songs could thus accelerate speciation. The Eastern and Spotted towhees are recently diverged sister species that now have partially overlapping ranges with evidence of some hybridization. Widespread community-science recordings of these species, including songs within their zone of overlap and from potential hybrids, enable us to investigate whether song differentiation might facilitate their reproductive isolation. Here, we quantify 16 song features to analyze geographic variation in Spotted and Eastern towhee songs and assess species-level differences. We then use several machine learning models to measure how accurately their songs can be classified by species. While no single song feature reliably distinguishes the two species, machine learning models classified songs with relatively high accuracy (random forest: 89.5%, deep learning: 90%, gradient boosting machine: 88%, convolutional neural network: 88%); interestingly, species classification was less accurate in their zone of overlap. Finally, our analysis of the limited publicly available genetic data from each species supports the hypothesis that the species are reproductively isolated. Together, our results suggest that small variations in multiple features may contribute to these sister species’ ability to recognize their species-specific songs. 

Bird song has historically been characterized as a primarily male behavior that evolves through sexual selection pressures involved in mate attraction. More recently, researchers showed that female song is far more prevalent in songbirds than previously thought, raising new questions about how other social functions of birdsong and sexual selection pressures on females might affect song evolution. Certain breeding systems, particularly cooperative breeding, are hypothesized to change social dynamics and sexual selection pressures on males and females and may thus influence song evolution in both. Here, we construct a large-scale database synthesizing species-level information on the presence of female song, the characteristics of presumably male song, social variables, and breeding systems, and we perform comparative phylogenetic analyses. Our results suggest that cooperative breeding and female song co-occur significantly more than expected and exhibit co-evolutionary dynamics; in particular, cooperative breeding appears to decrease the likelihood that female song is lost. Notably, we find evidence that these trends might be linked to certain social features associated with cooperative breeding, including social bond stability, but not others, such as increased group size. In addition, we observe that song repertoire size appears to evolve more slowly in cooperative breeding lineages. Overall, our findings demonstrate that cooperative breeding may have complex and sex-specific effects on song evolution, maintaining female song while slowing the rate of male song elaboration, suggesting that song in cooperatively breeding species could function in ways that differ from the traditional mate-attraction paradigm and that lesser-studied functions of songs may be evolutionarily consequential. 

Environmental stress, especially during development, can cause important phenotypic changes in individuals. In songbirds, these stress-induced changes have been shown to include impaired learning of song and reduced song complexity in several species. Typically, developmental stress has been studied in terms of individual variation within a population; however, birdsong, a culturally transmitted trait, may undergo relatively rapid changes if widespread learning impairment results from population-level environmental stress, and these changes could potentially be amplified if affected individuals become the song tutors for future generations. We thus hypothesize that ecosystem-wide stressors may cause population-level changes to birdsong. Here, we use publicly available birdsong recordings to determine whether song differences were evident after an abnormal and severe 2016 drought in western New York State. We analyzed birdsong recordings of two species, the Dark-eyed Junco (Junco hyemalis) and the Song Sparrow (Melospiza melodia), recorded between 2006–2020 in the drought-affected region and, for comparison, in two nearby regions less affected by the drought. The population-level song features of the species with more complex songs (Song Sparrow) changed in the drought area after 2016, but not in the control area. In the species with a more simple song (Dark-eyed Junco), we detected song changes in both regions, suggesting that the drought did not have an outsized effect on song in this species. These findings support a more nuanced hypothesis that stress-induced deficits may disproportionately affect species with songs that are more difficult to learn. These conclusions are tempered by the relatively sparse recording availability from years prior to 2016, but we predict that future longitudinal studies of song evolution in natural populations will be more tractable given the nearly exponential increase in the number of song recordings deposited in public repositories in recent years, making this experimental design a useful framework for future studies. 

Oscine songbirds learn vocalizations that function in mate attraction and territory defense. Sexual selection pressures on these learned songs could accelerate speciation. The Eastern and Spotted towhees are sister species that diverged recently (0.28 Ma) but now have partially overlapping ranges with evidence of some hybridization; widespread community-science recordings of these species, including songs within their zone of overlap and from potential hybrids, enable us to investigate whether song differentiation might facilitate their reproductive isolation. Here, we quantify 16 song features to analyze geographic variation in Spotted and Eastern towhee songs and test for species-level differences. We then use random-forest models to measure how accurately their songs can be classified by species, both within and outside the zone of overlap. While no single song feature reliably distinguishes the two species, a random-forest model trained on 16 features accurately classified 89.5% of songs; interestingly, species classification was less accurate in the zone of overlap. Finally, our analysis of the limited publicly available genetic data from each species supports the hypothesis that they are reproductively isolated. Together, our results suggest that, in combination, small variations in song features may contribute to these sister species’ ability to recognize their species-specific songs. 

