Multidimensional environmental predictors of variation in avian forest and city life histories

Optimal life-history decisions are shaped by prevailing environmental conditions. In the context of urbanization, environmental differences between urban and rural areas are known to vary across a multitude of axes. The relative roles of specific axes and whether they explain variation in avian life histories between forest and city populations have not often been studied empirically. This study comprehensively views urbanization from a multidimensional environmental perspective. For each of 13 nest box plots of a common passerine bird (the great tit Parus major), we quantified temperature, humidity, light, and noise, and subsequently assessed direct versus indirect effects of each environmental axis on components of annual reproductive success by applying a path analytical framework. All quantified environmental axes, and life-history traits, showed substantial repeatable variation between the plots. Forest and city plots differed tremendously in temperature, humidity, and light. We were able to attribute among-population variation in life history to variation in these environmental effects. However, the simple dichotomy between forest and city populations explained the data best. Birds in the city laid earlier, which indirectly resulted in smaller clutches, and their offspring fledged in poorer condition, compared to conspecifics in forests. Those differences persisted after controlling for temperature, humidity, light, and noise, which implies that they were shaped by other factors than the ones quantified in this study. In summary, our findings question the common interpretation that differences between forest and city areas relate to specific environmental axes that covary with urbanization, especially in in lieu of quantitative measurements.