Authors: Jensen, Johan Kjellberg; Ziegler, Ann-Kathrin;Isaxon, Christina;Jiménez Gallardo, Lucía Gloria;Garcia Dominguez, Susana;Rissler, Jenny;Nilsson, Jan-Åke;Isaksson, Caroline;

Urbanization is rapidly altering habitat characteristics across the world, with significant effects on wildlife. However, we still lack understanding of which urban environmental factors cause what effects, especially on a physiological level. Here, quantified several environmental factors linked to urbanization: air pollution (PM2.5 and NO2), ambient temperature, artificial light and tree composition within 29 territories of breeding great tits (Parus major). We linked these environmental factors to the physiology of the nestlings using three biomarkers: circulating fatty acid composition, antioxidant capacity and an oxidative damage marker (malondialdehyde), in addition to mass and wing length. Through this novel approach, we could connect several physiological responses to urban environmental factors, including a significant reduction of antioxidant capacity of nestlings with increased levels of PM2.5 but also number of oak trees, and an interaction between the local number of non-native trees and artificial light sources on nestling mass. Moreover, by performing a principal component analysis (PCA), we show that these results would likely not be detected if the common method of urbanization indices were to be used instead of specific environmental factors. Taken together, we show the need for detailed, mechanistic studies to fully understand the effects of urbanization on wildlife health.