The dark-eyed junco (Junco hyemalis) has experienced rapid phenotypic diversification within the last 18,000 years, resulting in several subspecies that reside in partially overlapping regions across North America. These subspecies have distinct plumage and morphology. If members of a subspecies disproportionately mate with one another, we would expect genetic differences to accumulate between the subspecies. In parallel, their learned songs could also accumulate changes. If song is used by individuals to recognize members of their own subspecies during mate selection, which would prevent the production of less fit hybrid offspring between subspecies, then song differences might co-localize with subspecies boundaries. Here, we quantify 10 song features to explore subspecies-level song variation using song recordings from community-science databases. We build a machine learning classifier to measure how accurately the subspecies’ songs can be distinguished from one another. Here, we show that songs of dark-eyed junco subspecies exhibit significant song-feature differences. However, these differences do not necessarily lead to distinguishability between subspecies. Notably, we find that subspecies pairs with adjacent ranges that do not hybridize have much more distinguishable songs, and also more evidence for genetic differentiation, than pairs that are known to hybridize. Thus, song distinguishability appears to have predictive power about which subspecies will hybridize, suggesting that song might play a role in reinforcing certain subspecies boundaries more than others. Finally, we analyze subspecies-level song differences alongside available genetic data and geographic coordinates to characterize the current evolutionary landscape of the dark-eyed junco subspecies complex. We observe geographic signal in the song and genetic data, indicating that individuals who share a range are more likely to share song characteristics and be genetically similar. This study illuminates the existence of subspecies-level song differences in the dark-eyed junco and provides further clarity on the role learned song plays in reinforcing reproductive boundaries between dark-eyed junco subspecies. 

The transition to specialization of knowledge within populations could have facilitated the accumulation of cultural complexity in humans. Specialization allows populations to increase their cultural repertoire without requiring that members of that population increase their individual capacity to accumulate knowledge. However, specialization also means that domain-specific knowledge can be concentrated in small subsets of the population, making it more susceptible to loss. Here, we use a model of cultural evolution to demonstrate that specialized populations can be more sensitive to stochastic loss of knowledge than populations without subdivision of knowledge, and that demographic and environmental changes have an amplified effect on populations with knowledge specialization. Finally, we suggest that specialization can be a double-edged sword; specialized populations may have an advantage in accumulating cultural traits but may also be less likely to expand and establish themselves successfully in new demes owing to the increased cultural loss that they experience during the population bottlenecks that often characterize such expansions. This article is part of the theme issue ‘Human socio-cultural evolution in light of evolutionary transitions’. 

Four zebra finches in a closed research colony presented with variable clinical signs, including masses, skin lesions,shivering, and/or ruffled feathers. These birds were not responsive to treatment efforts; 3 died and one was euthanized. All4 were submitted for necropsy to determine the cause of the clinical signs. Gross necropsy and histopathologic findings fromall birds resulted in a diagnosis of round cell neoplasia in multiple organs, including the skin, liver, kidney, and reproductivetract, with intranuclear inclusion bodies in the neoplastic cells. In all 4 cases, immunohistochemical staining showed strongimmunoreactivity for CD3 in 70% to 80% of the neoplastic round cells, with a relatively small subset that were immunopositivefor Pax5. These findings supported a diagnosis of T-cell lymphoma. Frozen liver tissue from one case was submittedfor next-generation sequencing (NGS), which revealed viral RNA with 100% sequence homology to canary polyomavirusstrain 34639 that had originally been identified in a European goldfinch. Formalin-fixed paraffin-embedded scrolls fromanother case were also submitted for NGS, which revealed viral RNA with 97.2% sequence homology to canary polyomavirusstrain 37273 that had originally been identified in a canary. To localize the virus in situ, RNAscope hybridizationwas performed using a probe designed to target the VP1 gene of the sequenced virus in frozen liver tissue. In all 4 cases,disseminated and robust hybridization signals were detected in neoplastic cells. These findings indicate that polyomaviruseshave the potential to be oncogenic in zebra finches. 

The development of rhythmicity is foundational to communicative and social behaviours in humans and many other species, and mechanisms of synchrony could be conserved across species. The goal of the current paper is to explore evolutionary hypotheses linking vocal learning and beat synchronization through genomic approaches, testing the prediction that genetic underpinnings of birdsong also contribute to the aetiology of human interactions with musical beat structure. We combined state-of-the-art-genomic datasets that account for underlying polygenicity of these traits: birdsong genome-wide transcriptomics linked to singing in zebra finches, and a human genome-wide association study of beat synchronization. Results of competitive gene set analysis revealed that the genetic architecture of human beat synchronization is significantly enriched for birdsong genes expressed in songbird Area X (a key nucleus for vocal learning, and homologous to human basal ganglia). These findings complement ethological and neural evidence of the relationship between vocal learning and beat synchronization, supporting a framework of some degree of common genomic substrates underlying rhythm-related behaviours in two clades, humans and songbirds (the largest evolutionary radiation of vocal learners). Future cross-species approaches investigating the genetic underpinnings of beat synchronization in a broad evolutionary context are discussed. This article is part of the theme issue ‘Synchrony and rhythm interaction: from the brain to behavioural ecology’.