This study analyses the extent to which human activity affects the morphology of urban bird populations. To do so, it draws on continuous monitoring of an urban population of black-capped chickadees, one of the most common passerine species in North America. The availability of standardised data collected before, during and after the COVID-19 pandemic on a university campus in Los Angeles allowed the authors to take advantage of the “anthropause” associated with restrictions as a natural experiment to separate the effect of human presence from that of the city itself in the absence of people. This design, which is rare in urban ecology, was complemented by a comparison with non-urban populations in the surrounding area.

The results show rapid and reversible changes in beak size and shape, a key feature for feeding. Birds born during the “anthropause” had beaks similar to those of non-urban populations, while, after normal campus activity resumed, the beaks regained their pre-pandemic urban morphology within a few years. These observations are interpreted as evidence of this population's responsiveness to changes in the environment associated with human presence, possibly related to the availability of food scraps or changes in access to different food resources.

The conclusions fit well with the accumulated evidence that many species respond rapidly to environmental changes related to urbanisation. This work adds a novel element by showing that these traits can not only appear rapidly, but also be reversed in a very short period of time when conditions change.

However, the study leaves some important questions unanswered. It is unclear what the adaptive advantage of having a proportionally smaller beak in the urban environment would be, if any. With the current design, it is also not possible to discriminate between possible underlying mechanisms, such as genetic adaptation through natural selection, phenotypic plasticity, or changes in population composition. The latter possibility is particularly suggestive given the rapidity with which the average beak size of the urban reed warbler population changed. Such a population change could be generated, for example, by an episode of immigration from non-urban areas (with individuals with relatively larger beaks) due to the temporary cessation of disturbances in the city during the pandemic, or by selective emigration of urban reed warblers with smaller beaks, potentially unable to access other trophic resources when food scraps are scarce.

Overall, the study clearly illustrates that the morphological traits of some urban populations can respond rapidly to changes in human activity. Taken together, these results reinforce the idea that cities are not evolutionarily static environments and that our daily activities can influence, even in the short term, the biology of the species with which we share urban space. The rapid response observed in these reeds highlights both the pressure we exert on urban biodiversity and the responsibility, and opportunity, we have to create cities that are more compatible with other species. To this end, it is essential to have sufficient support to maintain long-term systematic studies that allow us to document the responses of populations to environmental changes that sometimes occur rapidly and unpredictably.

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