Nicole Creanza

The Eastern towhee ( Pipilo erythrophthalmus ) and Spotted towhee ( Pipilo maculatus ) are large New World sparrows found across North America. These two species were previously classified as a single species, the Rufous-sided towhee, which was separated in 1995 based on differences in plumage, geographic range, and song. Previous studies have shown that ecological factors, such as urbanization and climate, can affect learned vocalizations, particularly frequency-related song characteristics such as minimum frequency; however, most studies have been conducted on one species in a specific location. The extensive geographic distributions of these towhee species, along with their ample publicly available song-recording data, give us the unique opportunity to assess whether ecological pressures influence song variation. Here, we extract frequency-related song features from 2916 Spotted and Eastern towhee recordings and investigate whether geography and ecology—including recording location (latitude and longitude), tree cover, urbanization (nighttime lights, human population density, and distance to road), climate zones, elevation, and ecoregions—explain patterns in these song frequency variables of this sister-species pair. Our results show that geographic location, particularly longitude, contributes more strongly to the variation of song frequency features than urbanization, environment, and climate, suggesting that culturally transmitted differences in learned song, not ecology or anthropogenic structure, drive this variation. However, there was not a clear pattern of isolation-by-distance despite the geographic patterns that we found in their songs. Further, we did not find strong support for behavioral adaptation to habitat structure, but we did find a weak signal that urbanization was associated with increased frequency in Spotted towhees. Overall, we provide a detailed study on the interactions between ecology and song evolution, and provide new insight into the evolution of birdsong.

Birdsong has historically been characterized as a sexually selected, primarily male behaviour. Recent findings suggest that female song is widespread, raising questions about how social functions of birdsong shape song evolution. Certain social behaviours, such as cooperative breeding, could alter selection pressures on both sexes and potentially influence the evolution of both female and male song. Here we use phylogenetic comparative analyses across 1,041 songbird species to examine relationships between cooperative breeding, female song and male song characteristics. We show robust bidirectional coevolutionary dynamics between cooperative breeding and female song that persist when controlling for territoriality, allometry, phylogenetic uncertainty, geographical sampling and analytical biases. While cooperative breeding and female song commonly co-occur in strongly territorial systems, their association is especially pronounced in weakly territorial systems, where they co-occur much more often than expected by chance. Additionally, we observe that male song repertoire size evolves more slowly in cooperative breeding lineages. These findings demonstrate that cooperative breeding shapes the evolution of vocal communication differently based on territorial context and sex, with female song potentially serving crucial but understudied functions related to social cohesion in cooperative systems, particularly in species where territorial conflict is reduced.

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.

Innovations, such as symbolic artifacts, are a product of cognitive abilities but also of cultural context. Factors that may determine the emergence and retention of an innovation include the population's pre-existing cultural repertoire, exposure to relevant ways of thinking, and the invention's utility. Thus, we suggest that the production of symbolic artifacts is not guaranteed even in cognitively advanced societies.

The history of people’s movements and interactions shapes both genetic and linguistic variation. Genes and languages are transmitted separately and their distributions reflect different aspects of human history, but some demographic processes can cause them to be similarly distributed. In particular, forms of societal organization, including movements in and out of a community, may have shaped the transmission of both genes and languages. If children were more likely to learn their mother’s language than their father’s when their parents were from populations that spoke different languages or dialects, then language variation might show a closer association with maternally transmitted genetic markers than autosomal ones; this association could be further reinforced if children reside with predominantly maternal kin. We analyze the worldwide relationship between linguistic and genomic variation, leveraging the sex-biased transmission of X chromosomes to assess whether language has tended to be preferentially transmitted along the male or female line. In addition, we measure the effects of postmarital residence with female kin, matrilineal descent, and endogamy on the covariation of mitochondrial DNA and languages, using mtDNA because genomic data were available for very few populations with these ethnographic traits. We find that while there is little evidence for a consistent or widespread sex bias in the transmission of language, such biased transmission may have occurred locally in several parts of the world and might have been influenced by population-level ethnographic characteristics, such as female-based descent or residence patterns. Our results highlight the complex relationships between genes, language, ethnography, and geography.

Birdsong has historically been characterized as a sexually selected, primarily male behavior. More recent findings suggest female song is widespread, raising questions about how social functions of birdsong shape song evolution. Certain breeding systems, like cooperative breeding, change social dynamics and selection pressures on both sexes, providing important contexts for studying song evolution. Here, we use phylogenetic comparative analyses across 1041 songbird species to examine relationships between cooperative breeding, female song, and male song characteristics. Here, we show robust bidirectional co-evolutionary dynamics between cooperative breeding and female song that persist when controlling for territoriality, allometry, and geographic sampling biases. Importantly, when examining intensity of territorial defense, we find this relationship is context-dependent: while cooperative breeding and female song commonly co-occur in strongly territorial systems, their association is especially pronounced in weakly territorial systems, where they co-occur much more often than expected by chance. Additionally, we observe that male song repertoire size evolves more slowly in cooperative-breeding lineages. These findings demonstrate that cooperative breeding shapes the evolution of vocal communication differently based on territorial context and sex, with female song potentially serving crucial but understudied functions related to social cohesion in cooperative systems, particularly in species where territorial conflict is reduced.