Humans have significantly impacted environmental conditions and landscapes for several decades through our urban expansion. These changes have produced selective pressures different from those present in non-urban areas. In response to these selective pressures, many species have diverged in physical, behavioral, and reproductive traits between their urban and non-urban populations. In addition, urban landscapes are typically more diverse and mosaic than non-urban environments, which produces different opportunities for local selection on a small spatial scale. Previous studies on singular species have shown that urban populations generally have more phenotypic variation as a result of this environmental variation. However, it remains to be seen how common this trend is across species.
Capilla-Lasheras et al. evaluated how urban conditions may alter the average value of phenology and reproductive phenotypes in bird species and how variable those traits are within populations. The authors conducted a literature review and analysis of various bird species from 399 urban and non-urban paired sites. They focused on life-history traits, specifically egg-laying date, clutch size, and the number of surviving fledglings. The referenced studies spanned publishing years from 1958 – 2020 and included 35 bird species. This wide range allowed the team to evaluate how urban expansion may have affected life-history trait values and variation over time. They predicted that urban populations of birds would lay their eggs earlier, have smaller clutch sizes, and fewer fledglings than their non-urban counterparts. They also hypothesized that all three life-history traits would have a broader range of values in the urban populations.
Urban birds have earlier and more variable breeding behavior
Through statistical analyses, Capilla-Lasheras et al. confirmed that urban bird populations had lower phenotypic values than non-urban populations for each life-history trait, meaning earlier lay dates, smaller clutch sizes, and fewer fledglings. Their results also showed correlations between these three traits, such that if there was little difference in clutch size between an urban and non-urban population, they also found little difference in the number of fledglings. Their analyses also confirmed that urban populations had overall higher phenotypic variation.
Looking to the future
This study is novel in its meta-analysis of the effects of urbanization on phenotypic variation. It confirmed that urbanization is correlated with a lower value in all three mean phenotypes and higher phenotypic variation. While this study cannot determine if these phenotypic shifts result from adaptive evolution, it demonstrates that they are consistently present and should continue to be investigated. Future researchers could tackle the question of adaptation vs. plasticity through long-term studies of individuals in paired urban and non-urban systems. Determining how growing urbanization affects trait variation can help us understand how those traits may evolve in the future and aid in the conservation efforts of urban species.
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Featured image: Turdus migratorius (Judy Gallagher)
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