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  1. Human activity around the globe is a growing source of selection pressure on animal behavior and communication systems. Some animals can modify their vocalizations to avoid masking from anthropogenic noise. However, such modifications can also affect the salience of these vocalizations in functional contexts such as competition and mate choice. Such is the case in the well-studied Nuttall's white-crowned sparrow ( Zonotrichia leucophrys nuttalli ), which lives year-round in both urban San Francisco and nearby rural Point Reyes. A performance feature of this species' song is salient in territorial defense, such that higher performance songs elicit stronger responses in simulated territorial intrusions; but songs with lower performance values transmit better in anthropogenic noise. A key question then is whether vocal performance signals male quality and ability to obtain high quality territories in urban populations. We predicted white-crowned sparrows with higher vocal performance will be in better condition and will tend to hold territories with lower noise levels and more species-preferred landscape features. Because white-crowned sparrows are adapted to coastal scrub habitats, we expect high quality territories to contain lower and less dense canopies, less drought, more greenness, and more flat open ground for foraging. To test our predictions, we recorded songs and measured vocal performance and body condition (scaled mass index and fat score) for a set of urban and rural birds ( N = 93), as well as ambient noise levels on their territories. Remote sensing metrics measured landscape features of territories, such as drought stress (NDWI), greenness (NDVI), mean canopy height, maximum height, leaf area density (understory and canopy), slope, and percent bare ground for a 50 m radius on each male territory. We did not find a correlation between body condition and performance but did find a relationship between noise levels and performance. Further, high performers held territories with lower canopies and less dense vegetation, which are species-preferred landscape features. These findings link together fundamental aspects of sexual selection in that habitat quality and the quality of sexually selected signals appear to be associated: males that have the highest performing songs are defending territories of the highest quality. 
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  3. Abstract Urbanization is one of the most extreme forms of land transformation and results in changes to ecosystems and species compositions. As a result, there are strong directional selection pressures compared to nearby rural areas. Despite a surge in research on the different selection pressures on acoustic communication in urban and rural areas, there has been comparatively little investigation into traits involved with visual communication. We measured the plumage of museum specimens of white-crowned sparrows (Zonotrichia leucophrys) from urban and adjacent rural habitats in San Francisco, CA, to assess the effects of divergent habitats on plumage. We found significant differences in dorsal plumage, but not crown plumage, between urban and rural populations that have been diverging over the past 100 years. Urban birds have increasingly darker and duller dorsal plumage, whereas rural birds in adjacent areas have plumage with richer hues and more color complexity. Our findings suggest a newly observed adaptation to urban environments by native species and suggest that many traits, in addition to acoustic signals, may be changing in response to urban selection pressures. Additional collections in urban areas are needed to explore likely divergences in plumage coloration between urban and rural environments. 
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  6. Abstract

    In a rapidly warming world, exposure to high temperatures may impact fitness, but the gene regulatory mechanisms that link sublethal heat to sexually selected traits are not well understood, particularly in endothermic animals. Our experiment used zebra finches (Taeniopygia guttata), songbirds that experience extreme temperature fluctuations in their native Australia. We exposed captive males to an acute thermal challenge (43°C) compared with thermoneutral (35°C) and lower (27°C) temperatures. We found significantly more heat dissipation behaviours at 43°C, a temperature previously shown to reduce song production and fertility, and more heat retention behaviours at 27°C. Next, we characterized transcriptomic responses in tissues important for mating effort—the posterior telencephalon, for its role in song production, and the testis, for its role in fertility and hormone production. Differential expression of hundreds of genes in the testes, but few in the brain, suggests the brain is less responsive to extreme temperatures. Nevertheless, gene network analyses revealed that expression related to dopaminergic signalling in the brain covaried with heat dissipation behaviours, providing a mechanism by which temporary thermal challenges may alter motivational circuits for song production. In both brain and testis, we observed correlations between thermally sensitive gene networks and individual differences in thermoregulatory behaviour. Although we cannot directly relate these gene regulatory changes to mating success, our results suggest that individual variation in response to thermal challenges could impact sexually selected traits in a warming world.

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  7. Candolin, Ulrika (Ed.)
    Abstract Learned traits, such as foraging strategies and communication signals, can change over time via cultural evolution. Using historical recordings, we investigate the cultural evolution of birdsong over nearly a 50-year period. Specifically, we examine the parts of white-crowned sparrow (Zonotrichia leucophrys nuttalli) songs used for mate attraction and territorial defense. We compared historical (early 1970s) recordings with contemporary (mid-2010s) recordings from populations within and near San Francisco, CA and assessed the vocal performance of these songs. Because birds exposed to anthropogenic noise tend to sing at higher minimum frequencies with narrower frequency bandwidths, potentially reducing one measure of song performance, we hypothesized that other song features, such as syllable complexity, might be exaggerated, as an alternative means to display performance capabilities. We found that vocal performance increased between historical and contemporary songs, with a larger effect size for urban songs, and that syllable complexity, measured as the number of frequency modulations per syllable, was historically low for urban males but increased significantly in urban songs. We interpret these results as evidence for males increasing song complexity and trilled performance over time in urban habitats, despite performance constraints from urban noise, and suggest a new line of inquiry into how environments alter vocal performance over time. 
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