Urbanization and temperature interactively shape laying dates, food abundance and diet, but have a limited impact on fledging success of Hirundo rustica (Barn swallow)

The food limitation hypothesis suggests that food scarcity is the main driver of direct negative effects on breeding success of urban animals. However, the mechanistic pathways by which phenological mismatches are created by the urban heat island effect, noise pollution, and artificial light at night (ALAN), which reduce food abundance and ultimately breeding success, are understudied. Additionally, these processes may have indirect positive effects on food abundance and breeding success. It is therefore critical to disentangle the relative importance of these contradictory effects of urbanization. Here, we implemented a multidimensional framework considering these multiple factors, using stable isotope analysis and structural equation modelling, to assess direct and indirect effects on food abundance, diet and fledgling success in 53 nests of Hirundo rustica (Barn Swallow) across an urban landscape. We found that urbanization had a direct positive influence on soft-bodied insect abundance and reduced phenological mismatch, while increased ambient temperature was indirectly related to reduced phenological mismatch. We also found direct positive effects of soft-bodied insect diets on fledging success of H. rustica. The direct negative effects of laying date on fledging success were stronger compared to the indirect positive effects of laying date on fledging success. We observed no positive effects of ALAN on food abundance and direct negative effects on fledging success. Finally, our results indicate no substantial differences in fledging success across an urban gradient. This suggests that H. rustica can adapt to diverse urbanization levels by adjusting their laying dates and foraging on optimal prey types, thereby maintaining similar levels of fledging success across urban gradients. These results emphasize the importance of employing multidimensional frameworks considering multiple environmental factors to understand the complex effects of urbanization on urban trophic webs. Our study also provides insights on how to minimize the detrimental effects of urbanization on aerial insectivores.