In a previous post titled Meta-analysis of Urban Bird Phenology, we discussed how urban conditions have affected the average value of phenology and reproductive phenotypes in bird species. Capilla-Lasheras et al. found that urban bird populations had lower values than their non-urban conspecifics for three life-history traits, indicating earlier lay dates, smaller clutch sizes, and fewer fledglings. Despite this consistency, available data have not shown many general patterns differentiating urban and non-urban birds, specifically with regard to the avian endocrine system. The endocrine system is an important determinant of both behavioral and physiological responses to environmental changes. Endocrine mechanisms are heritable and widely variable between species. Thus, they can be altered by evolution and could be a key factor in deciding which species thrive in urban systems, while others are negatively impacted. Deviche et al. aim to tackle that issue by evaluating the effectiveness and compatibility of the current literature on urbanization and avian endocrine systems.
No consistent findings across species
Deviche et al. searched for literature regarding several factors of urban systems that may impact avian endocrine systems, such as water and food availability and quality, ALAN, ambient noise, electromagnetic fields, chemical pollution, pathogens, and increased temperatures. While they found several studies that investigated the impact of these factors on birds, few studies directly searched for impacts on the avian endocrine system, despite its crucial role in physiological and behavioral responses. Additionally, most studies produced correlative data but did not search for causal links to specific urban conditions. The studies that researched endocrine responses found some correlations but did not produce consistent findings across taxa. This lack of uniformity makes it difficult to develop general hypotheses about the impact of urbanization on bird species. Deviche et al. provide several guidelines for future research to tackle this gap.
Future research
Primarily, Deviche et al. urge future studies to focus on determining causality. This is difficult due to the complexity and variability of urban ecosystems across the globe. They suggest creating networks of communication among national and international researchers studying these topics. This would allow experts in different endocrine systems to collaborate and help each other interpret data specific to their expertise. International collaboration can also help incorporate the greatest possible range of urban habitats. To create uniformity, these networks could be focused on species that are widely distributed, either continentally or globally, and standardize their experimental methods. However, Deviche et al. also acknowledge the importance of studying unique species and species that are declining in urban populations. Unique species can be used to challenge any premature generalizations found amongst the common species studied. And species that do not thrive in urban systems offer a critical contrast to urban exploiters and will help researchers understand the mechanisms facilitating success or failure in urban areas.
Looking forward
Human beings are the primary drivers of urbanization. While we are expanding our cities, some organisms are thriving while others are being negatively impacted. It is crucial to understand the mechanisms behind why some species prosper in urban areas to pinpoint what species may be at risk in the future from our continued expansion. Understanding these mechanisms can also help urban planners design future urban spaces to facilitate the needs of both human and wildlife residents.
Read the study
Featured image: Columba livia (Micah Carrick)
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