Sounds of the city: The effects of urbanization and noise on mountain and black-capped chickadee communication.

Urbanization leads to structural changes to habitats and introduction of anthropogenic noise, both of which can interfere with avian communication. While studies suggest that birds can adjust their vocal behaviour in response to urban noise, the relative effects of urban habitat structure vs. noise on avian communication are not well understood. Further, understanding which species can adjust to noise and the mechanisms they use to achieve this can help conservation efforts predict which species will be most negatively affected by increasing urbanization. Species that possess certain vocal attributes (e.g., repertoires or the ability to learn new songs types) may be more or less able to adjust to urban noise. Here, I investigate how two closely related species (mountain chickadees Poecile gambeli and black-capped chickadees Poecile atricapillus) adjust their vocalizations in response to noise. I look at relative effects of habitat structure and noise on signal transmission and transfer, how each species adjusts vocalizations in response to urbanization and noise (both naturally occurring and experimental), and whether these adjustments actually improve audibility in noise. I found that urban noise affects signal transmission and transfer more than habitat urbanization (Chapter 2), and both species adjust their vocalizations to noise by using mechanisms related to their natural vocalizing behaviour. Mountain chickadees change the proportion of time they spend singing vs. calling and spectrally adjust their songs to emphasize higher frequencies (Chapter 3). In contrast, black-capped chickadees use their natural pitch-shifting abilities to selectively sing higher-frequency songs in noisy conditions (Chapter 4). I also found evidence that these adjustments may improve audibility in noise. Among mountain chickadees, focal males from quiet areas responded more aggressively to playbacks of urban than rural songs when embedded in experimental noise (Chapter 5). Among black-capped chickadees, focal males responded more