Dominique A. Potvin

Geographically pervasive effects of urban noise on frequency and syllable rate of songs and calls in silvereyes (Zosterops lateralis)

Recent studies in the Northern Hemisphere have shown that songbirds living in noisy urban environments sing at higher frequencies than their rural counterparts. However, several aspects of this phenomenon remain poorly understood. These include the geographical scale over which such patterns occur (most studies have compared local populations), and whether they involve phenotypic plasticity or microevolutionary change. We conducted a field study of silvereye (Zosterops lateralis) vocalizations over more than 1 million km2 of urban and rural south-eastern Australia, and compared possible effects of urban noise on songs (which are learned) and contact calls (which are innate). Across 14 paired urban and rural populations, silvereyes consistently sang both songs and contact calls at higher frequencies in urban environments. Syllable rate (syllables per second) decreased in urban environments, consistent with the hypothesis that reflective structures degrade song and encourage longer intervals between syllables. This comprehensive study is, to our knowledge, the first to demonstrate varied adaptations of urban bird vocalizations over a vast geographical area, and to provide insight into the mechanism responsible for these changes.

Silvereyes decrease acoustic frequency but increase efficacy of alarm calls in urban noise

Many passerines adjust song attributes to avoid potential masking by anthropogenic noise. The costs of masking should be particularly high for vocalizations important for survival (e.g. alarm calls), but few studies have investigated how such calls are affected. We compared urban and rural silvereye, Zosterops lateralis, alarm calls across southeastern Australia, and found that urban calls had lower average, peak and maximum frequencies than rural calls. The average, peak and maximum frequency of alarm calls also decreased linearly with increasing background noise. The direction of this frequency shift runs contrary to expectations and previous findings of higher-pitched avian vocal signals in urban habitats, including higher-pitched song and contact calls in urban silvereyes. However, assuming no change in call amplitude, acoustic modelling indicates that the observed frequency shift would lead to a 20% increase in the predicted active space of alarm calls (i.e. the distance over which the calls can be detected by a conspecific bird) in urban noise, and therefore may be potentially adaptive. Our findings highlight the importance of considering behavioural and ecological contexts in urban acoustic-adaptation studies.

Song convergence in multiple urban populations of silvereyes (Zosterops lateralis)

Recent studies have revealed differences between urban and rural vocalizations of numerous bird species. These differences include frequency shifts, amplitude shifts, altered song speed, and selective meme use. If particular memes sung by urban populations are adapted to the urban soundscape, “urban-typical” calls, memes, or repertoires should be consistently used in multiple urban populations of the same species, regardless of geographic location. We tested whether songs or contact calls of silvereyes (Zosterops lateralis) might be subject to such convergent cultural evolution by comparing syllable repertoires of geographically dispersed urban and rural population pairs throughout southeastern Australia. Despite frequency and tempo differences between urban and rural calls, call repertoires were similar between habitat types. However, certain song syllables were used more frequently by birds from urban than rural populations. Partial redundancy analysis revealed that both geographic location and habitat characteristics were important predictors of syllable repertoire composition. These findings suggest convergent cultural evolution: urban populations modify both song and call syllables from their local repertoire in response to noise.

Paternal Song Complexity Predicts Offspring Sex Ratios Close to Fledging, but not Hatching, in Song Sparrows

Abstract Sex allocation theory predicts that population sex ratios should be generally stable and close to unity, but individuals may benefit by adjusting the sex ratio of their offspring. For example, females paired with attractive males may benefit by overproducing sons relative to daughters, as sons inherit their fathers attractive ornaments (“sexy son” hypothesis). Similarly, if compatible gene effects on fitness are more pronounced in males than females, genetically dissimilar mated pairs may enhance fitness by overproducing sons (“outbred son” hypothesis). We tested these hypotheses in Song Sparrows (Melospiza melodia) by examining offspring sex ratios of 64 complete families shortly after hatching (“early-stage”) and again shortly before fledging (“late-stage”) in relation to paternal song complexity and the genetic similarity of social mates. Neither early nor late-stage offspring sex ratio was related to parental genetic similarity. Nests of males with larger song repertoires contained more male-biased broods by the late-stage nestling period, but not in the early-stage nestling period. These findings suggest that attractive males may be better able to successfully raise male-biased broods, but not that females adaptively adjust primary sex ratios in response to their social mates attractiveness.

The relative roles of cultural drift and acoustic adaptation in shaping syllable repertoires of island bird populations change with time since colonization.

