Urbanization is associated with differences in age class structure in black-capped chickadees (Poecile atricapillus)

Urbanization has a tremendous impact on the environment from landscape features to distribution of food resources. Such drastic environmental changes can result in community, population, and individual differences between urban and non-urban animals. Urbanization has been associated with differential mortality and reproduction and therefore, differences in age structure may also be expected across urban gradients. Additionally, many traits studied along urban gradients, such as morphology, can also differ across age classes, and as such, age is an important factor to consider. Despite this, differences in age structure along urbanization gradients have only rarely been examined. Here, we use black-capped chickadee (Poecile atricapillus) age, morphological, and feeder visitation data to address whether age structure, morphology, and age-related winter survival differ across a gradient of urban land use in and around Ottawa, ON. We test the hypothesis that urbanization is associated with higher proportions of juveniles, because of an increased exploratory tendency by juveniles against the hypothesis that urbanization favors higher proportions of adults because of a slower pace of life. We further hypothesize that urban chickadees will be smaller than non-urban ones, because anthropogenic environments will attract both younger and worse quality individuals. We found that urban environments were associated with significantly higher proportions of first year individuals and these proportions remained stable from late fall through all of winter. We did not, however, find evidence that age related differences explained variation in morphology. Instead variation in morphology was small and inconsistently associated with both urbanization and age. The results stand in contrast to results for two species of European birds. The present study calls for using broadly available but under-exploited data to better understand urbanization-related differences in age structure and its implications for population-level processes such as disease transmission and information flow.

Automated summary

Urbanization has significant impacts on the environment, leading to differences in community, population, and individual characteristics between urban and non-urban animals. In a study on black-capped chickadees, it was found that urban environments have higher proportions of first year individuals, but age-related differences did not explain variation in morphology.