City life and chickadees: Effects of urbanization on vocal output and reproductive success of the mountain chickadee (Poecile gambeli)

Urbanization creates new types of habitats with novel benefits and challenges that are not found in natural habitats. How a species fares in urban habitats is largely dependent on its life history, yet predicting the response of individual species to urbanization remains a challenge. While “urban exploiter” species thrive in urban areas, other “urban avoiders” do poorly or are not present at all, and “urban adaptors” are able to adapt to some degree of urbanization. Mountain chickadees are year-round residents of montane regions of western North America. Commonly found in higher-elevation coniferous forests, these birds will also visit bird feeders in urbanized areas and readily nest in nest boxes. We monitored mountain chickadees along a habitat gradient, from natural habitat to suburban areas, to determine if the degree of urbanization was associated with: vocal output; clutch size and success; nestling growth rates; or variation in parental size and condition. Males in habitats with more urban features began vocalizing earlier, for longer periods, at higher rates, and produced a greater number of vocalizations than males in natural habitats. Females nesting in areas with more urban features and deciduous trees (non-native vegetation) initiated clutches earlier than those in natural areas, but neither fledging success nor the rate of nestling mass-change differed between habitats. Nestling feather growth-rate increased with later first egg dates, and the magnitude of this increase was greater in urban habitats than in rural ones. We suggest that these differences may be due to increased food abundance in both the pre-breeding and breeding seasons, allowing individuals to attain higher condition during the winter, maintain this condition through the breeding season, and better provision their offspring. Our results indicate no detriment to nesting in urban habitats, and suggest that mountain chickadees fall into the intermediate “urban adaptor” species category.