In birds, song divergence often precedes and facilitates divergence of other traits. We assessed the relative roles of cultural drift, innovation, and acoustic adaptation in divergence of island bird dialects, using silvereyes (Zosterops lateralis). In recently colonized populations, syllable diversity was not significantly lower than source populations, shared syllables between populations decreased with increasing number of founder events, and dialect variation displayed contributions from both habitat features and drift. The breadth of multivariate space occupied by recently colonized Z. l. lateralis populations was comparable to evolutionarily old forms that have diverged over thousands to hundreds of thousands of years. In evolutionarily old subspecies, syllable diversity was comparable to the mainland and the amount of variation in syllable composition explained by habitat features increased by two‐ to threefold compared to recently colonized populations. Together these results suggest that cultural drift influences syllable repertoires in recently colonized populations, but innovation likely counters syllable loss from colonization. In evolutionarily older populations, the influence of acoustic adaptation increases, possibly favoring a high diversity of syllables. These results suggest that the relative importance of cultural drift and acoustic adaptation changes with time since colonization in island bird populations, highlighting the value of considering multiple mechanisms and timescale of divergence when investigating island song divergence.

Genetic erosion and escalating extinction risk in frogs with increasing wildfire frequency

Wildfires are increasing in both frequency and intensity in many ecosystems, with climate change models predicting further escalations in fire-prone environments. Set against this background is the global decline of amphibians, with up to 40% of species facing extinction from multiple additive threats. Despite these disturbing figures, it is currently unclear how increasing fire frequency may impact the long-term persistence of frog populations. Following a severe wildfire in south-eastern Australia in 2009, field surveys indicated healthy tree frog populations. However, the 2009 fire had significant impacts on genetic diversity, including increased levels of inbreeding and declines in effective population size. Using stochastic population modelling under a range of fire-frequency scenarios, we demonstrate that amphibian populations in fire-prone environments may be increasingly vulnerable to extinction, particularly where rates of immigration are low. Synthesis and applications. This study of amphibian population genetics before and after a major wildfire emphasizes the importance of integrating both ecological and genetic data into population models. This will help managers make more appropriate conservation decisions regarding fire management of natural environments, especially those containing threatened populations. Priorities for agencies involved in planning controlled burns should consider carefully the timing of controlled burns, along with maintaining habitat connectivity.

Limited genetic differentiation between acoustically divergent populations of urban and rural silvereyes (Zosterops lateralis)

The bioacoustic attributes of vocalisations made by birds in urban environments often differ markedly from those of rural conspecifics. Whether such differences are result from genetic divergence between urban and rural populations, or from plasticity or cultural evolution of song remains poorly understood. Silvereyes (Zosterops lateralis) show evidence of acoustic adaptation to urban noise, modifying both their songs and calls in cities when compared to rural areas. We investigated whether these differences were associated with corresponding morphological and neutral genetic differences. Across six pairs of geographically separate urban and rural populations, all morphological traits measured were similar. Furthermore, genetic analyses of variation at nine microsatellite loci revealed high levels of genetic connectivity between populations, and similar levels of heterozygosity in both habitat types. Consistent directional shifts in song attributes of city birds across large geographic areas thus do not appear to be accompanied by associated morphological or neutral genetic divergence.

Identifying hybridization and admixture using SNPs: application of the DArTseq platform in phylogeographic research on vertebrates

Next-generation sequencing (NGS) approaches are increasingly being used to generate multi-locus data for phylogeographic and evolutionary genetics research. We detail the applicability of a restriction enzyme-mediated genome complexity reduction approach with subsequent NGS (DArTseq) in vertebrate study systems at different evolutionary and geographical scales. We present two case studies using SNP data from the DArTseq molecular marker platform. First, we used DArTseq in a large phylogeographic study of the agamid lizard Ctenophorus caudicinctus, including 91 individuals and spanning the geographical range of this species across arid Australia. A low-density DArTseq assay resulted in 28u2009960 SNPs, with low density referring to a comparably reduced set of identified and sequenced markers as a cost-effective approach. Second, we applied this approach to an evolutionary genetics study of a classic frog hybrid zone (Litoria ewingii–Litoria paraewingi) across 93 individuals, which resulted in 48u2009117 and 67u2009060 SNPs for a low- and high-density assay, respectively. We provide a docker-based workflow to facilitate data preparation and analysis, then analyse SNP data using multiple methods including Bayesian model-based clustering and conditional likelihood approaches. Based on comparison of results from the DArTseq platform and traditional molecular approaches, we conclude that DArTseq can be used successfully in vertebrates and will be of particular interest to researchers working at the interface between population genetics and phylogenetics, exploring species boundaries, gene exchange and hybridization.